PI(3,4,5)P3 and PI(4,5)P2 lipids target proteins with polybasic clusters to the plasma membrane

Vibrio MARTX toxin processing and degradation of cellular Rab GTPases by the cytotoxic effector Makes Caterpillars Floppy

Vibrio vulnificus causes life-threatening wound and gastrointestinal infections, mediated primarily by the production of a Multifunctional-Autoprocessing Repeats-In-Toxin (MARTX) toxin. The most commonly present MARTX effector domain, the Makes Caterpillars Floppy-like (MCF) toxin, is a cysteine protease stimulated by host adenosine diphosphate (ADP) ribosylation factors (ARFs) to autoprocess. Here, we show processed MCF then binds and cleaves host Ras-related proteins in brain (Rab) guanosine triphosphatases within their C-terminal tails resulting in Rab degradation. We demonstrate MCF binds Rabs at the same interface occupied by ARFs. Moreover, we show MCF preferentially binds to ARF1 prior to autoprocessing and is active to cleave Rabs only subsequent to autoprocessing. We then use structure prediction algorithms to demonstrate that structural composition, rather than sequence, determines Rab target specificity. We further determine a crystal structure of aMCF as a swapped dimer, revealing an alternative conformation we suggest represents the open, activated state of MCF with reorganized active site residues. The cleavage of Rabs results in Rab1B dispersal within cells and loss of Rab1B density in the intestinal tissue of infected mice. Collectively, our work describes an extracellular bacterial mechanism whereby MCF is activated by ARFs and subsequently induces the degradation of another small host guanosine triphosphatase (GTPase), Rabs, to drive organelle damage, cell death, and promote pathogenesis of these rapidly fatal infections.

Syntaxin 17 recruitment to mature autophagosomes is temporally regulated by PI4P accumulation

During macroautophagy, cytoplasmic constituents are engulfed by autophagosomes. Lysosomes fuse with closed autophagosomes but not with unclosed intermediate structures. This is achieved in part by the late recruitment of the autophagosomal SNARE syntaxin 17 (STX17) to mature autophagosomes. However, how STX17 recognizes autophagosome maturation is not known. Here, we show that this temporally regulated recruitment of STX17 depends on the positively charged C-terminal region of STX17. Consistent with this finding, mature autophagosomes are more negatively charged compared with unclosed intermediate structures. This electrostatic maturation of autophagosomes is likely driven by the accumulation of phosphatidylinositol 4-phosphate (PI4P) in the autophagosomal membrane. Accordingly, dephosphorylation of autophagosomal PI4P prevents the association of STX17 to autophagosomes. Furthermore, molecular dynamics simulations support PI4P-dependent membrane insertion of the transmembrane helices of STX17. Based on these findings, we propose a model in which STX17 recruitment to mature autophagosomes is temporally regulated by a PI4P-driven change in the surface charge of autophagosomes.

Nonlinear dynamics in phosphoinositide metabolism

Phosphoinositides broadly impact membrane dynamics, signal transduction and cellular physiology. The orchestration of signaling complexity by this seemingly simple metabolic pathway remains an open question. It is increasingly evident that comprehending the complexity of the phosphoinositides metabolic network requires a systems view based on nonlinear dynamics, where the products of metabolism can either positively or negatively modulate enzymatic function. These feedback and feedforward loops may be paradoxical, leading to counterintuitive effects. In this review, we introduce the framework of nonlinear dynamics, emphasizing distinct dynamical regimes such as the excitable state, oscillations, and mixed-mode oscillations-all of which have been experimentally observed in phosphoinositide metabolisms. We delve into how these dynamical behaviors arise from one or multiple network motifs, including positive and negative feedback loops, coherent and incoherent feedforward loops. We explore the current understanding of the molecular circuits responsible for these behaviors. While mapping these circuits presents both conceptual and experimental challenges, redefining cellular behavior based on dynamical state, lipid fluxes, time delay, and network topology is likely essential for a comprehensive understanding of this fundamental metabolic network.

Hippo and PI5P4K signaling intersect to control the transcriptional activation of YAP

Phosphoinositides are essential signaling molecules. The PI5P4K family of phosphoinositide kinases and their substrates and products, PI5P and PI4,5P2, respectively, are emerging as intracellular metabolic and stress sensors. We performed an unbiased screen to investigate the signals that these kinases relay and the specific upstream regulators controlling this signaling node. We found that the core Hippo pathway kinases MST1/2 phosphorylated PI5P4Ks and inhibited their signaling in vitro and in cells. We further showed that PI5P4K activity regulated several Hippo- and YAP-related phenotypes, specifically decreasing the interaction between the key Hippo proteins MOB1 and LATS and stimulating the YAP-mediated genetic program governing epithelial-to-mesenchymal transition. Mechanistically, we showed that PI5P interacted with MOB1 and enhanced its interaction with LATS, thereby providing a signaling connection between the Hippo pathway and PI5P4Ks. These findings reveal how these two important evolutionarily conserved signaling pathways are integrated to regulate metazoan development and human disease.

Let it stick: Strategies and applications for intracellular plasma membrane targeting of proteins in Saccharomyces cerevisiae

Lipid binding domains and protein lipidations are essential features to recruit proteins to intracellular membranes, enabling them to function at specific sites within the cell. Membrane association can also be exploited to answer fundamental and applied research questions, from obtaining insights into the understanding of lipid metabolism to employing them for metabolic engineering to redirect fluxes. This review presents a broad catalog of membrane binding strategies focusing on the plasma membrane of Saccharomyces cerevisiae. Both lipid binding domains (pleckstrin homology, discoidin-type C2, kinase associated-1, basic-rich and bacterial phosphoinositide-binding domains) and co- and post-translational lipidations (prenylation, myristoylation and palmitoylation) are introduced as tools to target the plasma membrane. To provide a toolset of membrane targeting modules, respective candidates that facilitate plasma membrane targeting are showcased including their in vitro and in vivo properties. The relevance and versatility of plasma membrane targeting modules are further highlighted by presenting a selected set of use cases.

Activation mechanisms and structural dynamics of STIM proteins

The family of stromal interaction molecules (STIM) includes two widely expressed single-pass endoplasmic reticulum (ER) transmembrane proteins and additional splice variants that act as precise ER-luminal Ca2+ sensors. STIM proteins mainly function as one of the two essential components of the so-called Ca2+ release-activated Ca2+ (CRAC) channel. The second CRAC channel component is constituted by pore-forming Orai proteins in the plasma membrane. STIM and Orai physically interact with each other to enable CRAC channel opening, which is a critical prerequisite for various downstream signalling pathways such as gene transcription or proliferation. Their activation commonly requires the emptying of the intracellular ER Ca2+ store. Using their Ca2+ sensing capabilities, STIM proteins confer this Ca2+ content-dependent signal to Orai, thereby linking Ca2+ store depletion to CRAC channel opening. Here we review the conformational dynamics occurring along the entire STIM protein upon store depletion, involving the transition from the quiescent, compactly folded structure into an active, extended state, modulation by a variety of accessory components in the cell as well as the impairment of individual steps of the STIM activation cascade associated with disease.

Native architecture of a human GBP1 defense complex for cell-autonomous immunity to infection

All living organisms deploy cell-autonomous defenses to combat infection. In plants and animals, large supramolecular complexes often activate immune proteins for protection. In this work, we resolved the native structure of a massive host-defense complex that polymerizes 30,000 guanylate-binding proteins (GBPs) over the surface of gram-negative bacteria inside human cells. Construction of this giant nanomachine took several minutes and remained stable for hours, required guanosine triphosphate hydrolysis, and recruited four GBPs plus caspase-4 and Gasdermin D as a cytokine and cell death immune signaling platform. Cryo-electron tomography suggests that GBP1 can adopt an extended conformation for bacterial membrane insertion to establish this platform, triggering lipopolysaccharide release that activated coassembled caspase-4. Our "open conformer" model provides a dynamic view into how the human GBP1 defense complex mobilizes innate immunity to infection.

A temperature-inducible protein module for control of mammalian cell fate

Inducible protein switches are used throughout the biosciences to allow on-demand control of proteins in response to chemical or optical inputs. However, these inducers either cannot be controlled with precision in space and time or cannot be applied in optically dense settings, limiting their application in tissues and organisms. Here we introduce a protein module whose active state can be reversibly toggled with a small change in temperature, a stimulus that is both penetrant and dynamic. This protein, called Melt (Membrane localization through temperature), exists as a monomer in the cytoplasm at elevated temperatures but both oligomerizes and translocates to the plasma membrane when temperature is lowered. Using custom devices for rapid and high-throughput temperature control during live-cell microscopy, we find that the original Melt variant fully switches states between 28-32°C, and state changes can be observed within minutes of temperature changes. Melt was highly modular, permitting thermal control over diverse intracellular processes including signaling, proteolysis, and nuclear shuttling through straightforward end-to-end fusions with no further engineering. Melt was also highly tunable, giving rise to a library of Melt variants with switch point temperatures ranging from 30-40°C. The variants with higher switch points allowed control of molecular circuits between 37°C-41°C, a well-tolerated range for mammalian cells. Finally, Melt could thermally regulate important cell decisions over this range, including cytoskeletal rearrangement and apoptosis. Thus Melt represents a versatile thermogenetic module that provides straightforward, temperature-based, real-time control of mammalian cells with broad potential for biotechnology and biomedicine.

Probing the interactions of the HIV-1 matrix protein-derived polybasic region with lipid bilayers: insights from AFM imaging and force spectroscopy

The human immunodeficiency virus type 1 (HIV-1) matrix protein contains a highly basic region, MA-HBR, crucial for various stages of viral replication. To elucidate the interactions between the polybasic peptide MA-HBR and lipid bilayers, we employed liquid-based atomic force microscopy (AFM) imaging and force spectroscopy on lipid bilayers of differing compositions. In 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, AFM imaging revealed the formation of annulus-shaped protrusions upon exposure to the polybasic peptide, accompanied by distinctive mechanical responses characterized by enhanced bilayer puncture forces. Importantly, our AFM-based force spectroscopy measurements unveiled that MA-HBR induces interleaflet decoupling within the cohesive bilayer organization. This is evidenced by a force discontinuity observed within the bilayer's elastic deformation regime. In POPC/cholesterol bilayers, MA-HBR caused similar yet smaller annular protrusions, demonstrating an intriguing interplay with cholesterol-rich membranes. In contrast, in bilayers containing anionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS) lipids, MA-HBR induced unique annular protrusions, granular nanoparticles, and nanotubules, showcasing its distinctive effects in anionic lipid-enriched environments. Notably, our force spectroscopy data revealed that anionic POPS lipids weakened interleaflet adhesion within the bilayer, resulting in interleaflet decoupling, which potentially contributes to the specific bilayer perturbations induced by MA-HBR. Collectively, our findings highlight the remarkable variations in how the polybasic peptide, MA-HBR, interacts with lipid bilayers of differing compositions, shedding light on its role in host membrane restructuring during HIV-1 infection.

Occurrence and evolutionary conservation analysis of α-helical cationic amphiphilic segments in the human proteome

The existence of encrypted fragments with antimicrobial activity in human proteins has been thoroughly demonstrated in the literature. Recently, algorithms for the large-scale identification of these segments in whole proteomes were developed, and the pervasiveness of this phenomenon was stated. These algorithms typically mine encrypted cationic and amphiphilic segments of proteins, which, when synthesized as individual polypeptide sequences, exert antimicrobial activity by membrane disruption. In the present report, the human reference proteome was submitted to the software kamal for the uncovering of protein segments that correspond to putative intragenic antimicrobial peptides (IAPs). The assessment of the identity of these segments, frequency, functional classes of parent proteins, structural relevance, and evolutionary conservation of amino acid residues within their corresponding proteins was conducted in silico. Additionally, the antimicrobial and anticancer activity of six selected synthetic peptides was evaluated. Our results indicate that cationic and amphiphilic segments can be found in 2% of all human proteins, but are more common in transmembrane and peripheral membrane proteins. These segments are surface-exposed basic patches whose amino acid residues present similar conservation scores to other residues with similar solvent accessibility. Moreover, the antimicrobial and anticancer activity of the synthetic putative IAP sequences was irrespective to whether these are associated to membranes in the cellular setting. Our study discusses these findings in light of the current understanding of encrypted peptide sequences, offering some insights into the relevance of these segments to the organism in the context of their harboring proteins or as separate polypeptide sequences.

Rapid Optogenetic Clustering in the Cytoplasm with BcLOVclust

Protein clustering is a powerful form of optogenetic control, yet remarkably few proteins are known to oligomerize with light. Recently, the photoreceptor BcLOV4 was found to form protein clusters in mammalian cells in response to blue light, although clustering coincided with its translocation to the plasma membrane, potentially constraining its application as an optogenetic clustering module. Herein we identify key amino acids that couple BcLOV4 clustering to membrane binding, allowing us to engineer a variant that clusters in the cytoplasm and does not associate with the membrane in response to blue light. This variant-called BcLOVclust-clustered over many cycles with substantially faster clustering and de-clustering kinetics compared to the widely used optogenetic clustering protein Cry2. The magnitude of clustering could be strengthened by appending an intrinsically disordered region from the fused in sarcoma (FUS) protein, or by selecting the appropriate fluorescent protein to which it was fused. Like wt BcLOV4, BcLOVclust activity was sensitive to temperature: light-induced clusters spontaneously dissolved at a rate that increased with temperature despite constant illumination. At low temperatures, BcLOVclust and Cry2 could be multiplexed in the same cells, allowing light control of independent protein condensates. BcLOVclust could also be applied to control signaling proteins and stress granules in mammalian cells. While its usage is currently best suited in cells and organisms that can be cultured below ∼30 °C, a deeper understanding of BcLOVclust thermal response will further enable its use at physiological mammalian temperatures.

A membrane-associated phosphoswitch in Rad controls adrenergic regulation of cardiac calcium channels

The ability to fight or flee from a threat relies on an acute adrenergic surge that augments cardiac output, which is dependent on increased cardiac contractility and heart rate. This cardiac response depends on β-adrenergic-initiated reversal of the small RGK G protein Rad-mediated inhibition of voltage-gated calcium channels (CaV) acting through the Cavβ subunit. Here, we investigate how Rad couples phosphorylation to augmented Ca2+ influx and increased cardiac contraction. We show that reversal required phosphorylation of Ser272 and Ser300 within Rad's polybasic, hydrophobic C-terminal domain (CTD). Phosphorylation of Ser25 and Ser38 in Rad's N-terminal domain (NTD) alone was ineffective. Phosphorylation of Ser272 and Ser300 or the addition of 4 Asp residues to the CTD reduced Rad's association with the negatively charged, cytoplasmic plasmalemmal surface and with CaVβ, even in the absence of CaVα, measured here by FRET. Addition of a posttranslationally prenylated CAAX motif to Rad's C-terminus, which constitutively tethers Rad to the membrane, prevented the physiological and biochemical effects of both phosphorylation and Asp substitution. Thus, dissociation of Rad from the sarcolemma, and consequently from CaVβ, is sufficient for sympathetic upregulation of Ca2+ currents.

Invasion of glioma cells through confined space requires membrane tension regulation and mechano-electrical coupling via Plexin-B2

Glioblastoma (GBM) is a malignant brain tumor with uncontrolled invasive growth. Here, we demonstrate how GBM cells usurp guidance receptor Plexin-B2 to gain biomechanical plasticity for polarized migration through confined space. Using live-cell imaging to track GBM cells negotiating microchannels, we reveal active endocytosis at cell front and filamentous actin assembly at rear to propel GBM cells through constrictions. These two processes are interconnected and governed by Plexin-B2 that orchestrates cortical actin and membrane tension, shown by biomechanical assays. Molecular dynamics simulations predict that balanced membrane and actin tension are required for optimal migratory velocity and consistency. Furthermore, Plexin-B2 mechanosensitive function requires a bendable extracellular ring structure and affects membrane internalization, permeability, phospholipid composition, as well as inner membrane surface charge. Together, our studies unveil a key element of membrane tension and mechanoelectrical coupling via Plexin-B2 that enables GBM cells to adapt to physical constraints and achieve polarized confined migration.

A new role for phosphoinositides in regulating mitochondrial dynamics

Phosphoinositides are a minor group of membrane-associated phospholipids that are transiently generated on the cytoplasmic leaflet of many organelle membranes and the plasma membrane. There are seven functionally distinct phosphoinositides, each derived via the reversible phosphorylation of phosphatidylinositol in various combinations on the inositol ring. Their generation and termination is tightly regulated by phosphatidylinositol-kinases and -phosphatases. These enzymes can function together in an integrated and coordinated manner, whereby the phosphoinositide product of one enzyme may subsequently serve as a substrate for another to generate a different phosphoinositide species. This regulatory mechanism not only enables the transient generation of phosphoinositides on membranes, but also more complex sequential or bidirectional conversion pathways, and phosphoinositides can also be transferred between organelles via membrane contacts. It is this capacity to fine-tune phosphoinositide signals that makes them ideal regulators of membrane organization and dynamics, through their recruitment of signalling, membrane altering and lipid transfer proteins. Research spanning several decades has provided extensive evidence that phosphoinositides are major gatekeepers of membrane organization, with roles in endocytosis, exocytosis, autophagy, lysosome dynamics, vesicular transport and secretion, cilia, inter-organelle membrane contact, endosome maturation and nuclear function. By contrast, there has been remarkably little known about the role of phosphoinositides at mitochondria - an enigmatic and major knowledge gap, with challenges in reliably detecting phosphoinositides at this site. Here we review recent significant breakthroughs in understanding the role of phosphoinositides in regulating mitochondrial dynamics and metabolic function.

Changes of plasma Rap1A levels in patients with in-stent restenosis after percutaneous coronary intervention and the underlying mechanisms

OBJECTIVES

Percutaneous coronary intervention (PCI) is one of the most important treatments for coronary artery disease (CAD). However, in-stent restenosis (ISR) after PCI is a serious complication without effective measures for prevention and treatment. This study aims to investigate the Ras-related protein 1A (Rap1A) level in ISR patients and in the tumor necrosis factor-α (TNF-α)-induced inflammatory injury model of human umbilical vein endothelial cells (HUVECs), to explore the role of Rap1A in regulating TNF-α-induced inflammation in HUVECs and to provide a new potential target for ISR prevention and treatment.

METHODS

A total of 60 CAD patients, who underwent PCI between December 2020 and July 2022 from the Department of Cardiovascular Medicine of Xiangya Hospital, Central South University, and re-examined coronary angiography (CAG) 1 year after the operation, were included. After admission, 27 patients were diagnosed with ISR and 33 patients were diagnosed with non-in-stent restenosis (non-ISR) according to the CAG. Clinical data were collected, and the plasma Rap1A level was determined by enzyme linked immunosorbent assay (ELISA). In cell experiments, an inflammatory injury model was established with TNF-α treatment (10 ng/mL, 24 h) in HUVECs. The mRNA and protein expression levels of Rap1A, interlukin-6 (IL-6), and vascular cell adhesion molecule-1 (VCAM-1) were measured by real-time reverse transcription PCR and Western blotting. Small interfering RNA (siRNA) was used to explore the role of Rap1A in regulating TNF-α-induced inflammation in HUVECs.

RESULTS

Compared with the non-ISR patients, a higher proportion of ISR patients had a history of smoking (P=0.005) and diabetes (P=0.028), and higher levels of glycosylated hemoglobin (HbA1c) (P=0.012), low-density lipoprotein cholesterol (LDL-c) (P=0.014), and hypersensitive C-reactive protein (hs-CRP) (P=0.027). The remaining projects did not show significant differences (all P>0.05). The plasma level of Rap1A in the ISR group was significantly higher than that in the non-ISR group [942.14 (873.28 to 1 133.81) μg/mL vs 886.93 (812.61 to 930.98) μg/mL; P=0.004]. Diabetes, LDL-c, and Rap1A were risk factors for ISR by univariate logistic regression analysis (all P<0.05). The mRNA and protein expression levels of inflammatory factors IL-6 and VCAM-1 were increased in HUVECs after 10 ng/mL TNF-α treatment for 24 h compared with the control group (all P<0.05), while the mRNA and protein levels of Rap1A were increased (both P<0.05). After inhibition of Rap1A in HUVECs, the mRNA and protein expression levels of IL-6 and VCAM-1 were significantly decreased (all P<0.05).

CONCLUSIONS

The plasma Rap1A level was significantly elevated in patients with ISR, suggesting that Rap1A may be a potential biomarker for predicting ISR. In the TNF-α- induced HUVECs inflammatory injury model, the expression level of Rap1A was increased. The level of TNF-α-induced endothelial cell inflammation was decreased after inhibition of Rap1A expression, suggesting that Rap1A may be a potential target for the treatment of endothelial cell inflammation in ISR.

Charging CAR by electrostatic power

Chimeric antigen receptor (CAR)-T cell therapy has emerged as a promising approach for cancer treatment. CAR is a synthetic immune receptor that recognizes tumor antigen and activates T cells through multiple signaling pathways. However, the current CAR design is not as robust as T cell receptor (TCR), a natural antigen receptor with high sensitivity and efficiency. TCR signaling relies on specific molecular interactions, and thus electrostatic force, the major force of molecular interactions, play critical roles. Understanding how electrostatic charge regulates TCR/CAR signaling events will facilitate the development of next-generation T cell therapies. This review summarizes recent findings on the roles of electrostatic interactions in both natural and synthetic immune receptor signaling, specifically that in CAR clustering and effector molecule recruitments, and highlights potential strategies for engineering CAR-T cell therapy by leveraging charge-based interactions.

The Ralstonia pseudosolanacearum effector RipE1 is recognized at the plasma membrane by NbPtr1, the Nicotiana benthamiana homologue of Pseudomonas tomato race 1

The bacterial wilt disease caused by soilborne bacteria of the Ralstonia solanacearum species complex (RSSC) threatens important crops worldwide. Only a few immune receptors conferring resistance to this devastating disease are known so far. Individual RSSC strains deliver around 70 different type III secretion system effectors into host cells to manipulate the plant physiology. RipE1 is an effector conserved across the RSSC and triggers immune responses in the model solanaceous plant Nicotiana benthamiana. Here, we used multiplexed virus-induced gene silencing of the nucleotide-binding and leucine-rich repeat receptor family to identify the genetic basis of RipE1 recognition. Specific silencing of the N. benthamiana homologue of Solanum lycopersicoides Ptr1 (confers resistance to Pseudomonas syringae pv. tomato race 1) gene (NbPtr1) completely abolished RipE1-induced hypersensitive response and immunity to Ralstonia pseudosolanacearum. The expression of the native NbPtr1 coding sequence was sufficient to restore RipE1 recognition in Nb-ptr1 knockout plants. Interestingly, RipE1 association with the host cell plasma membrane was necessary for NbPtr1-dependent recognition. Furthermore, NbPtr1-dependent recognition of RipE1 natural variants is polymorphic, providing additional evidence for the indirect mode of activation of NbPtr1. Altogether, this work supports NbPtr1 relevance for resistance to bacterial wilt disease in Solanaceae.

Rapid optogenetic clustering of a cytoplasmic BcLOV4 variant

Protein clustering is a powerful form of optogenetic control, yet there is currently only one protein -Cry2-whose light-induced clustering has been harnessed for these purposes. Recently, the photoreceptor BcLOV4 was found to form protein clusters in mammalian cells in response to blue light, although clustering coincided with its translocation to the plasma membrane, potentially constraining its application as an optogenetic clustering module. Herein we identify key amino acids that couple clustering to membrane binding, allowing us to engineer a variant of BcLOV4 that clusters in the cytoplasm and does not associate with the membrane in response to blue light. This variant, BcLOVclust, clustered over many cycles with dramatically faster clustering and de-clustering kinetics compared to Cry2. The magnitude of BcLOVclust clustering could be strengthened by appending an intrinsically disordered region from the fused in sarcoma (FUS) protein, or by optimizing the fluorescent protein to which it was fused. BcLOVclust retained the temperature sensitivity of BcLOV4 such that light induced clustering was transient, and the rate of spontaneous declustering increased with temperature. At low temperatures, BcLOVclust and Cry2 could be multiplexed in the same cells, allowing light control of independent protein condensates. BcLOVclust could also be applied to control signaling proteins and stress granules in mammalian cells. Thus BcLOVclust provides an alternative to Cry2 for optogenetic clustering and a method for multiplexed clustering. While its usage is currently suited for organisms that can be cultured below ~30 °C, a deeper understanding of BcLOVclust thermal response will further enable its use at physiological mammalian temperatures.

The phosphatidylinositol (4,5)-bisphosphate-Rab35 axis regulates migrasome formation

Migrasomes are recently discovered organelles, which are formed on the ends or branch points of retraction fibers at the trailing edge of migrating cells. Previously, we showed that recruitment of integrins to the site of migrasome formation is essential for migrasome biogenesis. In this study, we found that prior to migrasome formation, PIP5K1A, a PI4P kinase which converts PI4P into PI(4,5)P2, is recruited to migrasome formation sites. The recruitment of PIP5K1A results in generation of PI(4,5)P2 at the migrasome formation site. Once accumulated, PI(4,5)P2 recruits Rab35 to the migrasome formation site by interacting with the C-terminal polybasic cluster of Rab35. We further demonstrated that active Rab35 promotes migrasome formation by recruiting and concentrating integrin α5 at migrasome formation sites, which is likely mediated by the interaction between integrin α5 and Rab35. Our study identifies the upstream signaling events orchestrating migrasome biogenesis.

RAS-dependent RAF-MAPK hyperactivation by pathogenic RIT1 is a therapeutic target in Noonan syndrome-associated cardiac hypertrophy

RIT1 is a RAS guanosine triphosphatase (GTPase) that regulates different aspects of signal transduction and is mutated in lung cancer, leukemia, and in the germline of individuals with Noonan syndrome. Pathogenic RIT1 proteins promote mitogen-activated protein kinase (MAPK) hyperactivation; however, this mechanism remains poorly understood. Here, we show that RAF kinases are direct effectors of membrane-bound mutant RIT1 necessary for MAPK activation. We identify critical residues in RIT1 that facilitate interaction with membrane lipids and show that these are necessary for association with RAF kinases and MAPK activation. Although mutant RIT1 binds to RAF kinases directly, it fails to activate MAPK signaling in the absence of classical RAS proteins. Consistent with aberrant RAF/MAPK activation as a driver of disease, we show that pathway inhibition alleviates cardiac hypertrophy in a mouse model of RIT1 mutant Noonan syndrome. These data shed light on the function of pathogenic RIT1 and identify avenues for therapeutic intervention.

Impaired activity and membrane association of most calpain-5 mutants causal for neovascular inflammatory vitreoretinopathy

Neovascular inflammatory vitreoretinopathy (NIV) is a rare eye disease that ultimately leads to complete blindness and is caused by mutations in the gene encoding calpain-5 (CAPN5), with six pathogenic mutations identified. In transfected SH-SY5Y cells, five of the mutations resulted in decreased membrane association, diminished S-acylation, and reduced calcium-induced autoproteolysis of CAPN5. CAPN5 proteolysis of the autoimmune regulator AIRE was impacted by several NIV mutations. R243, L244, K250 and the adjacent V249 are on β-strands in the protease core 2 domain. Conformational changes induced by Ca²⁺binding result in these β-strands forming a β-sheet and a hydrophobic pocket which docks W286 side chain away from the catalytic cleft, enabling calpain activation based on comparison with the Ca²⁺-bound CAPN1 protease core. The pathologic variants R243L, L244P, K250N, and R289W are predicted to disrupt the β-strands, β-sheet, and hydrophobic pocket, impairing calpain activation. The mechanism by which these variants impair membrane association is unclear. G376S impacts a conserved residue in the CBSW domain and is predicted to disrupt a loop containing acidic residues which may contribute to membrane binding. G267S did not impair membrane association and resulted in a slight but significant increase in autoproteolytic and proteolytic activity. However, G267S is also identified in individuals without NIV. Combined with the autosomal dominant pattern of NIV inheritance and evidence that CAPN5 may dimerize, the results are consistent with a dominant negative mechanism for the five pathogenic variants which resulted in impaired CAPN5 activity and membrane association and a gain-of-function for the G267S variant.

Vibrio MARTX toxin processing and degradation of cellular Rab GTPases by the cytotoxic effector Makes Caterpillars Floppy

Vibrio vulnificus causes life threatening infections dependent upon the effectors released from the Multifunctional-Autoprocessing Repeats-In-Toxin (MARTX) toxin. The Makes Caterpillars Floppy-like (MCF) cysteine protease effector is activated by host ADP ribosylation factors (ARFs), although the targets of processing activity were unknown. In this study we show MCF binds Ras-related proteins in brain (Rab) GTPases at the same interface occupied by ARFs and then cleaves and/or degrades 24 distinct members of the Rab GTPases family. The cleavage occurs in the C-terminal tails of Rabs. We determine the crystal structure of MCF as a swapped dimer revealing the open, activated state of MCF and then use structure prediction algorithms to show that structural composition, rather than sequence or localization, determine Rabs selected as MCF proteolytic targets. Once cleaved, Rabs become dispersed in cells to drive organelle damage and cell death to promote pathogenesis of these rapidly fatal infections.

Myo-Inositol and D-Chiro-Inositol as Modulators of Ovary Steroidogenesis: A Narrative Review

Myo-inositol is a natural polyol, the most abundant among the nine possible structural isomers available in living organisms. Inositol confers some distinctive traits that allow for a striking distinction between prokaryotes and eukaryotes, the basic clusters into which organisms are partitioned. Inositol cooperates in numerous biological functions where the polyol participates or by furnishing the fundamental backbone of several related derived metabolites, mostly obtained through the sequential addition of phosphate groups (inositol phosphates, phosphoinositides, and pyrophosphates). Overall myo-inositol and its phosphate metabolites display an entangled network, which is involved in the core of the biochemical processes governing critical transitions inside cells. Noticeably, experimental data have shown that myo-inositol and its most relevant epimer D-chiro-inositol are both necessary to permit a faithful transduction of insulin and of other molecular factors. This improves the complete breakdown of glucose through the citric acid cycle, especially in glucose-greedy tissues, such as the ovary. In particular, while D-chiro-inositol promotes androgen synthesis in the theca layer and down-regulates aromatase and estrogen expression in granulosa cells, myo-inositol strengthens aromatase and FSH receptor expression. Inositol effects on glucose metabolism and steroid hormone synthesis represent an intriguing area of investigation, as recent results have demonstrated that inositol-related metabolites dramatically modulate the expression of several genes. Conversely, treatments including myo-inositol and its isomers have proven to be effective in the management and symptomatic relief of a number of diseases associated with the endocrine function of the ovary, namely polycystic ovarian syndrome.

Conserved but not critical: Trafficking and function of NaV1.7 are independent of highly conserved polybasic motifs

Non-addictive treatment of chronic pain represents a major unmet clinical need. Peripheral voltage-gated sodium (NaV) channels are an attractive target for pain therapy because they initiate and propagate action potentials in primary afferents that detect and transduce noxious stimuli. NaV1.7 sets the gain on peripheral pain-signaling neurons and is the best validated peripheral ion channel involved in human pain, and previous work has shown that it is transported in vesicles in sensory axons which also carry Rab6a, a small GTPase known to be involved in vesicular packaging and axonal transport. Understanding the mechanism of the association between Rab6a and NaV1.7 could inform therapeutic modalities to decrease trafficking of NaV1.7 to the distal axonal membrane. Polybasic motifs (PBM) have been shown to regulate Rab-protein interactions in a variety of contexts. In this study, we explored whether two PBMs in the cytoplasmic loop that joins domains I and II of human NaV1.7 were responsible for association with Rab6a and regulate axonal trafficking of the channel. Using site-directed mutagenesis we generated NaV1.7 constructs with alanine substitutions in the two PBMs. Voltage-clamp recordings showed that the constructs retain wild-type like gating properties. Optical Pulse-chase Axonal Long-distance (OPAL) imaging in live sensory axons shows that mutations of these PBMs do not affect co-trafficking of Rab6a and NaV1.7, or the accumulation of the channel at the distal axonal surface. Thus, these polybasic motifs are not required for interaction of NaV1.7 with the Rab6a GTPase, or for trafficking of the channel to the plasma membrane.

The SOAR of STIM1 interacts with plasma membrane lipids to form ER-PM contact sites

Depletion of Ca²⁺ from the endoplasmic reticulum (ER) causes the ER Ca²⁺ sensor STIM1 to form membrane contact sites (MCSs) with the plasma membrane (PM). At the ER-PM MCS, STIM1 binds to Orai channels to induce cellular Ca²⁺ entry. The prevailing view of this sequential process is that STIM1 interacts with the PM and with Orai1 using two separate modules: a C-terminal polybasic domain (PBD) for the interaction with PM phosphoinositides and the STIM-Orai activation region (SOAR) for the interaction with Orai channels. Here, using electron and fluorescence microscopy and protein-lipid interaction assays, we show that oligomerization of the SOAR promotes direct interaction with PM phosphoinositides to trap STIM1 at ER-PM MCSs. The interaction depends on a cluster of conserved lysine residues within the SOAR and is co-regulated by the STIM1 coil-coiled 1 and inactivation domains. Collectively, our findings uncover a molecular mechanism for formation and regulation of ER-PM MCSs by STIM1.

IQ67 DOMAIN protein 21 is critical for indentation formation in pavement cell morphogenesis

In plants, cortical microtubules anchor to the plasma membrane in arrays and play important roles in cell shape. However, the molecular mechanism of microtubule binding proteins, which connect the plasma membrane and cortical microtubules in cell morphology remains largely unknown. Here, we report that a plasma membrane and microtubule dual-localized IQ67 domain protein, IQD21, is critical for cotyledon pavement cell (PC) morphogenesis in Arabidopsis. iqd21 mutation caused increased indentation width, decreased lobe length, and similar lobe number of PCs, whereas IQD21 overexpression had a different effect on cotyledon PC shape. Weak overexpression led to increased lobe number, decreased indentation width, and similar lobe length, while moderate or great overexpression resulted in decreased lobe number, indentation width, and lobe length of PCs. Live-cell observations revealed that IQD21 accumulation at indentation regions correlates with lobe initiation and outgrowth during PC development. Cell biological and genetic approaches revealed that IQD21 promotes transfacial microtubules anchoring to the plasma membrane via its polybasic sites and bundling at the indentation regions in both periclinal and anticlinal walls. IQD21 controls cortical microtubule organization mainly through promoting Katanin 1-mediated microtubule severing during PC interdigitation. These findings provide the genetic evidence that transfacial microtubule arrays play a determinant role in lobe formation, and the insight into the molecular mechanism of IQD21 in transfacial microtubule organization at indentations and puzzle-shaped PC development.

Ena/VASP clustering at microspike tips involves lamellipodin but not I-BAR proteins, and absolutely requires unconventional myosin-X

Sheet-like membrane protrusions at the leading edge, termed lamellipodia, drive 2D-cell migration using active actin polymerization. Microspikes comprise actin-filament bundles embedded within lamellipodia, but the molecular mechanisms driving their formation and their potential functional relevance have remained elusive. Microspike formation requires the specific activity of clustered Ena/VASP proteins at their tips to enable processive actin assembly in the presence of capping protein, but the factors and mechanisms mediating Ena/VASP clustering are poorly understood. Systematic analyses of B16-F1 melanoma mutants lacking potential candidate proteins revealed that neither inverse BAR-domain proteins, nor lamellipodin or Abi is essential for clustering, although they differentially contribute to lamellipodial VASP accumulation. In contrast, unconventional myosin-X (MyoX) identified here as proximal to VASP was obligatory for Ena/VASP clustering and microspike formation. Interestingly, and despite the invariable distribution of other relevant marker proteins, the width of lamellipodia in MyoX-KO mutants was significantly reduced as compared with B16-F1 control, suggesting that microspikes contribute to lamellipodium stability. Consistently, MyoX removal caused marked defects in protrusion and random 2D-cell migration. Strikingly, Ena/VASP-deficiency also uncoupled MyoX cluster dynamics from actin assembly in lamellipodia, establishing their tight functional association in microspike formation.

Ena/VASP clustering at microspike tips involves lamellipodin but not I-BAR proteins, and absolutely requires unconventional myosin-X

Sheet-like membrane protrusions at the leading edge, termed lamellipodia, drive 2D-cell migration using active actin polymerization. Microspikes comprise actin-filament bundles embedded within lamellipodia, but the molecular mechanisms driving their formation and their potential functional relevance have remained elusive. Microspike formation requires the specific activity of clustered Ena/VASP proteins at their tips to enable processive actin assembly in the presence of capping protein, but the factors and mechanisms mediating Ena/VASP clustering are poorly understood. Systematic analyses of B16-F1 melanoma mutants lacking potential candidate proteins revealed that neither inverse BAR-domain proteins, nor lamellipodin or Abi is essential for clustering, although they differentially contribute to lamellipodial VASP accumulation. In contrast, unconventional myosin-X (MyoX) identified here as proximal to VASP was obligatory for Ena/VASP clustering and microspike formation. Interestingly, and despite the invariable distribution of other relevant marker proteins, the width of lamellipodia in MyoX-KO mutants was significantly reduced as compared with B16-F1 control, suggesting that microspikes contribute to lamellipodium stability. Consistently, MyoX removal caused marked defects in protrusion and random 2D-cell migration. Strikingly, Ena/VASP-deficiency also uncoupled MyoX cluster dynamics from actin assembly in lamellipodia, establishing their tight functional association in microspike formation.

Phosphatidylinositol-4-phosphate signaling regulates dense granule biogenesis and exocytosis in Toxoplasma gondii

Phosphoinositide metabolism defines the foundation of a major signaling pathway that is conserved throughout the eukaryotic kingdom. The 4-OH phosphorylated phosphoinositides such as phosphatidylinositol-4-phosphate (PtdIns4P) and phosphatidylinositol-4,5-bisphosphate are particularly important molecules as these execute intrinsically essential activities required for the viability of all eukaryotic cells studied thus far. Using intracellular tachyzoites of the apicomplexan parasite Toxoplasma gondii as model for assessing primordial roles for PtdIns4P signaling, we demonstrate the presence of PtdIns4P pools in Golgi/trans-Golgi (TGN) system and in post-TGN compartments of the parasite. Moreover, we show that deficits in PtdIns4P signaling result in structural perturbation of compartments that house dense granule cargo with accompanying deficits in dense granule exocytosis. Taken together, the data report a direct role for PtdIns4P in dense granule biogenesis and exocytosis. The data further indicate that the biogenic pathway for secretion-competent dense granule formation in T. gondii is more complex than simple budding of fully matured dense granules from the TGN.

Sympathetic Nervous System Regulation of Cardiac Calcium Channels

Calcium influx through voltage-gated calcium channels, Ca_v1.2, in cardiomyocytes initiates excitation-contraction coupling in the heart. The force and rate of cardiac contraction are modulated by the sympathetic nervous system, mediated substantially by changes in intracellular calcium. Norepinephrine released from sympathetic neurons innervating the heart and epinephrine secreted by the adrenal chromaffin cells bind to β-adrenergic receptors on the sarcolemma of cardiomyocytes initiating a signaling cascade that generates cAMP and activates protein kinase A, the targets of which control calcium influx. For decades, the mechanisms by which PKA regulated calcium channels in the heart were not known. Recently, these mechanisms have been elucidated. In this chapter, we will review the history of the field and the studies that led to the identification of the evolutionarily conserved process.

KIR signaling is regulated by electrostatic interaction of its cytosolic tail with the plasma membrane despite being neutral polyampholyte

Many receptors signal upon phosphorylation of tyrosine-based motifs in their cytosolic tail, with intrinsic disorder as a common feature. Studies on CD3ζ and CD3ε tails, which are disordered and polybasic, suggested regulation of phosphorylation through accessibility of tyrosines, governed by electrostatic interactions with membrane anionic lipids. We noticed characteristics of intrinsic disorder and previously unappreciated features in tyrosine-based motif-bearing cytosolic tails of many, especially, inhibitory receptors. They are neutral or acidic polyampholytes, with acidic and basic residues linearly segregated. To explore roles of these electrostatic features, we studied inhibitory killer-cell immunoglobulin-like receptor (KIR). Its cytosolic tail is a disordered neutrally charged polyampholyte, wherein juxtamembrane and membrane distal stretches are basic, and the intervening stretch is acidic. Despite lacking net charge, it interacted electrostatically with the plasma membrane. The juxtamembrane stretch was crucial for overall binding, which sequestered tyrosines in the lipid bilayer and restrained their constitutive phosphorylation. Human leukocyte antigen-C ligand binding to KIR released its tail from the plasma membrane to initiate signaling. Tail release occurred independently of KIR polymerization, clustering, or tyrosine phosphorylation, but required acidic residues of the acidic stretch. Tail interaction with the plasma membrane dictated signaling strength of KIR. These results revealed an electrostatic protein-lipid interaction that is unusual in being governed by segregated clusters of acidic and basic residues in polyampholytic disordered region of protein. In contrast to previously known, segregated distribution of oppositely charged residues made both binding and unbinding modules inherent to receptor tail, which could make the interaction an independent signaling switch.

CRAC and SK Channels: Their Molecular Mechanisms Associated with Cancer Cell Development

Cancer represents a major health burden worldwide. Several molecular targets have been discovered alongside treatments with positive clinical outcomes. However, the reoccurrence of cancer due to therapy resistance remains the primary cause of mortality. Endeavors in pinpointing new markers as molecular targets in cancer therapy are highly desired. The significance of the co-regulation of Ca²⁺-permeating and Ca²⁺-regulated ion channels in cancer cell development, proliferation, and migration make them promising molecular targets in cancer therapy. In particular, the co-regulation of the Orai1 and SK3 channels has been well-studied in breast and colon cancer cells, where it finally leads to an invasion-metastasis cascade. Nevertheless, many questions remain unanswered, such as which key molecular components determine and regulate their interplay. To provide a solid foundation for a better understanding of this ion channel co-regulation in cancer, we first shed light on the physiological role of Ca²⁺ and how this ion is linked to carcinogenesis. Then, we highlight the structure/function relationship of Orai1 and SK3, both individually and in concert, their role in the development of different types of cancer, and aspects that are not yet known in this context.

Lipid Polarization during Cytokinesis

The plasma membrane of eukaryotic cells is composed of a large number of lipid species that are laterally segregated into functional domains as well as asymmetrically distributed between the outer and inner leaflets. Additionally, the spatial distribution and organization of these lipids dramatically change in response to various cellular states, such as cell division, differentiation, and apoptosis. Division of one cell into two daughter cells is one of the most fundamental requirements for the sustenance of growth in all living organisms. The successful completion of cytokinesis, the final stage of cell division, is critically dependent on the spatial distribution and organization of specific lipids. In this review, we discuss the properties of various lipid species associated with cytokinesis and the mechanisms involved in their polarization, including forward trafficking, endocytic recycling, local synthesis, and cortical flow models. The differences in lipid species requirements and distribution in mitotic vs. male meiotic cells will be discussed. We will concentrate on sphingolipids and phosphatidylinositols because their transbilayer organization and movement may be linked via the cytoskeleton and thus critically regulate various steps of cytokinesis.

Phosphatidylinositol 4,5-Bisphosphate Mediates the Co-Distribution of Influenza A Hemagglutinin and Matrix Protein M1 at the Plasma Membrane

The fully assembled influenza A virus (IAV) has on its surface the highest density of a single membrane protein found in nature-the glycoprotein hemagglutinin (HA) that mediates viral binding, entry, and assembly. HA clusters at the plasma membrane of infected cells, and the HA density (number of molecules per unit area) of these clusters correlates with the infectivity of the virus. Dense HA clusters are considered to mark the assembly site and ultimately lead to the budding of infectious IAV. The mechanism of spontaneous HA clustering, which occurs with or without other viral components, has not been elucidated. Using super-resolution fluorescence photoactivation localization microscopy (FPALM), we have previously shown that these HA clusters are interdependent on phosphatidylinositol 4,5-biphosphate (PIP2). Here, we show that the IAV matrix protein M1 co-clusters with PIP2, visualized using the pleckstrin homology domain. We find that cetylpyridinium chloride (CPC), which is a positively charged quaternary ammonium compound known for its antibacterial and antiviral properties at millimolar concentrations, disrupts M1 clustering and M1-PIP2 co-clustering at micromolar concentrations well below the critical micelle concentration (CMC). CPC also disrupts the co-clustering of M1 with HA at the plasma membrane, suggesting the role of host cell PIP2 clusters as scaffolds for gathering and concentrating M1 and HA to achieve their unusually high cluster densities in the IAV envelope.

PI(4,5)P2: signaling the plasma membrane

In the almost 70 years since the first hints of its existence, the phosphoinositide, phosphatidyl-D-myo-inositol 4,5-bisphosphate has been found to be central in the biological regulation of plasma membrane (PM) function. Here, we provide an overview of the signaling, transport and structural roles the lipid plays at the cell surface in animal cells. These include being substrate for second messenger generation, direct modulation of receptors, control of membrane traffic, regulation of ion channels and transporters, and modulation of the cytoskeleton and cell polarity. We conclude by re-evaluating PI(4,5)P2's designation as a signaling molecule, instead proposing a cofactor role, enabling PM-selective function for many proteins.

A conserved megaprotein-based molecular bridge critical for lipid trafficking and cold resilience

Cells adapt to cold by increasing levels of unsaturated phospholipids and membrane fluidity through conserved homeostatic mechanisms. Here we report an exceptionally large and evolutionarily conserved protein LPD-3 in C. elegans that mediates lipid trafficking to confer cold resilience. We identify lpd-3 mutants in a mutagenesis screen for genetic suppressors of the lipid desaturase FAT-7. LPD-3 bridges the endoplasmic reticulum (ER) and plasma membranes (PM), forming a structurally predicted hydrophobic tunnel for lipid trafficking. lpd-3 mutants exhibit abnormal phospholipid distribution, diminished FAT-7 abundance, organismic vulnerability to cold, and are rescued by Lecithin comprising unsaturated phospholipids. Deficient lpd-3 homologues in Zebrafish and mammalian cells cause defects similar to those observed in C. elegans. As mutations in BLTP1, the human orthologue of lpd-3, cause Alkuraya-Kucinskas syndrome, LPD-3 family proteins may serve as evolutionarily conserved highway bridges critical for ER-associated non-vesicular lipid trafficking and resilience to cold stress in eukaryotic cells.

The brain-specific splice variant of the CDC42 GTPase works together with the kinase ACK to downregulate the EGF receptor in promoting neurogenesis

The small GTPase CDC42 plays essential roles in neurogenesis and brain development. Previously, we showed that a CDC42 splice variant that has a ubiquitous tissue distribution specifically stimulates the formation of neural progenitor cells, whereas a brain-specific CDC42 variant, CDC42b, is essential for promoting the transition of neural progenitor cells to neurons. These specific roles of CDC42 and CDC42b in neurogenesis are ascribed to their opposing effects on mTORC1 activity. Specifically, the ubiquitous form of CDC42 stimulates mTORC1 activity and thereby upregulates tissue-specific transcription factors that are essential for neuroprogenitor formation, whereas CDC42b works together with activated CDC42-associated kinase (ACK) to downregulate mTOR expression. Here, we demonstrate that the EGF receptor (EGFR) is an additional and important target of CDC42b and ACK, which is downregulated by their combined actions in promoting neurogenesis. The activation status of the EGFR determines the timing by which neural progenitor cells derived from P19 embryonal carcinoma terminally differentiate into neurons. By promoting EGFR degradation, we found that CDC42b and ACK stimulate autophagy, which protects emerging neurons from apoptosis and helps trigger neural progenitor cells to differentiate into neurons. Moreover, our results reveal that CDC42b is localized in phosphatidylinositol (3,4,5)-triphosphate-enriched microdomains on the plasma membrane, mediated through its polybasic sequence ¹⁸⁵KRK¹⁸⁷, which is essential for determining its distinct functions. Overall, these findings now highlight a molecular mechanism by which CDC42b and ACK regulate neuronal differentiation and provide new insights into the functional interplay between EGFR degradation and autophagy that occurs during embryonic neurogenesis.

Spatiotemporal dynamics of membrane surface charge regulates cell polarity and migration

During cell migration and polarization, numerous signal transduction and cytoskeletal components self-organize to generate localized protrusions. Although biochemical and genetic analyses have delineated many specific interactions, how the activation and localization of so many different molecules are spatiotemporally orchestrated at the subcellular level has remained unclear. Here we show that the regulation of negative surface charge on the inner leaflet of the plasma membrane plays an integrative role in the molecular interactions. Surface charge, or zeta potential, is transiently lowered at new protrusions and within cortical waves of Ras/PI3K/TORC2/F-actin network activation. Rapid alterations of inner leaflet anionic phospholipids-such as PI(4,5)P2, PI(3,4)P2, phosphatidylserine and phosphatidic acid-collectively contribute to the surface charge changes. Abruptly reducing the surface charge by recruiting positively charged optogenetic actuators was sufficient to trigger the entire biochemical network, initiate de novo protrusions and abrogate pre-existing polarity. These effects were blocked by genetic or pharmacological inhibition of key signalling components such as AKT and PI3K/TORC2. Conversely, increasing the negative surface charge deactivated the network and locally suppressed chemoattractant-induced protrusions or subverted EGF-induced ERK activation. Computational simulations involving excitable biochemical networks demonstrated that slight changes in feedback loops, induced by recruitment of the charged actuators, could lead to outsized effects on system activation. We propose that key signalling network components act on, and are in turn acted upon, by surface charge, closing feedback loops, which bring about the global-scale molecular self-organization required for spontaneous protrusion formation, cell migration and polarity establishment.

The phospholipid flippase ALA3 regulates pollen tube growth and guidance in Arabidopsis

Pollen tube guidance regulates the growth direction and ovule targeting of pollen tubes in pistils, which is crucial for the completion of sexual reproduction in flowering plants. The Arabidopsis (Arabidopsis thaliana) pollen-specific receptor kinase (PRK) family members PRK3 and PRK6 are specifically tip-localized and essential for pollen tube growth and guidance. However, the mechanisms controlling the polar localization of PRKs at the pollen tube tip are unclear. The Arabidopsis P4-ATPase ALA3 helps establish the polar localization of apical phosphatidylserine (PS) in pollen tubes. Here, we discovered that loss of ALA3 function caused pollen tube defects in growth and ovule targeting and significantly affected the polar localization pattern of PRK3 and PRK6. Both PRK3 and PRK6 contain two polybasic clusters in the intracellular juxtamembrane domain, and they bound to PS in vitro. PRK3 and PRK6 with polybasic cluster mutations showed reduced or abolished binding to PS and altered polar localization patterns, and they failed to effectively complement the pollen tube-related phenotypes of prk mutants. These results suggest that ALA3 influences the precise localization of PRK3, PRK6, and other PRKs by regulating the distribution of PS, which plays a key role in regulating pollen tube growth and guidance.

SAUR63 stimulates cell growth at the plasma membrane

In plants, regulated cell expansion determines organ size and shape. Several members of the family of redundantly acting Small Auxin Up RNA (SAUR) proteins can stimulate plasma membrane (PM) H⁺-ATPase proton pumping activity by inhibiting PM-associated PP2C.D phosphatases, thereby increasing the PM electrochemical potential, acidifying the apoplast, and stimulating cell expansion. Similarly, Arabidopsis thaliana SAUR63 was able to increase growth of various organs, antagonize PP2C.D5 phosphatase, and increase H⁺-ATPase activity. Using a gain-of-function approach to bypass genetic redundancy, we dissected structural requirements for SAUR63 growth-promoting activity. The divergent N-terminal domain of SAUR63 has a predicted basic amphipathic α-helix and was able to drive partial PM association. Deletion of the N-terminal domain decreased PM association of a SAUR63 fusion protein, as well as decreasing protein level and eliminating growth-promoting activity. Conversely, forced PM association restored ability to promote H⁺-ATPase activity and cell expansion, indicating that SAUR63 is active when PM-associated. Lipid binding assays and perturbations of PM lipid composition indicate that the N-terminal domain can interact with PM anionic lipids. Mutations in the conserved SAUR domain also reduced PM association in root cells. Thus, both the N-terminal domain and the SAUR domain may cooperatively mediate the SAUR63 PM association required to promote growth.

Plant organs reach their final shape and size after substantial cell expansion. Proton pumps at the plasma membrane promote cell expansion by acidifying the cell wall to loosen it, and by increasing electrochemical potential across the plasma membrane for solute uptake that maintains intracellular turgor. Plasma-membrane-associated proteins tightly regulate proton pump activity, in order for organs to grow to an appropriate extent. We have studied requirements for activity of one such regulatory protein in the model plant Arabidopsis called SAUR63. This protein is made rapidly in response to plant growth hormones, and it increases proton pump activity to promote organ growth. These activities depend on its binding to anionic lipids in the plasma membrane, and forced plasma membrane association of SAUR63 can increase growth. Many proteins in the same family are found within Arabidopsis and in all land plants, and likely differ in their affinity for the plasma membrane or in other properties. Further studies of other family members may show how such proteins regulate growth under diverse physiological contexts.

Two cooperative binding sites sensitize PI(4,5)P2 recognition by the tubby domain

Phosphoinositides (PIs) are lipid signaling molecules that operate by recruiting proteins to cellular membranes via PI recognition domains. The dominant PI of the plasma membrane is phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂]. One of only two PI(4,5)P₂ recognition domains characterized in detail is the tubby domain. It is essential for targeting proteins into cilia involving reversible membrane association. However, the PI(4,5)P₂ binding properties of tubby domains have remained enigmatic. Here, we used coarse-grained molecular dynamics simulations to explore PI(4,5)P₂ binding by the prototypic tubby domain. The comparatively low PI(4,5)P₂ affinity of the previously described canonical binding site is underpinned in a cooperative manner by a previously unknown, adjacent second binding site. Mutations in the previously unknown site impaired PI(4,5)P₂-dependent plasma membrane localization in living cells and PI(4,5)P₂ interaction in silico, emphasizing its importance for PI(4,5)P₂ affinity. The two-ligand binding mode may serve to sharpen the membrane association-dissociation cycle of tubby-like proteins that underlies delivery of ciliary cargo.

A proteomic approach identifies isoform-specific and nucleotide-dependent RAS interactions

Active mutations in the RAS genes are found in ∼30% of human cancers. Although thought to have overlapping functions, RAS isoforms show preferential activation in human tumors, which prompted us to employ a comparative and quantitative proteomics approach to generate isoform-specific and nucleotide-dependent interactomes of the four RAS isoforms, KRAS4A, KRAS4B, HRAS, and NRAS. Many isoform-specific interacting proteins were identified, including HRAS-specific CARM1 and CHK1 and KRAS-specific PIP4K2C and IPO7. Comparing the interactomes of WT and constitutively active G12D mutant of RAS isoforms, we identified several potential previously unknown effector proteins of RAS, one of which was recently reported while this article was in preparation, RADIL. These interacting proteins play important roles as knockdown or pharmacological inhibition leads to potent inhibition of cancer cells. The HRAS-specific interacting protein CARM1 plays a role in HRAS-induced senescence, with CARM1 knockdown or inhibition selectively increasing senescence in HRAS-transformed cells but not in KRAS4B-transformed cells. By revealing new isoform-specific and nucleotide-dependent RAS interactors, the study here provides insights to help understand the overlapping functions of the RAS isoforms.

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RAS interactome uncovers isoform-specific and nucleotide-dependent interactors.

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Potential novel RAS effector proteins are introduced.

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RAS interactors are possible new targets for RAS-driven cancer cells.

A chemogenetic platform for controlling plasma membrane signaling and synthetic signal oscillation

Chemogenetic methods enabling the rapid translocation of specific proteins to the plasma membrane (PM) in a single protein-single ligand manner are useful tools in cell biology. We recently developed a technique, in which proteins fused to an Escherichia coli dihydrofolate reductase (eDHFR) variant carrying N-terminal hexalysine residues are recruited from the cytoplasm to the PM using the synthetic myristoyl-d-Cys-tethered trimethoprim (m^DcTMP) ligand. However, this system achieved PM-specific translocation only when the eDHFR tag was fused to the N terminus of proteins, thereby limiting its application. In this report, we engineered a universal PM-targeting tag for m^DcTMP-induced protein translocation by grafting the hexalysine motif into an intra-loop region of eDHFR. We demonstrate the broad applicability of the new loop-engineered eDHFR tag and m^DcTMP pair for conditional PM recruitment and activation of various tag-fused signaling proteins with different fusion configurations and for reversibly and repeatedly controlling protein localization to generate synthetic signal oscillations.

Kinase-independent synthesis of 3-phosphorylated phosphoinositides by a phosphotransferase

Despite their low abundance, phosphoinositides play a central role in membrane traffic and signalling. PtdIns(3,4,5)P3 and PtdIns(3,4)P2 are uniquely important, as they promote cell growth, survival and migration. Pathogenic organisms have developed means to subvert phosphoinositide metabolism to promote successful infection and their survival in host organisms. We demonstrate that PtdIns(3,4)P2 is a major product generated in host cells by the effectors of the enteropathogenic bacteria Salmonella and Shigella. Pharmacological, gene silencing and heterologous expression experiments revealed that, remarkably, the biosynthesis of PtdIns(3,4)P2 occurs independently of phosphoinositide 3-kinases. Instead, we found that the Salmonella effector SopB, heretofore believed to be a phosphatase, generates PtdIns(3,4)P2 de novo via a phosphotransferase/phosphoisomerase mechanism. Recombinant SopB is capable of generating PtdIns(3,4,5)P3 and PtdIns(3,4)P2 from PtdIns(4,5)P2 in a cell-free system. Through a remarkable instance of convergent evolution, bacterial effectors acquired the ability to synthesize 3-phosphorylated phosphoinositides by an ATP- and kinase-independent mechanism, thereby subverting host signalling to gain entry and even provoke oncogenic transformation.

Cetylpyridinium chloride (CPC) reduces zebrafish mortality from influenza infection: Super-resolution microscopy reveals CPC interference with multiple protein interactions with phosphatidylinositol 4,5-bisphosphate in immune function

The COVID-19 pandemic raises significance for a potential influenza therapeutic compound, cetylpyridinium chloride (CPC), which has been extensively used in personal care products as a positively-charged quaternary ammonium antibacterial agent. CPC is currently in clinical trials to assess its effects on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) morbidity. Two published studies have provided mouse and human data indicating that CPC may alleviate influenza infection, and here we show that CPC (0.1 μM, 1 h) reduces zebrafish mortality and viral load following influenza infection. However, CPC mechanisms of action upon viral-host cell interaction are currently unknown. We have utilized super-resolution fluorescence photoactivation localization microscopy to probe the mode of CPC action. Reduction in density of influenza viral protein hemagglutinin (HA) clusters is known to reduce influenza infectivity: here, we show that CPC (at non-cytotoxic doses, 5-10 μM) reduces HA density and number of HA molecules per cluster within the plasma membrane of NIH-3T3 mouse fibroblasts. HA is known to colocalize with the negatively-charged mammalian lipid phosphatidylinositol 4,5-bisphosphate (PIP2); here, we show that nanoscale co-localization of HA with the PIP2-binding Pleckstrin homology (PH) reporter in the plasma membrane is diminished by CPC. CPC also dramatically displaces the PIP2-binding protein myristoylated alanine-rich C-kinase substrate (MARCKS) from the plasma membrane of rat RBL-2H3 mast cells; this disruption of PIP2 is correlated with inhibition of mast cell degranulation. Together, these findings offer a PIP2-focused mechanism underlying CPC disruption of influenza and suggest potential pharmacological use of this drug as an influenza therapeutic to reduce global deaths from viral disease.

Functional diversity in the RAS subfamily of small GTPases

RAS small GTPases regulate important signalling pathways and are notorious drivers of cancer development and progression. While most research to date has focused on understanding and addressing the oncogenic potential of three RAS oncogenes: HRAS, KRAS, and NRAS; the full RAS subfamily is composed of 35 related GTPases with diverse cellular functions. Most remain deeply understudied despite strong evolutionary conservation. Here, we highlight a group of 17 poorly characterized RAS GTPases that are frequently down-regulated in cancer and evidence suggests may function not as oncogenes, but as tumour suppressors. These GTPases remain largely enigmatic in terms of their cellular function, regulation, and interaction with effector proteins. They cluster within two families we designate as 'distal-RAS' (D-RAS; comprised of DIRAS, RASD, and RASL10) and 'CaaX-Less RAS' (CL-RAS; comprised of RGK, NKIRAS, RERG, and RASL11/12 GTPases). Evidence of a tumour suppressive role for many of these GTPases supports the premise that RAS subfamily proteins may collectively regulate cellular proliferation.

Selective Recognition of Phosphatidylinositol Phosphate Receptors by C-Terminal Tail of Mitotic Kinesin-like Protein 2 (MKlp2)

The mitotic kinesin-like protein 2 (MKlp2) plays a key role in the proper completion of cytokinetic abscission. Specifically, the C-terminal tail of MKlp2 (CTM peptides) offers a stable tethering on the plasma membrane and microtubule cytoskeleton in the midbody during abscission. However, little is known about the underlying mechanism of how the CTM peptides bind to the plasma membrane of the intercellular bridge. Herein, we identify the specific molecular interaction between the CTM peptides and phosphatidylinositol phosphate (PIP) receptors using quartz crystal microbalance-dissipation and atomic force microscopy force spectroscopic measurements. To systematically examine the effects of amino acids, we designed a series of synthetic 33-mer peptides derived from the wild-type (CTM1). First, we evaluated the peptide binding amount caused by electrostatic interactions based on 100% zwitterionic and 30% negatively charged model membranes, whereby the nonspecific attractions were nearly proportional to the net charge of peptides. Upon incubating with PIP-containing model membranes, the wild-type CTM1 and its truncated mutation showed significant PI(3)P-specific binding, which was evidenced by a 15-fold higher binding mass and 6-fold stronger adhesion force compared to other negatively charged membranes. The extent of the specific binding was predominantly dependent on the existence of S21, whereby substitution or deletion of S21 significantly hindered the binding affinity. Taken together, our findings based on a correlative measurement platform enabled the quantification of the nonelectrostatic, selective binding interactions of the C-terminal of MKlp2 to certain PIP receptors and contributed to understanding the molecular mechanisms on complete cytokinetic abscission in cells.

Biochemical and NMR studies reveal specific interaction between STIMATE C-tail and PI(4,5)P2 or PI(3,4,5)P3-containing membrane

STIMATE is an endoplasmic reticulum (ER) resident membrane protein that plays key roles in regulating calcium signaling occurring at ER-plasma membrane (PM) junctions. It is also involved in the regulation of ER-PM junction maintenance. STIMATE contains multiple putative transmembrane domains with a polybasic C tail (STIMATE-CT) that directly interacts with stromal interaction molecule 1 (STIM1) to promote STIM1 conformational switch. Here using liposome pulldown assay, we show that STIMATE-CT can specifically interact with PI(4,5)P2 or PI(3,4,5)P3-containing membrane. NMR analysis indicates that STIMATE-CT is intrinsically disordered. Furthermore, NMR titration with bicelles and mutation analysis reveal that the regions of 242VRYR245 and 284KKKK287 in STIMATE-CT are both essential for its membrane binding.

Modulation of the cell membrane lipid milieu by peroxisomal β-oxidation induces Rho1 signaling to trigger inflammatory responses

Phagocytosis, signal transduction, and inflammatory responses require changes in lipid metabolism. Peroxisomes have key roles in fatty acid homeostasis and in regulating immune function. We find that Drosophila macrophages lacking peroxisomes have perturbed lipid profiles, which reduce host survival after infection. Using lipidomic, transcriptomic, and genetic screens, we determine that peroxisomes contribute to the cell membrane glycerophospholipid composition necessary to induce Rho1-dependent signals, which drive cytoskeletal remodeling during macrophage activation. Loss of peroxisome function increases membrane phosphatidic acid (PA) and recruits RhoGAPp190 during infection, inhibiting Rho1-mediated responses. Peroxisome-glycerophospholipid-Rho1 signaling also controls cytoskeleton remodeling in mouse immune cells. While high levels of PA in cells without peroxisomes inhibit inflammatory phenotypes, large numbers of peroxisomes and low amounts of cell membrane PA are features of immune cells from patients with inflammatory Kawasaki disease and juvenile idiopathic arthritis. Our findings reveal potential metabolic markers and therapeutic targets for immune diseases and metabolic disorders.

IQGAP1 scaffolding links phosphoinositide kinases to cytoskeletal reorganization

IQGAP1 is a multidomain scaffold protein that coordinates the direction and impact of multiple signaling pathways by scaffolding its various binding partners. However, the spatial and temporal resolution of IQGAP1 scaffolding remains unclear. Here, we use fluorescence imaging and correlation methods that allow for real-time live-cell changes in IQGAP1 localization and complex formation during signaling. We find that IQGAP1 and PIPKIγ interact on both the plasma membrane and in cytosol. Epidermal growth factor (EGF) stimulation, which can initiate cytoskeletal changes, drives the movement of the cytosolic pool toward the plasma membrane to promote cytoskeletal changes. We also observe that a significant population of cytosolic IQGAP1-PIPKIγ complexes localize to early endosomes, and in some instances form aggregated clusters which become highly mobile upon EGF stimulation. Our imaging studies show that PIPKIγ and PI3K bind simultaneously to IQGAP1, which may accelerate conversion of PI4P to PI(3,4,5)P₃ that is required for cytoskeletal changes. Additionally, we find that IQGAP1 is responsible for PIPKIγ association with two proteins associated with cytoskeletal changes, talin and Cdc42, during EGF stimulation. These results directly show that IQGAP1 provides a physical link between phosphoinositides (through PIPKIγ), focal adhesion formation (through talin), and cytoskeletal reorganization (through Cdc42) upon EGF stimulation. Taken together, our results support the importance of IQGAP1 in regulating cell migration by linking phosphoinositide lipid signaling with cytoskeletal reorganization.