Coordinated regulation of phosphatidylinositol 4-phosphate and phosphatidylserine levels by Osh4p and Osh5p is an essential regulatory mechanism in autophagy

Macroautophagy (hereafter autophagy) is an intracellular degradative pathway in budding yeast cells. Certain lipid types play essential roles in autophagy; yet the precise mechanisms regulating lipid composition during autophagy remain unknown. Here, we explored the role of the Osh family proteins in the modulating lipid composition during autophagy in budding yeast. Our results showed that osh1-osh7∆ deletions lead to autophagic dysfunction, with impaired GFP-Atg8 processing and the absence of autophagosomes and autophagic bodies in the cytosol and vacuole, respectively. Freeze-fracture electron microscopy (EM) revealed elevated phosphatidylinositol 4-phosphate (PtdIns(4)P) levels in cytoplasmic and luminal leaflets of autophagic bodies and vacuolar membranes in all deletion mutants. Phosphatidylserine (PtdSer) levels were significantly decreased in the autophagic bodies and vacuolar membranes in osh4∆ and osh5∆ mutants, whereas no significant changes were observed in other osh deletion mutants. Furthermore, we identified defects in autophagic processes in the osh4∆ and osh5∆ mutants, including rare autophagosome formation in the osh5∆ mutant and accumulation of autophagic bodies in the vacuole in the osh4∆ mutant, even in the absence of the proteinase inhibitor PMSF. These findings suggest that Osh4p and Osh5p play crucial roles in the transport of PtdSer to autophagic bodies and autophagosome membranes, respectively. The precise control of lipid composition in the membranes of autophagosomes and autophagic bodies by Osh4p and Osh5p represents an important regulatory mechanism in autophagy.

Glucose controls lipolysis through Golgi PtdIns4P-mediated regulation of ATGL

Metabolic crosstalk of the major nutrients glucose, amino acids and fatty acids (FAs) ensures systemic metabolic homeostasis. The coordination between the supply of glucose and FAs to meet various physiological demands is especially important as improper nutrient levels lead to metabolic disorders, such as diabetes and metabolic dysfunction-associated steatohepatitis (MASH). In response to the oscillations in blood glucose levels, lipolysis is thought to be mainly regulated hormonally to control FA liberation from lipid droplets by insulin, catecholamine and glucagon. However, whether general cell-intrinsic mechanisms exist to directly modulate lipolysis via glucose sensing remains largely unknown. Here we report the identification of such an intrinsic mechanism, which involves Golgi PtdIns4P-mediated regulation of adipose triglyceride lipase (ATGL)-driven lipolysis via intracellular glucose sensing. Mechanistically, depletion of intracellular glucose results in lower Golgi PtdIns4P levels, and thus reduced assembly of the E3 ligase complex CUL7FBXW8 in the Golgi apparatus. Decreased levels of the E3 ligase complex lead to reduced polyubiquitylation of ATGL in the Golgi and enhancement of ATGL-driven lipolysis. This cell-intrinsic mechanism regulates both the pool of intracellular FAs and their extracellular release to meet physiological demands during fasting and glucose deprivation. Moreover, genetic and pharmacological manipulation of the Golgi PtdIns4P-CUL7FBXW8-ATGL axis in mouse models of simple hepatic steatosis and MASH, as well as during ex vivo perfusion of a human steatotic liver graft leads to the amelioration of steatosis, suggesting that this pathway might be a promising target for metabolic dysfunction-associated steatotic liver disease and possibly MASH.

GRAF1 Acts as a Downstream Mediator of Parkin to Regulate Mitophagy in Cardiomyocytes

Cardiomyocytes rely on proper mitochondrial homeostasis to maintain contractility and achieve optimal cardiac performance. Mitochondrial homeostasis is controlled by mitochondrial fission, fusion, and mitochondrial autophagy (mitophagy). Mitophagy plays a particularly important role in promoting the degradation of dysfunctional mitochondria in terminally differentiated cells. However, the precise mechanisms by which this is achieved in cardiomyocytes remain opaque. Our study identifies GRAF1 as an important mediator in PINK1-Parkin pathway-dependent mitophagy. Depletion of GRAF1 (Arhgap26) in cardiomyocytes results in actin remodeling defects, suboptimal mitochondria clustering, and clearance. Mechanistically, GRAF1 promotes Parkin-LC3 complex formation and directs autophagosomes to damaged mitochondria. Herein, we found that these functions are regulated, at least in part, by the direct binding of GRAF1 to phosphoinositides (PI(3)P, PI(4)P, and PI(5)P) on autophagosomes. In addition, PINK1-dependent phosphorylation of Parkin promotes Parkin-GRAF1-LC3 complex formation, and PINK1-dependent phosphorylation of GRAF1 (on S668 and S671) facilitates the clustering and clearance of mitochondria. Herein, we developed new phosphor-specific antibodies to these sites and showed that these post-translational modifications are differentially modified in human hypertrophic cardiomyopathy and dilated cardiomyopathy. Furthermore, our metabolic studies using serum collected from isoproterenol-treated WT and GRAF1CKO mice revealed defects in mitophagy-dependent cardiomyocyte fuel flexibility that have widespread impacts on systemic metabolism. In summary, our study reveals that GRAF1 co-regulates actin and membrane dynamics to promote cardiomyocyte mitophagy and that dysregulation of GRAF1 post-translational modifications may underlie cardiac disease pathogenesis.

Shigella generates distinct IAM subpopulations during epithelial cell invasion to promote efficient intracellular niche formation

The facultative intracellular pathogen Shigella flexneri invades non-phagocytic epithelial gut cells. Through a syringe-like apparatus called type 3 secretion system, it injects effector proteins into the host cell triggering actin rearrangements leading to its uptake within a tight vacuole, termed the bacterial-containing vacuole (BCV). Simultaneously, Shigella induces the formation of large vesicles around the entry site, which we refer to as infection-associated macropinosomes (IAMs). After entry, Shigella ruptures the BCV and escapes into the host cytosol by disassembling the BCV remnants. Previously, IAM formation has been shown to be required for efficient BCV escape, but the molecular events associated with BCV disassembly have remained unclear. To identify host components required for BCV disassembly, we performed a microscopy-based screen to monitor the recruitment of BAR domain-containing proteins, which are a family of host proteins involved in membrane shaping and sensing (e.g. endocytosis and recycling) during Shigella epithelial cell invasion. We identified endosomal recycling BAR protein Sorting Nexin-8 (SNX8) localized to IAMs in a PI(3)P-dependent manner before BCV disassembly. At least two distinct IAM subpopulations around the BCV were found, either being recycled back to cellular compartments such as the plasma membrane or transitioning to become RAB11A positive "contact-IAMs" involved in promoting BCV rupture. The IAM subpopulation duality was marked by the exclusive recruitment of either SNX8 or RAB11A. Hindering PI(3)P production at the IAMs led to an inhibition of SNX8 recruitment at these compartments and delayed both, the step of BCV rupture time and successful BCV disassembly. Finally, siRNA depletion of SNX8 accelerated BCV rupture and unpeeling of BCV remnants, indicating that SNX8 is involved in controlling the timing of the cytosolic release. Overall, our work sheds light on how Shigella establishes its intracellular niche through the subversion of a specific set of IAMs.

Research progress on phosphatidylinositol 4-kinase inhibitors

Phosphatidylinositol 4-kinases (PI4Ks) could phosphorylate phosphatidylinositol (PI) to produce phosphatidylinositol 4-phosphate (PI4P) and maintain its metabolic balance and location. PI4P, the most abundant monophosphate inositol in eukaryotic cells, is a precursor of higher phosphoinositols and an essential substrate for the PLC/PKC and PI3K/Akt signaling pathways. PI4Ks regulate vesicle transport, signal transduction, cytokinesis, and cell unity, and are involved in various physiological and pathological processes, including infection and growth of parasites such as Plasmodium and Cryptosporidium, replication and survival of RNA viruses, and the development of tumors and nervous system diseases. The development of novel drugs targeting PI4Ks and PI4P has been the focus of the research and clinical application of drugs, especially in recent years. In particular, PI4K inhibitors have made great progress in the treatment of malaria and cryptosporidiosis. We describe the biological characteristics of PI4Ks; summarize the physiological functions and effector proteins of PI4P; and analyze the structural basis of selective PI4K inhibitors for the treatment of human diseases in this review. Herein, this review mainly summarizes the developments in the structure and enzyme activity of PI4K inhibitors.

The temporal association of CapZ with early endosomes regulates endosomal trafficking and viral entry into host cells

BACKGROUND

Many viruses enter host cells by hijacking endosomal trafficking. CapZ, a canonical actin capping protein, participates in endosomal trafficking, yet its precise role in endocytosis and virus infection remains elusive.

RESULTS

Here, we showed that CapZ was transiently associated with early endosomes (EEs) and was subsequently released from the matured EEs after the fusion of two EEs, which was facilitated by PI(3)P to PI(3,5)P2 conversion. Vacuolin-1 (a triazine compound) stabilized CapZ at EEs and thus blocked the transition of EEs to late endosomes (LEs). Likewise, artificially tethering CapZ to EEs via a rapamycin-induced protein-protein interaction system blocked the early-to-late endosome transition. Remarkably, CapZ knockout or artificially tethering CapZ to EEs via rapamycin significantly inhibited flaviviruses, e.g., Zika virus (ZIKV) and dengue virus (DENV), or beta-coronavirus, e.g., murine hepatitis virus (MHV), infection by preventing the escape of RNA genome from endocytic vesicles.

CONCLUSIONS

These results indicate that the temporal association of CapZ with EEs facilitates early-to-late endosome transition (physiologically) and the release of the viral genome from endocytic vesicles (pathologically).

Unexpected inhibition of the lipid kinase PIKfyve reveals an epistatic role for p38 MAPKs in endolysosomal fission and volume control

p38 mitogen-activated protein kinases (MAPKs) participate in autophagic signaling; and previous reports suggest that pyridinyl imidazole p38 MAPK inhibitors, including SB203580 and SB202190, induce cell death in some cancer cell-types through unrestrained autophagy. Subsequent studies, however, have suggested that the associated cytoplasmic vacuolation resulted from off-target inhibition of an unidentified enzyme. Herein, we report that SB203580-induced vacuolation is rapid, reversible, and relies on the class III phosphatidylinositol 3-kinase (PIK3C3) complex and the production of phosphatidylinositol 3-phosphate [PI(3)P] but not on autophagy per se. Rather, vacuolation resulted from the accumulation of Rab7 on late endosome and lysosome (LEL) membranes, combined with an osmotic imbalance that triggered severe swelling in these organelles. Inhibition of PIKfyve, the lipid kinase that converts PI(3)P to PI(3,5)P2 on LEL membranes, produced a similar phenotype in cells; therefore, we performed in vitro kinase assays and discovered that both SB203580 and SB202190 directly inhibited recombinant PIKfyve. Cancer cells treated with either drug likewise displayed significant reductions in the endogenous levels of PI(3,5)P2. Despite these results, SB203580-induced vacuolation was not entirely due to off-target inhibition of PIKfyve, as a drug-resistant p38α mutant suppressed vacuolation; and combined genetic deletion of both p38α and p38β dramatically sensitized cells to established PIKfyve inhibitors, including YM201636 and apilimod. The rate of vacuole dissolution (i.e., LEL fission), following the removal of apilimod, was also significantly reduced in cells treated with BIRB-796, a structurally unrelated p38 MAPK inhibitor. Thus, our studies indicate that pyridinyl imidazole p38 MAPK inhibitors induce cytoplasmic vacuolation through the combined inhibition of both PIKfyve and p38 MAPKs, and more generally, that p38 MAPKs act epistatically to PIKfyve, most likely to promote LEL fission.

Molding a PI(3,5)P2 biosensor

The lipid phosphatidylinositol 3,5-bisphosphate-PI(3,5)P2-is known to be a key regulator of cellular traffic in health and disease, but its cellular localization was somewhat enigmatic until now, with the discovery of a new PI(3,5)P2 biosensor reported in this issue of JCB by Vines et al. (2023. J. Cell Biol.https://doi.org/10.1083/jcb.202209077).

De novo missense variants in phosphatidylinositol kinase PIP5KIγ underlie a neurodevelopmental syndrome associated with altered phosphoinositide signaling

Phosphoinositides (PIs) are membrane phospholipids produced through the local activity of PI kinases and phosphatases that selectively add or remove phosphate groups from the inositol head group. PIs control membrane composition and play key roles in many cellular processes including actin dynamics, endosomal trafficking, autophagy, and nuclear functions. Mutations in phosphatidylinositol 4,5 bisphosphate [PI(4,5)P2] phosphatases cause a broad spectrum of neurodevelopmental disorders such as Lowe and Joubert syndromes and congenital muscular dystrophy with cataracts and intellectual disability, which are thus associated with increased levels of PI(4,5)P2. Here, we describe a neurodevelopmental disorder associated with an increase in the production of PI(4,5)P2 and with PI-signaling dysfunction. We identified three de novo heterozygous missense variants in PIP5K1C, which encodes an isoform of the phosphatidylinositol 4-phosphate 5-kinase (PIP5KIγ), in nine unrelated children exhibiting intellectual disability, developmental delay, acquired microcephaly, seizures, visual abnormalities, and dysmorphic features. We provide evidence that the PIP5K1C variants result in an increase of the endosomal PI(4,5)P2 pool, giving rise to ectopic recruitment of filamentous actin at early endosomes (EEs) that in turn causes dysfunction in EE trafficking. In addition, we generated an in vivo zebrafish model that recapitulates the disorder we describe with developmental defects affecting the forebrain, including the eyes, as well as craniofacial abnormalities, further demonstrating the pathogenic effect of the PIP5K1C variants.

Phosphoinositide acyl chain saturation drives CD8+ effector T cell signaling and function

How lipidome changes support CD8+ effector T (Teff) cell differentiation is not well understood. Here we show that, although naive T cells are rich in polyunsaturated phosphoinositides (PIPn with 3-4 double bonds), Teff cells have unique PIPn marked by saturated fatty acyl chains (0-2 double bonds). PIPn are precursors for second messengers. Polyunsaturated phosphatidylinositol bisphosphate (PIP2) exclusively supported signaling immediately upon T cell antigen receptor activation. In late Teff cells, activity of phospholipase C-γ1, the enzyme that cleaves PIP2 into downstream mediators, waned, and saturated PIPn became essential for sustained signaling. Saturated PIP was more rapidly converted to PIP2 with subsequent recruitment of phospholipase C-γ1, and loss of saturated PIPn impaired Teff cell fitness and function, even in cells with abundant polyunsaturated PIPn. Glucose was the substrate for de novo PIPn synthesis, and was rapidly utilized for saturated PIP2 generation. Thus, separate PIPn pools with distinct acyl chain compositions and metabolic dependencies drive important signaling events to initiate and then sustain effector function during CD8+ T cell differentiation.

ORP9 and ORP10 form a heterocomplex to transfer phosphatidylinositol 4-phosphate at ER-TGN contact sites

Oxysterol-binding protein (OSBP) and its related proteins (ORPs) are a family of lipid transfer proteins (LTPs) that mediate non-vesicular lipid transport. ORP9 and ORP10, members of the OSBP/ORPs family, are located at the endoplasmic reticulum (ER)-trans-Golgi network (TGN) membrane contact sites (MCSs). It remained unclear how they mediate lipid transport. In this work, we discovered that ORP9 and ORP10 form a binary complex through intermolecular coiled-coil (CC) domain-CC domain interaction. The PH domains of ORP9 and ORP10 specially interact with phosphatidylinositol 4-phosphate (PI4P), mediating the TGN targeting. The ORP9-ORP10 complex plays a critical role in regulating PI4P levels at the TGN. Using in vitro reconstitution assays, we observed that while full-length ORP9 efficiently transferred PI4P between two apposed membranes, the lipid transfer kinetics was further accelerated by ORP10. Interestingly, our data showed that the PH domains of ORP9 and ORP10 participate in membrane tethering simultaneously, whereas ORDs of both ORP9 and ORP10 are required for lipid transport. Furthermore, our data showed that the depletion of ORP9 and ORP10 led to increased vesicle transport to the plasma membrane (PM). These findings demonstrate that ORP9 and ORP10 form a binary complex through the CC domains, maintaining PI4P homeostasis at ER-TGN MCSs and regulating vesicle trafficking.

A Comparative Analysis of the Membrane Binding and Remodeling Properties of Two Related Sorting Nexin Complexes Involved in Autophagy

The sorting nexin (SNX) proteins, Atg20 and Atg24, are involved in nonselective autophagy, are necessary for efficient selective autophagy, and are required for the cytoplasm-to-vacuole transport pathway. However, the specific roles of these proteins in autophagy are not well understood. Atg20 and Atg24 each contain a Phox homology domain that facilitates phosphoinositide binding. They also each contain an SNX-Bin/Amphiphysin/Rvs domain that forms a cup-shaped dimer, capable of binding to curved membranes and remodeling those membranes in some cases. Atg20 and Atg24 form two distinct complexes, an Atg24/Atg24 homodimer and an Atg20/Atg24 heterodimer. Despite the presence of Atg24 in both complexes, it is currently unclear if these complexes have different membrane binding and remodeling properties. Therefore, in this study, we explored the membrane binding and shaping properties of these two dimeric complexes. We found that Atg24/Atg24 and Atg20/Atg24 have distinct membrane binding preferences. Both dimers recognized membranes containing phosphatidylinositol 3-phosphate [PI(3)P] and phosphatidylinositol 3,5-bisphosphate, but Atg20/Atg24 bound to a broader array of liposomes, including those lacking phosphorylated phosphatidylinositol. In addition, we discovered that while both complexes bound to autophagosomal-like liposomes containing at least 5% PI(3)P, Atg20/Atg24 was capable of binding to autophagosomal-like liposomes lacking PI(3)P. Lastly, we observed that the Atg20/Atg24 heterodimer tubulates PI(3)P-containing and autophagosomal-like liposomes, but the Atg24/Atg24 homodimer could not tubulate these liposomes. Our findings suggest that these two dimers contain distinct membrane binding and shaping properties.

Localization of phosphatidylinositol phosphate 5 kinase γ, phospholipase β3 and diacylglycerol kinase ζ in corneal epithelium in comparison with conjunctival epithelium of mice

Based on the theory that the phosphoinositide (PI) signal is involved in the physiology of cornea and conjunctiva, we examined the localization in the mouse anterior ocular epithelia of immunoreactivities for phosphatidylinositol 4-phosphate 5-kinase (PIP5K), phospholipase C (PLC) and diacylglycerol kinase (DGK), enzymes that work sequentially in PI cycle. Immunoreactivity for PIP5Kγ in the corneal epithelium, including the limbus, was distinct in adults in contrast to faint or negligible immunoreactivity in the conjunctival epithelium in neonatal mice. This adult localization pattern was first recognized at the postnatal time of eyelid opening. Immunoreactivity for PLCβ3 was rather equally distinct throughout the entire corneal and conjunctival epithelia in adults. DGKζ-immunoreactive nuclei were mainly localized in the basal half domain of the corneal epithelium but in both basal and apical domains of the conjunctival epithelium in adults. This nuclear immunoreactivity was at weak or negligible levels in the peripheral and limbus cornea and in a considerable portion of the bulbar conjunctival epithelium continuous with the limbus. The adult patterns for PLCβ3 and DGKζ were already present at birth. The present findings suggest the following possibilities on the functional significance of the three enzyme molecules. PIP5Kγ is involved in cornea-specific functions such as bright-field vision, including corneal transparency, and in the stability of epithelial junctions, for which there seems to be a much higher requirement in the corneal epithelium than in the conjunctival epithelium. PLCβ3 is involved from birth in as-yet undefined functions exerted ubiquitously from birth in both corneal and conjunctival epithelia. DGKζ is involved in regulation from birth of the transcription in epithelial cells, including apoptosis as well as regulation of mitosis of epithelial cells in both cornea and conjunctiva, with the transcription involvement more apparent in the conjunctiva, although it does not work in stem cells of the corneal limbus.

Self-transducible LRS-UNE-L peptide enhances muscle regeneration

BACKGROUND

Muscle regeneration includes proliferation and differentiation of muscle satellite cells, which involves the mammalian target of rapamycin (mTOR). We identified the C-terminal unique attached sequence motif (UNE) domain of leucyl-tRNA synthetase (LRS-UNE-L) as an mTORC1 (mTOR complex1)-activating domain that acts through Vps34 and phospholipase D1 (PLD1) when introduced in the form of a muscle-enhancing peptide.

METHODS

In vitro Vps34 lipid kinase assay, phosphatidylinositol 3-phosphate (PI(3)P) measurement, in vivo PLD1 assay, and western blot assay were performed in HEK293 cells to test the effect of the LRS-UNE-L on the Vps34-PLD1-mTOR pathway. Adeno-associated virus (AAV)-LRS-UNE-L was transduced in C2C12 cells in vitro, in BaCl₂ -injured tibialis anterior (TA) muscles, and in 18-month-old TA muscles to analyse its effect on myogenesis, muscle regeneration, and aged muscle, respectively. The muscle-specific cell-permeable peptide M12 was fused with LRS-UNE-L and tested for cell integration in C2C12 and HEK293 cells using FACS analysis and immunocytochemistry. Finally, M12-LRS-UNE-L was introduced into BaCl₂ -injured TA muscles of 15-week-old Pld1^+/+ or Pld1^-/- mice, and its effect was analysed by measurement of cross-sectional area of regenerating muscle fibres.

RESULTS

The LRS-UNE-L expression restored amino acid-induced S6K1 phosphorylation in LRS knockdown cells in a RagD GTPases-independent manner (421%, P = 0.007 vs. LRS knockdown control cells). The LRS-UNE-L domain was directly bound to Vps34; this interaction was accompanied by increases in Vps34 activity (166%, P = 0.0352), PI(3)P levels (146%, P = 0.0039), and PLD1 activity (228%, P = 0.0294) compared with amino acid-treated control cells, but it did not affect autophagic flux. AAV-delivered LRS-UNE-L domain augmented S6K1 phosphorylation (174%, P = 0.0013), mRNA levels of myosin heavy chain (MHC) (122%, P = 0.0282) and insulin-like growth factor 2 (IGF2) (146%, P = 0.008), and myogenic fusion (133%, P = 0.0479) in C2C12 myotubes. AAV-LRS-UNE-L increased the size of regenerating muscle fibres in BaCl₂ -injured TA muscles (124%, P = 0.0279) (n = 9-10), but it did not change the muscle fibre size of TA muscles in old mice. M12-LRS-UNE-L was preferentially delivered into C2C12 cells compared with HEK293 cells and augmented regeneration of BaCl₂ -injured TA muscles in a PLD1-dependent manner (116%, P = 0.0022) (n = 6).

CONCLUSIONS

Our results provide compelling evidence that M12-LRS-UNE-L could be a muscle-enhancing protein targeting mTOR.

The phosphoinositide coincidence detector Phafin2 promotes macropinocytosis by coordinating actin organisation at forming macropinosomes

Uptake of large volumes of extracellular fluid by actin-dependent macropinocytosis has an important role in infection, immunity and cancer development. A key question is how actin assembly and disassembly are coordinated around macropinosomes to allow them to form and subsequently pass through the dense actin network underlying the plasma membrane to move towards the cell center for maturation. Here we show that the PH and FYVE domain protein Phafin2 is recruited transiently to newly-formed macropinosomes by a mechanism that involves coincidence detection of PtdIns3P and PtdIns4P. Phafin2 also interacts with actin via its PH domain, and recruitment of Phafin2 coincides with actin reorganization around nascent macropinosomes. Moreover, forced relocalization of Phafin2 to the plasma membrane causes rearrangement of the subcortical actin cytoskeleton. Depletion of Phafin2 inhibits macropinosome internalization and maturation and prevents KRAS-transformed cancer cells from utilizing extracellular protein as an amino acid source. We conclude that Phafin2 promotes macropinocytosis by controlling timely delamination of actin from nascent macropinosomes for their navigation through the dense subcortical actin network.

Vacuolar H+-ATPase dysfunction rescues intralumenal vesicle cargo sorting in yeast lacking PI(3,5)P2 or Doa4

Endosomes undergo a maturation process highlighted by a reduction in lumenal pH, a conversion of surface markers that prime endosome-lysosome fusion and the sequestration of ubiquitylated transmembrane protein cargos within intralumenal vesicles (ILVs). We investigated ILV cargo sorting in mutant strains of the budding yeast Saccharomyces cerevisiae that are deficient for either the lysosomal/vacuolar signaling lipid PI(3,5)P2 or the Doa4 ubiquitin hydrolase that deubiquitylates ILV cargos. Disruption of PI(3,5)P2 synthesis or Doa4 function causes a defect in sorting of a subset of ILV cargos. We show that these cargo-sorting defects are suppressed by mutations that disrupt Vph1, a subunit of vacuolar H+-ATPase (V-ATPase) complexes that acidify late endosomes and vacuoles. We further show that Vph1 dysfunction increases endosome abundance, and disrupts vacuolar localization of Ypt7 and Vps41, two crucial mediators of endosome-vacuole fusion. Because V-ATPase inhibition attenuates this fusion and rescues the ILV cargo-sorting defects in yeast that lack PI(3,5)P2 or Doa4 activity, our results suggest that the V-ATPase has a role in coordinating ILV cargo sorting with the membrane fusion machinery. This article has an associated First Person interview with the first author of the paper.

NCP2/RHD4/SAC7, SAC6 and SAC8 phosphoinositide phosphatases are required for PtdIns4P and PtdIns(4,5)P2 homeostasis and Arabidopsis development

Phosphoinositides play important roles in plant growth and development. Several SAC domain phosphoinositide phosphatases have been reported to be important for plant development. Here, we show functional analysis of SUPPRESSOR OF ACTIN 6 (SAC6) to SAC8 in Arabidopsis, a subfamily of phosphoinositide phosphatases containing SAC-domain and two transmembrane motifs. We isolated an Arabidopsis mutant ncp2 that lacked cotyledons in seedling and embryo in pid, a background defective in auxin signaling and transport. NCP2 encodes RHD4/SAC7 phosphoinositide phosphatase. SAC6, SAC7 and SAC8 exhibit overlapping and specific expression patterns in seedling and embryo. The sac6 sac7 embryos either fail to develop into seeds, or have three or four cotyledons. The embryo development of sac7 sac8 and sac6 sac7 sac8 mutants is significantly delayed or lethal, and the seedlings are arrested at early stages. Auxin maxima are decreased in double and triple sac mutants. The contents of PtdIns4P and PtdIns(4,5)P2 in sac6 sac7 and sac7 sac8 mutants are dramatically increased. Protein trafficking of the plasma membrane (PM)-localized protein PIN1 and PIN2 from trans-Golgi network/early endosome back to PM is delayed in sac7 sac8 mutants. These results indicate that SAC6-SAC8 are essential for maintaining homeostasis of PtdIns4P and PtdIns(4,5)P2, and auxin-mediated development in Arabidopsis.

A highly conserved glutamic acid in ALFY inhibits membrane binding to aid in aggregate clearance

Autophagy-linked FYVE protein (ALFY) is a large, multidomain protein involved in the degradation of protein aggregates by selective autophagy. The C-terminal FYVE domain of ALFY has been shown to bind phosphatidylinositol 3-phosphate (PI(3)P); however, ALFY only partially colocalizes with other FYVE domains in cells. Thus, we asked if the FYVE domain of ALFY has distinct membrane binding properties compared to other FYVE domains and whether these properties might affect its function in vivo. We found that the FYVE domain of ALFY binds weakly to PI(3)P containing membranes in vitro. This weak binding is the result of a highly conserved glutamic acid within the membrane insertion loop in the FYVE domain of ALFY that is not present in any other human FYVE domain. In addition, not only does this glutamic acid reduce binding to membranes in vitro and inhibits its targeting to membranes in vivo, but it is also important for the ability of ALFY to clear protein aggregates.

The phosphoinositide 3-kinase inhibitor alpelisib restores actin organization and improves proximal tubule dysfunction in vitro and in a mouse model of Lowe syndrome and Dent disease

Loss-of-function mutations in the OCRL gene, which encodes the phosphatidylinositol [PI] 4,5-bisphosphate [PI(4,5)P2] 5-phosphatase OCRL, cause defective endocytosis and proximal tubule dysfunction in Lowe syndrome and Dent disease 2. The defect is due to increased levels of PI(4,5)P2 and aberrant actin polymerization, blocking endosomal trafficking. PI 3-phosphate [PI(3)P] has been recently identified as a coactivator with PI(4,5)P2 in the actin pathway. Here, we tested the hypothesis that phosphoinositide 3-kinase (PI3K) inhibitors may rescue the endocytic defect imparted by OCRL loss, by rebalancing phosphoinositide signals to the actin machinery. The broad-range PI3K inhibitor copanlisib and class IA p110α PI3K inhibitor alpelisib reduced aberrant actin polymerization in OCRL-deficient human kidney cells in vitro. Levels of PI 3,4,5-trisphosphate, PI(4,5)P2 and PI(3)P were all reduced with alpelisib treatment, and siRNA knockdown of the PI3K catalytic subunit p110α phenocopied the actin phenotype. In a humanized OcrlY/- mouse model, alpelisib reduced endosomal actin staining while restoring stress fiber architecture and levels of megalin at the plasma membrane of proximal tubule cells, reflected by improved endocytic uptake of low molecular weight proteins in vivo. Thus, our findings support the link between phosphoinositide lipids, actin polymerization and endocytic trafficking in the proximal tubule and represent a proof-of-concept for repurposing alpelisib in Lowe syndrome/Dent disease 2.

Vps34 derived phosphatidylinositol 3-monophosphate modulates megakaryocyte maturation and proplatelet production through late endosomes/lysosomes

BACKGROUND

Development of platelet precursor cells, megakaryocytes (MKs), implies an increase in their size; formation of the elaborate demarcation membrane system (DMS); and extension of branched cytoplasmic structures, proplatelets, that will release platelets. The membrane source(s) for MK expansion and proplatelet formation have remained elusive.

OBJECTIVE

We hypothesized that traffic of membranes regulated by phosphatidylinositol 3-monophosphate (PI3P) contributes to MK maturation and proplatelet formation.

RESULTS

In immature MKs, PI3P produced by the lipid kinase Vps34 is confined to perinuclear early endosomes (EE), while in mature MKs PI3P shifts to late endosomes and lysosomes (LE/Lys). PI3P partially colocalized with the plasma membrane marker phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2 ) and with LE/Lys in mature MKs, suggests that PI3P-containing LE/Lys membranes contribute to MK expansion and proplatelet formation. Consistently, we found that sequestration of PI3P, specific pharmacological inhibition of Vps34-mediated PI3P production, or depletion of PI3P by PI3-phosphatase (MTM1)-mediated hydrolysis potently blocked proplatelet formation. Moreover, Vps34 inhibition led to the intracellular accumulation of enlarged LE/Lys, and decreased expression of surface LE/Lys markers. Inhibiting Vps34 at earlier MK stages caused aberrant DMS development. Finally, inhibition of LE/Lys membrane fusion by a dominant negative mutant of the small GTPase Rab7 or pharmacological inhibition of PI3P conversion into PI(3,5)P2 led to enlarged LE/Lys, reduced surface levels of LE/Lys markers, and decreased proplatelet formation.

CONCLUSION

Our results suggest that PI3P-positive LE/Lys contribute to the membrane growth and proplatelet formation in MKs by their translocation to the cell periphery and fusion with the plasma membrane.

Characterization of the c10orf76-PI4KB complex and its necessity for Golgi PI4P levels and enterovirus replication

The lipid kinase PI4KB, which generates phosphatidylinositol 4-phosphate (PI4P), is a key enzyme in regulating membrane transport and is also hijacked by multiple picornaviruses to mediate viral replication. PI4KB can interact with multiple protein binding partners, which are differentially manipulated by picornaviruses to facilitate replication. The protein c10orf76 is a PI4KB-associated protein that increases PI4P levels at the Golgi and is essential for the viral replication of specific enteroviruses. We used hydrogen-deuterium exchange mass spectrometry to characterize the c10orf76-PI4KB complex and reveal that binding is mediated by the kinase linker of PI4KB, with formation of the heterodimeric complex modulated by PKA-dependent phosphorylation. Complex-disrupting mutations demonstrate that PI4KB is required for membrane recruitment of c10orf76 to the Golgi, and that an intact c10orf76-PI4KB complex is required for the replication of c10orf76-dependent enteroviruses. Intriguingly, c10orf76 also contributed to proper Arf1 activation at the Golgi, providing a putative mechanism for the c10orf76-dependent increase in PI4P levels at the Golgi.

A Complex Interplay of Anionic Phospholipid Binding Regulates 3'-Phosphoinositide-Dependent-Kinase-1 Homodimer Activation

3'-Phosphoinositide-dependent-Kinase-1 (PDK1) is a master regulator whereby its PI3-kinase-dependent dysregulation in human pathologies is well documented. Understanding the direct role for PtdIns(3,4,5)P3 and other anionic phospholipids in the regulation of PDK1 conformational dynamics and its downstream activation remains incomplete. Using advanced quantitative-time-resolved imaging (Fluorescence Lifetime Imaging and Fluorescence Correlation Spectroscopy) and molecular modelling, we show an interplay of antagonistic binding effects of PtdIns(3,4,5)P3 and other anionic phospholipids, regulating activated PDK1 homodimers. We demonstrate that phosphatidylserine maintains PDK1 in an inactive conformation. The dysregulation of the PI3K pathway affects the spatio-temporal and conformational dynamics of PDK1 and the activation of its downstream substrates. We have established a new anionic-phospholipid-dependent model for PDK1 regulation, depicting the conformational dynamics of multiple homodimer states. We show that the dysregulation of the PI3K pathway perturbs equilibrium between the PDK1 homodimer conformations. Our findings provide a role for the PtdSer binding site and its previously unrewarding role in PDK1 downregulation, suggesting a possible therapeutic strategy where the constitutively active dimer conformer of PDK1 may be rendered inactive by small molecules that drive it to its PtdSer-bound conformer.

ATG9A supplies PtdIns4P to the autophagosome initiation site

The identity of the platform supporting the initiation and formation of the nascent autophagosome, the phagophore, is not fully understood. Nucleation and expansion of the phagophore membrane requires a coordinated flux or activation of specific proteins and membrane lipids at the initiation site. The transmembrane protein ATG9A is essential for macroautophagy/autophagy and proposed to be an initiator of the phagophore by directing or facilitating the delivery of proteins and lipids to the initiation site. Upon amino acid starvation, ATG9A-containing vesicles are formed from the Golgi complex and endosomal compartments and translocate to the initiation site. Unravelling the complement of proteins and lipids brought by ATG9A vesicles to the forming autophagosome is essential to further understand the initiation of autophagy.

A new genetic subgroup of chronic granulomatous disease with autosomal recessive mutations in p40 phox and selective defects in neutrophil NADPH oxidase activity

Chronic granulomatous disease (CGD), an immunodeficiency with recurrent pyogenic infections and granulomatous inflammation, results from loss of phagocyte superoxide production by recessive mutations in any 1 of 4 genes encoding subunits of the phagocyte NADPH oxidase. These include gp91(phox) and p22(phox), which form the membrane-integrated flavocytochrome b, and cytosolic subunits p47(phox) and p67(phox). A fifth subunit, p40(phox), plays an important role in phagocytosis-induced superoxide production via a phox homology (PX) domain that binds to phosphatidylinositol 3-phosphate (PtdIns(3)P). We report the first case of autosomal recessive mutations in NCF4, the gene encoding p40(phox), in a boy who presented with granulomatous colitis. His neutrophils showed a substantial defect in intracellular superoxide production during phagocytosis, whereas extracellular release of superoxide elicited by phorbol ester or formyl-methionyl-leucyl-phenylalanine (fMLF) was unaffected. Genetic analysis of NCF4 showed compound heterozygosity for a frameshift mutation with premature stop codon and a missense mutation predicting a R105Q substitution in the PX domain. Parents and a sibling were healthy heterozygous carriers. p40(phox)R105Q lacked binding to PtdIns(3)P and failed to reconstitute phagocytosis-induced oxidase activity in p40(phox)-deficient granulocytes, with premature loss of p40(phox)R105Q from phagosomes. Thus, p40(phox) binding to PtdIns(3)P is essential for phagocytosis-induced oxidant production in human neutrophils and its absence can be associated with disease.

Arabidopsis phosphatidylinositol phosphate kinase 1 binds F-actin and recruits phosphatidylinositol 4-kinase beta1 to the actin cytoskeleton

The actin cytoskeleton can be influenced by phospholipids and lipid-modifying enzymes. In animals the phosphatidylinositol phosphate kinases (PIPKs) are associated with the cytoskeleton through a scaffold of proteins; however, in plants such an interaction was not clear. Our approach was to determine which of the plant PIPKs interact with actin and determine whether the PIPK-actin interaction is direct. Our results indicate that AtPIPK1 interacts directly with actin and that the binding is mediated through a predicted linker region in the lipid kinase. AtPIPK1 also recruits AtPI4Kbeta1 to the cytoskeleton. Recruitment of AtPI4Kbeta1 to F-actin was dependent on the C-terminal catalytic domain of phosphatidylinositol-4-phosphate 5-kinase but did not require the presence of the N-terminal 251 amino acids, which includes 7 putative membrane occupation and recognition nexus motifs. In vivo studies confirm the interaction of plant lipid kinases with the cytoskeleton and suggest a role for actin in targeting PIPKs to the membrane.

Distinct targeting and fusion functions of the PX and SNARE domains of yeast vacuolar Vam7p

Regulated membrane fusion requires organelle tethering, enrichment of selected proteins and lipids at the fusion site, bilayer distortion, and lipid rearrangement. Yeast vacuole homotypic fusion requires regulatory lipids (ergosterol, diacylglycerol, and phosphoinositides), the Rab family GTPase Ypt7p, the multisubunit Ypt7p-effector complex HOPS (homotypic fusion and vacuole protein sorting), and four SNAREs. One SNARE, Vam7p, has an N-terminal PX domain which binds to phosphatidylinositol 3-phosphate (PI(3)P) and to HOPS and a C-terminal SNARE domain but no apolar membrane anchor. We have exploited an in vitro reaction of vacuole fusion to analyze the functions of each domain, removing the PX domain or mutating it to abolish its PI(3)P affinity. Lowering the PI(3)P affinity of the PX domain, or even deleting the PX domain, affects the fusion K(m) for Vam7p but not the maximal fusion rate. Fusion driven by the SNARE domain alone is strikingly enhanced by the PLC inhibitor U73122 through enhanced binding of Vam7p SNARE domain to vacuoles, and the further addition of Plc1p blocks this U73122 effect. The PX domain, through its affinities for phosphoinositides and HOPS, is thus exclusively required for enhancing the targeting of Vam7p rather than for execution of the Vam7p functions in HOPS.SNARE complex assembly and fusion.

The Phox (PX) domain proteins and membrane traffic

Phosphoinositides (PIs) are phosphorylated derivatives of phosphatidylinositol (PtdIns) that regulate many cellular and physiological processes. Most PIs act by serving as membrane docking sites for proteins harboring specific PI-binding domains so that the location and function of these proteins could be dynamically governed. The Phox (PX) domain represents a novel PI-binding module capable of regulating membrane targeting of about 47 mammalian proteins, 30 of which are tentatively referred to as sorting nexins (SNXs). Some SNXs have been implicated in regulating membrane trafficking in the endocytic pathway. We discuss here recent development and progress in the study of the PX domain-containing proteins.

Phospholipase D from photoreceptor rod outer segments is a downstream effector of RhoA: evidence of a light-dependent mechanism

Photoreceptor cells contain rod outer segments (ROS) which are specialized light-sensitive organelles. The biological function of ROS is to generate a photoresponse, which occurs via the classic transducin-mediated pathway. Moreover, ROS undergo light-regulated membrane turnover and protein translocation whose mechanisms have not been fully elucidated to date. Phospholipase D (PLD) is a key enzyme involved in lipid signal transduction and membrane trafficking. We have previously reported that PLD activity is present in purified ROS (Salvador, G.A., Giusto, N.M., 1998. Characterization of phospholipase D activity in bovine photoreceptor membranes. Lipids 33, 853-860). We now demonstrate that ROS PLD activity is enhanced by phosphatidylinositol bisphosphate (PIP2) and cytosolic factors in a GTP dependent-manner. Western blot analysis demonstrates the presence of PLD1 isoform in purified ROS. In ROS obtained from dark-adapted retinas (DROS), PIP2-dependent PLD activity was higher than that observed in ROS obtained from light-adapted retinas (LROS). In addition, experiments carried out in the presence of C3 toxin inhibited PLD activity from DROS whereas pertussis toxin did not affect the enzyme activity. Western blot analysis demonstrates the presence of RhoA, a PLD upstream-regulator. Moreover, RhoA levels were higher in DROS with respect to those in LROS. The present study reports evidence of the involvement of the small G-protein, RhoA, in ROS PLD regulation. Our data strongly suggest that RhoA regulates ROS PLD activity under a light-dependent mechanism.

Localised PtdIns(3,4,5)P3 or PtdIns(3,4)P2 at the phagocytic cup is required for both phagosome closure and Ca2+ signalling in HL60 neutrophils

Several events accompany integrin-mediated phagocytosis by myeloid cells. These include local pseudopod and phagocytic cup formation followed by Ca2+ signalling. However, there is also a role for localised phosphatidylinositol (3,4,5) trisphosphate [PtdIns(3,4,5)P3] production. Here we report that in neutrophilic HL-60 cells expressing PH-Akt-GFP, binding of iC3b-coated zymosan particles (2 μm in diameter) via β2 integrin induces an incomplete phagocytic cup to form before either PtdIns(3,4,5)P3 or phosphatidylinositol (3,4) bisphosphate [PtdIns(3,4)P2] production or Ca2+ signalling. These phosphoinositides then accumulated locally at the site of the phagocytic cup and Ca2+ signalling and phagosome closure follows immediately. Although photobleaching showed that PH-Akt-GFP was freely diffusible in the cytosol and able to dissociate from the phagocytic cup, it was restricted to the plasma membrane of the formed but open phagosome and failed to diffuse into the surrounding plasma membrane or neighbouring phagocytic cups even if connected. Inhibition of phosphoinositide (PI) 3-kinase or depletion of membrane cholesterol inhibited both Ca2+ signalling and phagosome closure, but had no effect on particle binding or phagocytic cup formation. We therefore conclude that PtdIns(3,4,5)P3 or PtdIns(3,4)P2 generation was not required for the events that initiate the formation of the phagocytic cup, but that anchoring of PtdIns(3,4,5)P3 at the phagocytic cup is an essential step for phagosome closure and Ca2+ signalling.

Expression of the brain-specific membrane adapter protein p42IP4/centaurin alpha, a Ins(1,3,4,5)P4/PtdIns(3,4,5)P3 binding protein, in developing rat brain

Inositolphosphates and phosphatidylinositides are important second messengers. Previously p42(IP4), a protein with high affinity for both Ins(1,3,4,5)P(4) and PtdIns(3,4,5)P(3) has been characterized in our laboratory. In the present study mRNA levels of p42(IP4) were quantified during development (ages: 7, 14, 21 days and adult) by means of ribonuclease protection assay in various rat brain regions (cerebellum, cortex, striatum, thalamus, hypothalamus, olfactory bulb, hippocampus and tectum (superior and inferior colliculus)). A high level of p42(IP4) mRNA was detected in the cortex (ca. 1 pg specific RNA per microg of total RNA) which stayed highly independent of the age of the animals. In hippocampus and in the thalamus, p42(IP4) mRNA levels were comparable to those in the cortex in the first and second week postnatally, but decreased to lower levels in the adult brain. In striatum, the mRNA increased, albeit less intensely than in hippocampus and thalamus, until day 21 postnatally, and then decreased in the adult rat brain. Cerebellar p42(IP4) mRNA showed a slow increase within the first 3 weeks postnatally, and remained rather high in the adult brain. The protein expression of p42(IP4), tested within the same samples by Western blot staining, was consistent with mRNA values. For comparison, glutamic acid decarboxylase (isoforms GAD65/GAD67), an enzyme, for which some regional brain specific distribution is already known, was also examined. The mRNA levels of GAD and its developmental regulation clearly differed from that of p42(IP4). In summary, p42(IP4) expressed in several neuronal cell types, did not seem to be restricted to specific developmental stages, but the high absolute expression levels at all developmental stages indicated that p42(IP4) is a protein fundamental for neuronal functioning.

Defective Hyphal induction of a Candida albicans phosphatidylinositol 3-phosphate 5-kinase null mutant on solid media does not lead to decreased virulence

A phosphatidylinositol 3-phosphate [PI(3)P] 5-kinase gene (CaFAB1) of the most important human pathogenic yeast, Candida albicans, was cloned and sequenced. An open reading frame was detected which encodes a 2,369-amino-acid protein with a calculated molecular mass of 268 kDa and a relative isoelectric point of 6.76. This protein exhibits 38% overall amino acid sequence identity with Saccharomyces cerevisiae Fab1p. We localized the CaFAB1 gene on chromosome R. To determine the influence of the PI(3)P 5-kinase CaFab1p on processes involved in C. albicans morphogenesis and pathogenicity, we sequentially disrupted both copies of the gene. Homozygous deletion of C. albicans CaFAB1 resulted in a mutant strain which exhibited defects in morphogenesis. A Cafab1 null mutant had enlarged vacuoles, an acidification defect, and increased generation times and was unable to form hyphae on different solid media. The sensitivities to hyperosmotic and high-temperature stresses, adherence, and virulence compared to those of wild-type strain SC5314 were not affected.

Discordant effects of glucosamine on insulin-stimulated glucose metabolism and phosphatidylinositol 3-kinase activity

The impact of increased GlcN availability on insulin-stimulated p85/p110 phosphatidylinositol 3-kinase (PI3K) activity in skeletal muscle was examined in relation to GlcN-induced defects in peripheral insulin action. Primed continuous GlcN infusion (750 micromol/kg bolus; 30 micromol/kg.min) in conscious rats limited both maximal stimulation of muscle PI3K by acute insulin (I) (1 unit/kg) bolus (I + GlcN = 1.9-fold versus saline = 3.3-fold above fasting levels; p < 0.01) and chronic activation of PI3K following 3-h euglycemic, hyperinsulinemic (18 milliunits/kg.min) clamp studies (I + GlcN = 1.2-fold versus saline = 2.6-fold stimulation; p < 0.01). To determine the time course of GlcN-induced defects in insulin-stimulated PI3K activity and peripheral insulin action, GlcN was administered for 30, 60, 90, or 120 min during 2-h euglycemic, hyperinsulinemic clamp studies. Activation of muscle PI3K by insulin was attenuated following only 30 min of GlcN infusion (GlcN 30 min = 1.5-fold versus saline = 2.5-fold stimulation; p < 0.05). In contrast, the first impairment in insulin-mediated glucose uptake (Rd) developed following 110 min of GlcN infusion (110 min = 39.9 +/- 1.8 versus 30 min = 42.8 +/- 1.4 mg/kg.min, p < 0.05). However, the ability of insulin to stimulate phosphatidylinositol 3,4, 5-trisphosphate production and to activate glycogen synthase in skeletal muscle was preserved following up to 180 min of GlcN infusion. Thus, increased GlcN availability induced (a) profound and early inhibition of proximal insulin signaling at the level of PI3K and (b) delayed effects on insulin-mediated glucose uptake, yet (c) complete sparing of insulin-mediated glycogen synthase activation. The pattern and time sequence of GlcN-induced defects suggest that the etiology of peripheral insulin resistance may be distinct from the rapid and marked impairment in insulin signaling.

Protein kinase C isotypes controlled by phosphoinositide 3-kinase through the protein kinase PDK1

Phosphorylation sites in members of the protein kinase A (PKA), PKG, and PKC kinase subfamily are conserved. Thus, the PKB kinase PDK1 may be responsible for the phosphorylation of PKC isotypes. PDK1 phosphorylated the activation loop sites of PKCzeta and PKCdelta in vitro and in a phosphoinositide 3-kinase (PI 3-kinase)-dependent manner in vivo in human embryonic kidney (293) cells. All members of the PKC family tested formed complexes with PDK1. PDK1-dependent phosphorylation of PKCdelta in vitro was stimulated by combined PKC and PDK1 activators. The activation loop phosphorylation of PKCdelta in response to serum stimulation of cells was PI 3-kinase-dependent and was enhanced by PDK1 coexpression.

Cloning and characterization of a novel class II phosphoinositide 3-kinase containing C2 domain

Phosphoinositide 3-kinases (PI3Ks) have been shown to play critical roles in cell growth, differentiation, survival, and vesicular transport. Class II PI3Ks have been recently identified in mouse and human (PI3K-C2 alpha/m-p170/m-cpk and HsC2-PI3K) and in Drosophila (PI3K 68D/cpk) which contain C2 domain at the C-terminus. However, their physiological function is largely unknown. We report here cloning and characterization of murine PI3K-C2 gamma, a novel class II PI3K. The catalytic domain as well as C2 domain are highly conserved in the Class II PI3K family, while the N-terminal regions of these proteins share little similarity. Unlike other Class II PI3Ks, PI3K-C2 gamma exclusively expressed in the liver, and a N-terminal truncated form was found in lung and a certain hematopoietic cell line. Specific antiserum against PI3K-C2 gamma precipitated PI3K activity from the membrane fraction of mouse liver but not from heart. Recombinant PI3K-C2 gamma exhibited a restricted lipid substrate specificity; it phosphorylated phosphatidylinositol (PtdIns) and PtdIns4P but not PtdIns(4,5)P2. Deletion mutations revealed that both the N-terminal region and the C2 domain were critical for enzymatic activity. The murine PI3K-C2 gamma gene locus was mapped to the distal region of mouse chromosome 6 in a region of homology with human chromosome 12p, which is distinct from the position of HsC2-PI3K. Cloning and biochemical characterization of the third member of class II PI3Ks provide a new insight into the function of this subfamily of PI3Ks.

Effects of profilin-annexin I association on some properties of both profilin and annexin I: modification of the inhibitory activity of profilin on actin polymerization and inhibition of the self-association of annexin I and its interactions with liposomes

We have previously shown that annexin I, a member of a family of calcium-dependent phospholipid and membrane binding proteins, interacts with profilin with high specificity and affinity. This finding further suggests that annexin I is involved through profilin in the regulation of membrane-cytoskeleton organization. We have investigated the consequences of a complex formed by these two proteins on the functions of both profilin and annexin I. Annexin I is able to modify the inhibitory effect of profilin on actin polymerization. This action is partial and the mechanism involved appears to be complex. On the other hand, the association between annexin I and profilin is sufficiently strong to inhibit the self-association of annexin I. The binding capacity of annexin I to liposomes containing phosphatidylserine, which mimics annexin I binding to membranes, is also decreased by profilin. This binding is nevertheless restored when phosphatidylinositol 4,5-biphosphate (PtdInsP2) is included in the liposomes. Finally, the capacity of annexin I to aggregate liposomes is also modified. It is worthwhile mentioning that the liposomes-binding and liposomes-aggregating activities of annexin I are independently regulated. The cell localization and functions of annexin I and profilin suggest that interaction between these two proteins may be directly implicated in the regulation of membrane-cytoskeleton. The phospholipid composition of membranes may be one of the modulating factors.

Polyphosphoinositide inclusion in artificial lipid bilayer vesicles promotes divalent cation-dependent membrane fusion

Recent studies suggest that phosphoinositide kinases may participate in intracellular trafficking or exocytotic events. Because both of these events ultimately require fusion of biological membranes, the susceptibility of membranes containing polyphosphoinositides (PPIs) to divalent cation-induced fusion was investigated. Results of these investigations indicated that artificial liposomes containing PPI or phosphatidic acid required lower Ca2+ concentrations for induction of membrane fusion than similar vesicles containing phosphatidylserine, phosphatidylinositol, or phosphatidylcholine. This trend was first observed in liposomes composed solely of one type of phospholipid. In addition, however, liposomes designed to mimic the phospholipid composition of the endofacial leaflet of plasma membranes (i.e., liposomes composed of combinations of PPI, phosphatidylethanolamine, and phosphatidylcholine) also required lower Ca2+ concentrations for induction of aggregation and fusion. Liposomes containing PPI and phosphatidic acid also had increased sensitivity to Mg(2+)-induced fusion, an observation that is particularly intriguing given the intracellular concentration of Mg2+ ions. Moreover, the fusogenic effects of Ca2+ and Mg2+ were additive in vesicles containing phosphatidylinositol bisphosphate. These data suggest that enzymatic modification of the PPI content of intracellular membranes could be an important mechanism of fusion regulation.