An intact SH3 domain is required for myosin I-induced actin polymerization

Myosin-based nucleation of actin filaments contributes to stereocilia development critical for hearing

Assembly of actin-based stereocilia is critical for cochlear hair cells to detect sound. To tune their mechanosensivity, stereocilia form bundles composed of graded rows of ascending height, necessitating the precise control of actin polymerization. Myosin 15 (MYO15A) drives hair bundle development by delivering critical proteins to growing stereocilia that regulate actin polymerization via an unknown mechanism. Here, we show that MYO15A is itself an actin nucleation-promoting factor. Moreover, a deafness-causing mutation in the MYO15A actin-binding interface inhibits nucleation activity but still preserves some movement on filaments in vitro and partial trafficking on stereocilia in vivo. Stereocilia fail to elongate correctly in this mutant mouse, providing evidence that MYO15A-driven actin nucleation contributes to hair bundle biogenesis. Our work shows that in addition to generating force and motility, the ATPase domain of MYO15A can directly regulate actin polymerization and that disrupting this activity can promote cytoskeletal disease, such as hearing loss.

Myosin-I's motor and actin assembly activation activities are modular and separable in budding yeast clathrin-mediated endocytosis

The myosin-Is, Myo3 and Myo5 in budding yeast, are implicated in force generation and actin assembly during clathrin-mediated endocytosis (CME). The myosin-Is have motor activity, bind the plasma membrane, and activate the Arp2/3 complex to promote branched actin assembly. We reveal that Myo5 's force-generating motor activity and nucleation-promoting factor (NPF) activity each must be coupled to membrane binding for successful CME. However, the motor and NPF activities are modular and separable, showing that these activities function independently rather than in an obligatorily integrated manner to provide myosin-I's essential functions in actin network assembly and force generation during budding yeast CME.

Dissection of the role of a Src homology 3 domain in the evolution of binding preference of paralogous proteins

Protein-protein interactions (PPIs) drive many cellular processes. Some interactions are directed by Src homology 3 (SH3) domains that bind proline-rich motifs on other proteins. The evolution of the binding specificity of SH3 domains is not completely understood, particularly following gene duplication. Paralogous genes accumulate mutations that can modify protein functions and, for SH3 domains, their binding preferences. Here, we examined how the binding of the SH3 domains of 2 paralogous yeast type I myosins, Myo3 and Myo5, evolved following duplication. We found that the paralogs have subtly different SH3-dependent interaction profiles. However, by swapping SH3 domains between the paralogs and characterizing the SH3 domains freed from their protein context, we find that very few of the differences in interactions, if any, depend on the SH3 domains themselves. We used ancestral sequence reconstruction to resurrect the preduplication SH3 domains and examined, moving back in time, how the binding preference changed. Although the most recent ancestor of the 2 domains had a very similar binding preference as the extant ones, older ancestral domains displayed a gradual loss of interaction with the modern interaction partners when inserted in the extant paralogs. Molecular docking and experimental characterization of the free ancestral domains showed that their affinity with the proline motifs is likely not the cause for this loss of binding. Taken together, our results suggest that a SH3 and its host protein could create intramolecular or allosteric interactions essential for the SH3-dependent PPIs, making domains not functionally equivalent even when they have the same binding specificity.

Screening the complex biological behavior of late lanthanides through genome-wide interactions

Despite their similar physicochemical properties, recent studies have demonstrated that lanthanides can display different biological behaviors. Hence, the lanthanide series can be divided into three parts, namely early, mid, and late lanthanides, based on their interactions with biological systems. In particular, the late lanthanides demonstrate distinct, but poorly understood biological activity. In the current study, we employed genome-wide functional screening to help understand biological effects of exposure to Yb(III) and Lu(III), which were selected as representatives of the late lanthanides. As a model organism, we used Saccharomyces cerevisiae, since it shares many biological functions with humans. Analysis of the functional screening results indicated toxicity of late lanthanides is consistent with disruption of vesicle-mediated transport, and further supported a role for calcium transport processes and mitophagy in mitigating toxicity. Unexpectedly, our analysis suggested that late lanthanides target proteins with SH3 domains, which may underlie the observed toxicity. This study provides fundamental insights into the unique biological chemistry of late lanthanides, which may help devise new avenues toward the development of decorporation strategies and bio-inspired separation processes.

Endocytic myosin-1 is a force-insensitive, power-generating motor

Myosins are required for clathrin-mediated endocytosis, but their precise molecular roles in this process are not known. This is, in part, because the biophysical properties of the relevant motors have not been investigated. Myosins have diverse mechanochemical activities, ranging from powerful contractility against mechanical loads to force-sensitive anchoring. To better understand the essential molecular contribution of myosin to endocytosis, we studied the in vitro force-dependent kinetics of the Saccharomyces cerevisiae endocytic type I myosin called Myo5, a motor whose role in clathrin-mediated endocytosis has been meticulously studied in vivo. We report that Myo5 is a low-duty-ratio motor that is activated ∼10-fold by phosphorylation, and that its working stroke and actin-detachment kinetics are relatively force-insensitive. Strikingly, the in vitro mechanochemistry of Myo5 is more like that of cardiac myosin than like that of slow anchoring myosin-1s found on endosomal membranes. We therefore propose that Myo5 generates power to augment actin assembly-based forces during endocytosis in cells.

Summary

Pedersen, Snoberger et al. measure the force-sensitivity of the yeast endocytic the myosin-1 called Myo5 and find that it is more likely to generate power than to serve as a force-sensitive anchor in cells. Implications for Myo5's role in clathrin-mediated endocytosis are discussed.

Kazrin promotes dynein/dynactin-dependent traffic from early to recycling endosomes

Kazrin is a protein widely expressed in vertebrates whose depletion causes a myriad of developmental defects, in part derived from altered cell adhesion and migration, as well as failure to undergo epidermal to mesenchymal transition. However, the primary molecular role of kazrin, which might contribute to all these functions, has not been elucidated yet. We previously identified one of its isoforms, kazrin C, as a protein that potently inhibits clathrin-mediated endocytosis when overexpressed. We now generated kazrin knock-out mouse embryonic fibroblasts to investigate its endocytic function. We found that kazrin depletion delays juxtanuclear enrichment of internalized material, indicating a role in endocytic traffic from early to recycling endosomes. Consistently, we found that the C-terminal domain of kazrin C, predicted to be an intrinsically disordered region, directly interacts with several early endosome (EE) components, and that kazrin depletion impairs retrograde motility of these organelles. Further, we noticed that the N-terminus of kazrin C shares homology with dynein/dynactin adaptors and that it directly interacts with the dynactin complex and the dynein light intermediate chain 1. Altogether, the data indicate that one of the primary kazrin functions is to facilitate endocytic recycling by promoting dynein/dynactin-dependent transport of EEs or EE-derived transport intermediates to the recycling endosomes.

The endocytic protein machinery as an actin-driven membrane-remodeling machine

In clathrin-mediated endocytosis, a principal membrane trafficking route of all eukaryotic cells, forces are applied to invaginate the plasma membrane and form endocytic vesicles. These forces are provided by specific endocytic proteins and the polymerizing actin cytoskeleton. One of the best-studied endocytic systems is endocytosis in yeast, known for its simplicity, experimental amenability, and overall similarity to human endocytosis. Importantly, the yeast endocytic protein machinery generates and transmits tremendous force to bend the plasma membrane, making this system beneficial for mechanistic studies of cellular force-driven membrane reshaping. This review summarizes important protein players, molecular functions, applied forces, and open questions and perspectives of this robust, actin-powered membrane-remodeling protein machine.

Does the Actin Network Architecture Leverage Myosin-I Functions?

The actin cytoskeleton plays crucial roles in cell morphogenesis and functions. The main partners of cortical actin are molecular motors of the myosin superfamily. Although our understanding of myosin functions is heavily based on myosin-II and its ability to dimerize, the largest and most ancient class is represented by myosin-I. Class 1 myosins are monomeric, actin-based motors that regulate a wide spectrum of functions, and whose dysregulation mediates multiple human diseases. We highlight the current challenges in identifying the “pantograph” for myosin-I motors: we need to reveal how conformational changes of myosin-I motors lead to diverse cellular as well as multicellular phenotypes. We review several mechanisms for scaling, and focus on the (re-) emerging function of class 1 myosins to remodel the actin network architecture, a higher-order dynamic scaffold that has potential to leverage molecular myosin-I functions. Undoubtfully, understanding the molecular functions of myosin-I motors will reveal unexpected stories about its big partner, the dynamic actin cytoskeleton.

Coupled sterol synthesis and transport machineries at ER-endocytic contact sites

Sterols are unevenly distributed within cellular membranes. How their biosynthetic and transport machineries are organized to generate heterogeneity is largely unknown. We previously showed that the yeast sterol transporter Osh2 is recruited to endoplasmic reticulum (ER)–endocytic contacts to facilitate actin polymerization. We now find that a subset of sterol biosynthetic enzymes also localizes at these contacts and interacts with Osh2 and the endocytic machinery. Following the sterol dynamics, we show that Osh2 extracts sterols from these subdomains, which we name ERSESs (ER sterol exit sites). Further, we demonstrate that coupling of the sterol synthesis and transport machineries is required for endocytosis in mother cells, but not in daughters, where plasma membrane loading with accessible sterols and endocytosis are linked to secretion.

Type-I myosins promote actin polymerization to drive membrane bending in endocytosis

Clathrin-mediated endocytosis in budding yeast requires the formation of a dynamic actin network that produces the force to invaginate the plasma membrane against the intracellular turgor pressure. The type-I myosins Myo3 and Myo5 are important for endocytic membrane reshaping, but mechanistic details of their function remain scarce. Here, we studied the function of Myo3 and Myo5 during endocytosis using quantitative live-cell imaging and genetic perturbations. We show that the type-I myosins promote, in a dose-dependent way, the growth and expansion of the actin network, which controls the speed of membrane and coat internalization. We found that this myosin-activity is independent of the actin nucleation promoting activity of myosins, and cannot be compensated for by increasing actin nucleation. Our results suggest a new mechanism for type-I myosins to produce force by promoting actin filament polymerization.

Type I myosins anchor actin assembly to the plasma membrane during clathrin-mediated endocytosis

Actin assembly and type I myosins are both required for clathrin-mediated endocytosis. Here Pedersen and Drubin show that type I myosins anchor actin assembly factors to the plasma membrane at sites of clathrin-mediated endocytosis, facilitating force generation by actin assembly.

The actin cytoskeleton generates forces on membranes for a wide range of cellular and subcellular morphogenic events, from cell migration to cytokinesis and membrane trafficking. For each of these processes, filamentous actin (F-actin) interacts with membranes and exerts force through its assembly, its associated myosin motors, or both. These two modes of force generation are well studied in isolation, but how they are coordinated in cells is mysterious. During clathrin-mediated endocytosis, F-actin assembly initiated by the Arp2/3 complex and several proteins that compose the WASP/myosin complex generates the force necessary to deform the plasma membrane into a pit. Here we present evidence that type I myosin is the key membrane anchor for endocytic actin assembly factors in budding yeast. By mooring actin assembly factors to the plasma membrane, this myosin organizes endocytic actin networks and couples actin-generated forces to the plasma membrane to drive invagination and scission. Through this unexpected mechanism, myosin facilitates force generation independent of its motor activity.

Tail domains of myosin-1e regulate phosphatidylinositol signaling and F-actin polymerization at the ventral layer of podosomes

During podosome formation, distinct phosphatidylinositol 3,4,5-trisphosphate lipid (PI(3,4,5)P3) production and F-actin polymerization take place at integrin-mediated adhesions. Membrane-associated actin regulation factors, such as myosin-1, serve as key molecules to link phosphatidylinositol signals to podosome assembly. Here, we report that long-tailed myosin-1e (Myo1e) is enriched at the ventral layer of the podosome core in a PI(3,4,5)P3-dependent manner. The combination of TH1 and TH2 (TH12) of Myo1e tail domains contains the essential motif for PI(3,4,5)P3-dependent membrane association and ventral localization at the podosome. TH12 KR2A (K772A and R782A) becomes dissociated from the plasma membrane. While F-actin polymerizations are initialized from the ventral layer of the podosome, TH12 precedes the recruitment of N-WASP and Arp2/3 in the initial phase of podosome formation. Overexpression of TH12, not TH12 KR2A, impedes PI(3,4,5)P3 signaling, restrains F-actin polymerization, and inhibits podosome formation. TH12 also suppresses gelatin degradation and migration speed of invadopodia-forming A375 melanoma cells. Thus, TH12 domain of Myo1e serves as a regulatory component to connect phosphatidylinositol signaling to F-actin polymerization at the podosome.

The Sla1 adaptor-clathrin interaction regulates coat formation and progression of endocytosis

Clathrin-mediated endocytosis is a fundamental transport pathway that depends on numerous protein-protein interactions. Testing the importance of the adaptor protein-clathrin interaction for coat formation and progression of endocytosis in vivo has been difficult due to experimental constrains. Here, we addressed this question using the yeast clathrin adaptor Sla1, which is unique in showing a cargo endocytosis defect upon substitution of 3 amino acids in its clathrin-binding motif (sla1^AAA ) that disrupt clathrin binding. Live-cell imaging showed an impaired Sla1-clathrin interaction causes reduced clathrin levels but increased Sla1 levels at endocytic sites. Moreover, the rate of Sla1 recruitment was reduced indicating proper dynamics of both clathrin and Sla1 depend on their interaction. sla1^AAA cells showed a delay in progression through the various stages of endocytosis. The Arp2/3-dependent actin polymerization machinery was present for significantly longer time before actin polymerization ensued, revealing a link between coat formation and activation of actin polymerization. Ultimately, in sla1^AAA cells a larger than normal actin network was formed, dramatically higher levels of various machinery proteins other than clathrin were recruited, and the membrane profile of endocytic invaginations was longer. Thus, the Sla1-clathrin interaction is important for coat formation, regulation of endocytic progression and membrane bending.

ORP-Mediated ER Contact with Endocytic Sites Facilitates Actin Polymerization

Oxysterol binding protein-related proteins (ORPs) are conserved lipid binding polypeptides, enriched at ER contacts sites. ORPs promote non-vesicular lipid transport and work as lipid sensors in the context of many cellular tasks, but the determinants of their distinct localization and function are not understood. Here, we demonstrate that the yeast endocytic invaginations associate with the ER and that this association specifically requires the ORPs Osh2 and Osh3, which bridge the endocytic myosin-I Myo5 to the ER integral-membrane VAMP-associated protein (VAP) Scs2. Disruption of the ER contact with endocytic sites using ORP, VAP, myosin-I, or reticulon mutants delays and weakens actin polymerization and interferes with vesicle scission. Finally, we provide evidence suggesting that ORP-dependent sterol transfer facilitates actin polymerization at endocytic sites.

Switch-like Arp2/3 activation upon WASP and WIP recruitment to an apparent threshold level by multivalent linker proteins in vivo

Actin-related protein 2/3 (Arp2/3) complex activation by nucleation promoting factors (NPFs) such as WASP, plays an important role in many actin-mediated cellular processes. In yeast, Arp2/3-mediated actin filament assembly drives endocytic membrane invagination and vesicle scission. Here we used genetics and quantitative live-cell imaging to probe the mechanisms that concentrate NPFs at endocytic sites, and to investigate how NPFs regulate actin assembly onset. Our results demonstrate that SH3 (Src homology 3) domain-PRM (proline-rich motif) interactions involving multivalent linker proteins play central roles in concentrating NPFs at endocytic sites. Quantitative imaging suggested that productive actin assembly initiation is tightly coupled to accumulation of threshold levels of WASP and WIP, but not to recruitment kinetics or release of autoinhibition. These studies provide evidence that WASP and WIP play central roles in establishment of a robust multivalent SH3 domain-PRM network in vivo, giving actin assembly onset at endocytic sites a switch-like behavior.

DOI:http://dx.doi.org/10.7554/eLife.29140.001

Actin is one of the most abundant proteins in yeast, mammalian and other eukaryotic cells. It assembles into long chains known as filaments that the cell uses to generate forces for various purposes. For example, actin filaments are needed to pull part of the membrane surrounding the cell inwards to bring molecules from the external environment into the cell by a process called endocytosis.

In yeast, a member of the WASP family of proteins promotes the assembly of actin filaments around the site where endocytosis will occur. To achieve this, WASP interacts with several other proteins including WIP and myosin, a motor protein that moves along actin filaments to generate mechanical forces. However, it was not clear how these proteins work together to trigger actin filaments to assemble at the right place and time.

Sun et al. addressed this question by studying yeast cells with genetic mutations affecting one or more of these proteins. The experiments show that WASP, myosin and WIP are recruited to sites where endocytosis is about to occur through specific interactions with other proteins. For example, a region of WASP known as the proline-rich domain can bind to proteins that contain an “SH3” domain. WASP and WIP arrive first, stimulating actin to assemble in an “all and nothing” manner and attracting myosin to the actin. Further experiments indicate that WASP and WIP need to reach a threshold level before actin starts to assemble.

The findings of Sun et al. suggest that WASP and WIP play key roles in establishing the network of proteins needed for actin filaments to assemble during endocytosis. These proteins are needed for many other processes in yeast and other cells, including mammalian cells. Therefore, the next steps will be to investigate whether WASP and WIP use the same mechanism to operate in other situations.

DOI:http://dx.doi.org/10.7554/eLife.29140.002

An Engineered Minimal WASP-Myosin Fusion Protein Reveals Essential Functions for Endocytosis

Actin polymerization powers membrane deformation during many processes, including clathrin-mediated endocytosis (CME). During CME in yeast, actin polymerization is triggered and coordinated by a six-protein WASP/Myosin complex that includes WASP, class I myosins (Myo3 and Myo5), WIP (Vrp1), and two other proteins. We show that a single engineered protein can replace this entire complex while still supporting CME. This engineered protein reveals that the WASP/Myosin complex has four essential activities: recruitment to endocytic sites, anchorage to the plasma membrane, Arp2/3 activation, and transient actin filament binding by the motor domain. The requirement for both membrane and F-actin binding reveals that myosin-mediated coupling between actin filaments and the base of endocytic sites is essential for allowing actin polymerization to drive membrane invagination.

Evolution of the SH3 Domain Specificity Landscape in Yeasts

To explore the conservation of Src homology 3 (SH3) domain-mediated networks in evolution, we compared the specificity landscape of these domains among four yeast species, Saccharomyces cerevisiae, Ashbya gossypii, Candida albicans, and Schizosaccharomyces pombe, encompassing 400 million years of evolution. We first aligned and catalogued the families of SH3-containing proteins in these four species to determine the relationships between homologous domains. Then, we tagged and purified all soluble SH3 domains (82 in total) to perform a quantitative peptide assay (SPOT) for each SH3 domain. All SPOT readouts were hierarchically clustered and we observed that the organization of the SH3 specificity landscape in three distinct profile classes remains conserved across these four yeast species. We also produced a specificity profile for each SH3 domain from manually aligned top SPOT hits and compared the within-family binding motif consensus. This analysis revealed a striking example of binding motif divergence in a C. albicans Rvs167 paralog, which cannot be explained by overall SH3 sequence or interface residue divergence, and we validated this specificity change with a yeast two-hybrid (Y2H) assay. In addition, we show that position-weighted matrices (PWM) compiled from SPOT assays can be used for binding motif screening in potential binding partners and present cases where motifs are either conserved or lost among homologous SH3 interacting proteins. Finally, by comparing pairwise SH3 sequence identity to binding profile correlation we show that for ~75% of all analyzed families the SH3 specificity profile was remarkably conserved over a large evolutionary distance. Thus, a high sequence identity within an SH3 domain family predicts conserved binding specificity, whereas divergence in sequence identity often coincided with a change in binding specificity within this family. As such, our results are important for future studies aimed at unraveling complex specificity networks of peptide recognition domains in higher eukaryotes, including mammals.

Crosstalk between PI(4,5)P₂and CK2 modulates actin polymerization during endocytic uptake

A transient burst of actin polymerization assists endocytic budding. How actin polymerization is controlled in this context is not understood. Here, we show that crosstalk between PI(4,5)P₂and the CK2 catalytic subunit Cka2 controls actin polymerization at endocytic sites. We find that phosphorylation of the myosin-I Myo5 by Cka2 downregulates Myo5-induced Arp2/3-dependent actin polymerization, whereas PI(4,5)P₂cooperatively relieves Myo5 autoinhibition and inhibits the catalytic activity of Cka2. Cka2 and the PI(4,5)P₂-5-phosphatases Sjl1 and Sjl2, the yeast synaptojanins, exhibit genetic interactions indicating functional redundancy. The ultrastructural analysis of plasma membrane invaginations in CK2 and synaptojanin mutants demonstrates that both cooperate to initiate constriction of the invagination neck, a process coupled to the remodeling of the endocytic actin network. Our data demonstrate a holoenzyme-independent function of CK2 in endocytic budding and establish a robust genetic, functional, and molecular link between PI(4,5)P₂and CK2, two masters of intracellular signaling.

Modeling the dynamics of dendritic actin waves in living cells

The actin cytoskeleton in living cells exhibits a high degree of capacity for dynamic self-organization. Recent experiments have observed propagating actin waves in Dictyostelium cells recovering from complete depolymerization of their actin cytoskeleton. The propagation of these waves appear to be dependent on a programmed recruitment of a few proteins that control actin assembly and disassembly. Such waves also arise spontaneously along the plasma membrane of the cell, and it has been suggested that actin waves enable the cell to scan a surface for particles to engulf. Based on known molecular components involved in wave propagation, we present and study a minimal reaction-diffusion model for actin wave production observed in recovering cells.

Myosin 1 controls membrane shape by coupling F-Actin to membrane

Cellular functions are intimately associated with rapid changes in membrane shape. Different mechanisms interfering with the lipid bilayer, such as the insertion of proteins with amphipatic helices or the association of a protein scaffold, trigger membrane bending. By exerting force on membranes, molecular motors can also contribute to membrane remodeling. Previous studies have shown that actin and myosin 1 participate in the invagination of the plasma membrane during endocytosis while kinesins and dynein with microtubules provide the force to elongate membrane buds at recycling endosomes and at the trans-Golgi network (TGN). Using live cell imaging we have recently shown that a myosin 1 (myosin 1b) regulates the actin dependent post-Golgi traffic of cargo and generates force that controls the assembly of F-actin foci and promotes with the actin cytoskeleton the formation of tubules at the TGN. Our data provide evidence that actin and myosin 1 can regulate membrane remodeling of organelles as well as having an unexpected role in the spatial organization of the actin cytoskeleton. Here, we discuss our results together with the role of actin and other myosins that have been implicated in the traffic of cargo.

Lsb1 is a negative regulator of las17 dependent actin polymerization involved in endocytosis

The spatial and temporal regulation of actin polymerization is crucial for various cellular processes. Members of the Wiskott–Aldrich syndrome protein (WASP) family activate the Arp2/3-complex leading to actin polymerization. The yeast Saccharomyces cerevisiae contains only one WASP homolog, Las17, that requires additional factors for its regulation. Lsb1 and Lsb2/Pin3 are two yeast homologous proteins bearing an SH3 domain that were identified as Las17-binding proteins. Lsb2/Pin3 that promotes prion induction was suggested to link this prion formation to the actin cytoskeleton. However, the cellular role of Lsb1 and the molecular function of both Lsb1 and Lsb2 remain unknown. In this study, we show that Lsb1 and/or Lsb2 full-length proteins inhibit Las17-mediated actin polymerization in vitro, Lsb2 being a less potent inhibitor of Las17 activity compared to Lsb1. Addition of Lsb1 or Lsb2 to the corresponding full-length Lsb1/2 further inhibits Las17 activity. Lsb1 and Lsb2 form homo- and hetero-oligomeric complexes suggesting that these two proteins could regulate Las17 activity via dimerization or cooperative binding. In vivo, overexpressed Lsb1 and Lsb2 proteins cluster Las17-CFP in few cytoplasmic punctate structures that are also positive for other Arp2/3-dependent actin polymerization effectors like Sla1 or Abp1. But, only Lsb1 overexpression blocks the internalization step of receptor-mediated endocytosis. This shows a specific function of Lsb1 in endocytosis.

Ultrastructural dynamics of proteins involved in endocytic budding

Fluorescence live-cell imaging has temporally resolved the conserved choreography of more than 30 proteins involved in clathrin and actin-mediated endocytic budding from the plasma membrane. However, the resolution of these studies is insufficient to unveil how the endocytic machinery actually drives membrane deformation in vivo. In this study, we use quantitative immuno-EM to introduce the temporal dimension to the ultrastructural analysis of membrane budding and define changes in the topography of the lipid bilayer coupled to the dynamics of endocytic proteins with unprecedented spatiotemporal resolution. Using this approach, we frame the emergence of membrane curvature with respect to the recruitment of endocytic factors and show that constriction of the invaginations correlates with translocation of membrane-sculpting proteins. Furthermore, we show that initial bending of the plasma membrane is independent of actin and clathrin polymerization and precedes building of an actin cap branched by the Arp2/3 complex. Finally, our data indicate that constriction and additional elongation of the endocytic profiles require the mechanochemical activity of the myosins-I. Altogether, this work provides major insights into the molecular mechanisms driving membrane deformation in a cellular context.

Myosin 1E coordinates actin assembly and cargo trafficking during clathrin-mediated endocytosis

An actin-dependent role is shown for Myo1E in the trafficking of newly internalized cargo to early endosomes during CME. The results establish for mammalian cells, similar to budding yeast, interdependence in the recruitment of type I myosins, WIP/WIRE, and N-WASP to endocytic sites to assemble F-actin as endocytic vesicles are being formed.

Myosin 1E (Myo1E) is recruited to sites of clathrin-mediated endocytosis coincident with a burst of actin assembly. The recruitment dynamics and lifetime of Myo1E are similar to those of tagged actin polymerization regulatory proteins. Like inhibition of actin assembly, depletion of Myo1E causes reduced transferrin endocytosis and a significant delay in transferrin trafficking to perinuclear compartments, demonstrating an integral role for Myo1E in these actin-mediated steps. Mistargeting of GFP-Myo1E or its src-homology 3 domain to mitochondria results in appearance of WIP, WIRE, N-WASP, and actin filaments at the mitochondria, providing evidence for Myo1E's role in actin assembly regulation. These results suggest for mammalian cells, similar to budding yeast, interdependence in the recruitment of type I myosins, WIP/WIRE, and N-WASP to endocytic sites for Arp2/3 complex activation to assemble F-actin as endocytic vesicles are being formed.

Cell polarization and cytokinesis in budding yeast

Asymmetric cell division, which includes cell polarization and cytokinesis, is essential for generating cell diversity during development. The budding yeast Saccharomyces cerevisiae reproduces by asymmetric cell division, and has thus served as an attractive model for unraveling the general principles of eukaryotic cell polarization and cytokinesis. Polarity development requires G-protein signaling, cytoskeletal polarization, and exocytosis, whereas cytokinesis requires concerted actions of a contractile actomyosin ring and targeted membrane deposition. In this chapter, we discuss the mechanics and spatial control of polarity development and cytokinesis, emphasizing the key concepts, mechanisms, and emerging questions in the field.

Bridging membrane and cytoskeleton dynamics in the secretory and endocytic pathways

Transport carriers regulate membrane flow between compartments of the secretory and endocytic pathways in eukaryotic cells. Carrier biogenesis is assisted by microtubules, actin filaments and their associated motors that link to membrane-associated coats, adaptors and accessory proteins. We summarize here how the biochemical properties of membranes inform their interactions with cytoskeletal regulators. We also discuss how the forces generated by the cytoskeleton and motor proteins alter the biophysical properties and the shape of membranes. The interplay between the cytoskeleton and membrane proteins ensures tight spatial and temporal control of carrier biogenesis, which is essential for cellular homeostasis.

Evolutionary traces decode molecular mechanism behind fast pace of myosin XI

Background

Cytoplasmic class XI myosins are the fastest processive motors known. This class functions in high-velocity cytoplasmic streaming in various plant cells from algae to angiosperms. The velocities at which they process are ten times faster than its closest class V homologues.

Results

To provide sequence determinants and structural rationale for the molecular mechanism of this fast pace myosin, we have compared the sequences from myosin class V and XI through Evolutionary Trace (ET) analysis. The current study identifies class-specific residues of myosin XI spread over the actin binding site, ATP binding site and light chain binding neck region. Sequences for ET analysis were accumulated from six plant genomes, using literature based text search and sequence searches, followed by triple validation viz. CDD search, string-based searches and phylogenetic clustering. We have identified nine myosin XI genes in sorghum and seven in grape by sequence searches. Both the plants possess one gene product each belonging to myosin type VIII as well. During this process, we have re-defined the gene boundaries for three sorghum myosin XI genes using fgenesh program.

Conclusion

Molecular modelling and subsequent analysis of putative interactions involving these class-specific residues suggest a structural basis for the molecular mechanism behind high velocity of plant myosin XI. We propose a model of a more flexible switch I region that contributes to faster ADP release leading to high velocity movement of the algal myosin XI.

Function and regulation of Saccharomyces cerevisiae myosins-I in endocytic budding

Myosins-I are widely expressed actin-dependent motors which bear a phospholipid-binding domain. In addition, some members of the family can trigger Arp2/3 complex (actin-related protein 2/3 complex)-dependent actin polymerization. In the early 1990s, the development of powerful genetic tools in protozoa and mammals and discovery of these motors in yeast allowed the demonstration of their roles in membrane traffic along the endocytic and secretory pathways, in vacuole contraction, in cell motility and in mechanosensing. The powerful yeast genetics has contributed towards dissecting in detail the function and regulation of Saccharomyces cerevisiae myosins-I Myo3 and Myo5 in endocytic budding from the plasma membrane. In the present review, we summarize the evidence, dissecting their exact role in membrane budding and the molecular mechanisms controlling their recruitment and biochemical activities at the endocytic sites.

Clathrin light chain directs endocytosis by influencing the binding of the yeast Hip1R homologue, Sla2, to F-actin

The clathrin light-chain (LC) N-terminal region interacts with the Sla2/Hip1/Hip1R family of ANTH/talin–like proteins. In vivo evidence shows that LC–Sla2 binding is important for releasing Sla2 attachments to actin in the endocytic coat. Loss of this regulation can suppress major actin defects during endocytosis.

The role of clathrin light chain (CLC) in clathrin-mediated endocytosis is not completely understood. Previous studies showed that the CLC N-terminus (CLC-NT) binds the Hip1/Hip1R/Sla2 family of membrane/actin–binding factors and that overexpression of the CLC-NT in yeast suppresses endocytic defects of clathrin heavy-chain mutants. To elucidate the mechanistic basis for this suppression, we performed synthetic genetic array analysis with a clathrin CLC-NT deletion mutation (clc1-Δ19-76). clc1-Δ19-76 suppressed the internalization defects of null mutations in three late endocytic factors: amphiphysins (rvs161 and rvs167) and verprolin (vrp1). In actin sedimentation assays, CLC binding to Sla2 inhibited Sla2 interaction with F-actin. Furthermore, clc1-Δ19-76 suppression of the rvs and vrp phenotypes required the Sla2 actin-binding talin-Hip1/R/Sla2 actin-tethering C-terminal homology domain, suggesting that clc1-Δ19-76 promotes internalization by prolonging actin engagement by Sla2. We propose that CLC directs endocytic progression by pruning the Sla2-actin attachments in the clathrin lattice, providing direction for membrane internalization.

Principles of unconventional myosin function and targeting

Unconventional myosins are a superfamily of actin-based motors implicated in diverse cellular processes. In recent years, much progress has been made in describing their biophysical properties, and headway has been made into analyzing their cellular functions. Here, we focus on the principles that guide in vivo motor function and targeting to specific cellular locations. Rather than describe each motor comprehensively, we outline the major themes that emerge from research across the superfamily and use specific examples to illustrate each. In presenting the data in this format, we seek to identify open questions in each field as well as to point out commonalities between them. To advance our understanding of myosins' roles in vivo, clearly we must identify their cellular cargoes and the protein complexes that regulate motor attachment to fully appreciate their functions on the cellular and developmental levels.

Reconstitution and protein composition analysis of endocytic actin patches

BACKGROUND

Clathrin-actin-mediated endocytosis in yeast involves the progressive assembly of at least 60 different proteins at cortical sites. More than half of these proteins are involved in the assembly of a branched network of actin filaments to provide the forces required for plasma membrane invagination.

RESULTS

To gain insights into the regulation of endocytic actin patch dynamics, we developed an in vitro actin assembly assay using microbeads functionalized with the nucleation promoting factor (NPF) Las17 (yeast WASP). When incubated in a yeast extract, these beads assembled actin networks, and a significant fraction became motile. Multidimensional protein identification technology (MudPIT) showed that the recruitment of actin-binding proteins to these Las17-derived actin networks is selective. None of the proteins known to exclusively regulate the in vivo formation of actin cables or the actin contractile ring were identified. Our analysis also identified components of three other cortical structures, eisosomes, phosphoinositide kinase (PIK) patches, and the TORC2 complex, establishing intriguing biochemical connections between four different yeast cortical complexes. Finally, we identified Aim3 as a regulator of actin dynamics at endocytic sites.

CONCLUSIONS

WASP is sufficient to trigger assembly of actin networks composed selectively of actin patch proteins. These experiments establish that the protein composition of different F-actin structures is determined by the protein factor that initiates the network. The identification of binding partners revealed new biochemical connections between WASP-derived networks and other cortical complexes and identified Aim3 as a novel regulator of the endocytic actin patch.

Vrp1p-Las17p interaction is critical for actin patch polarization but is not essential for growth or fluid phase endocytosis in S. cerevisiae

Vrp1p (yeast WIP) forms a protein complex with Las17p (yeast WASP), however the physiological significance of the interaction has not been fully characterized. Vrp1p residues, (788)MPKPR(792) are essential for Vrp1p-Las17p interaction. While C-Vrp1p(364-817) complements all the defects of the vrp1Δ strain, C-Vrp1p(364-817)(5A) ((788)AAAAA(792)) does not complement any of the defects, due to its inability to localize to cortical patches. Targeting C-Vrp1p(364-817)(5A) to membranes using CAAX motif (C-Vrp1p(364-817)(5A)-CAAX) rescued the growth and endocytosis defect but not the actin patch polarization defect of vrp1Δ. Vrp1p can localize to cortical patches, either by binding to Las17p through LBD (Las17 Binding Domain, Vrp1p(760-817)) or independent of Las17p through residues in N-Vrp1p(1-364). Unlike Vrp1p, Vrp1p(5A) localizes poorly to cortical patches and complements all the defects of vrp1Δ strain except actin patch polarization at elevated temperature. N-Vrp1p(1-364) complements all the defects of vrp1Δ strain except the actin patch polarization defect while N-Vrp1p(1-364)-LBD fusion protein complements all the defects. Thus our results show that while both Vrp1p and Las17p are essential for many cellular processes, the two proteins do not necessarily have to bind to each other to carry out these cellular functions. However, Las17p-Vrp1p interaction is essential for actin patch polarization at elevated temperature.

Calmodulin dissociation regulates Myo5 recruitment and function at endocytic sites

Myosins-I are conserved proteins that bear an N-terminal motor head followed by a Tail Homology 1 (TH1) lipid-binding domain. Some myosins-I have an additional C-terminal extension (C(ext)) that promotes Arp2/3 complex-dependent actin polymerization. The head and the tail are separated by a neck that binds calmodulin or calmodulin-related light chains. Myosins-I are known to participate in actin-dependent membrane remodelling. However, the molecular mechanisms controlling their recruitment and their biochemical activities in vivo are far from being understood. In this study, we provided evidence suggesting the existence of an inhibitory interaction between the TH1 domain of the yeast myosin-I Myo5 and its C(ext). The TH1 domain prevented binding of the Myo5 C(ext) to the yeast WIP homologue Vrp1, Myo5 C(ext)-induced actin polymerization and recruitment of the Myo5 C(ext) to endocytic sites. Our data also indicated that calmodulin dissociation from Myo5 weakened the interaction between the neck and TH1 domains and the C(ext). Concomitantly, calmodulin dissociation triggered Myo5 binding to Vrp1, extended the myosin-I lifespan at endocytic sites and activated Myo5-induced actin polymerization.

Contributions of Wiskott-Aldrich syndrome family cytoskeletal regulatory adapters to immune regulation

Cytoskeletal structure and dynamic rearrangement are integrally involved in coupling external stimuli to the orchestrated network of molecular interactions and cellular responses required for T-cell effector function. Members of the Wiskott-Aldrich syndrome protein (WASp) family are now widely recognized as cytoskeletal scaffolding adapters that coordinate the transmission of stimulatory signals to downstream induction of actin remodeling and cytoskeletal-dependent T-cell responses. In this review, we discuss the structural and functional properties of the WASp family members, with an emphasis on the roles of these proteins in the molecular pathways underpinning T-cell activation. The contributions of WASp family proteins and the cytoskeletal reorganization they evoke to expression of specific T-cell effector functions and the implications of such activity to normal immune responses and to the immunologic deficits manifested by Wiskott-Aldrich syndrome patients are also described.

Functions of actin in endocytosis

Endocytosis is a fundamental eukaryotic process required for remodelling plasma-membrane lipids and protein to ensure appropriate membrane composition. Increasing evidence from a number of cell types reveals that actin plays an active, and often essential, role at key endocytic stages. Much of our current mechanistic understanding of the endocytic process has come from studies in budding yeast and has been facilitated by yeast's genetic amenability and by technological advances in live cell imaging. While endocytosis in metazoans is likely to be subject to a greater array of regulatory signals, recent reports indicate that spatiotemporal aspects of vesicle formation requiring actin are likely to be conserved across eukaryotic evolution. In this review we focus on the 'modular' model of endocytosis in yeast before highlighting comparisons with other cell types. Our discussion is limited to endocytosis involving clathrin as other types of endocytosis have not been demonstrated in yeast.

Las17p-Vrp1p but not Las17p-Arp2/3 interaction is important for actin patch polarization in yeast

The actin cytoskeleton plays a central role in many important cellular processes such as cell polarization, cell division and endocytosis. The dynamic changes to the actin cytoskeleton that accompany these processes are regulated by actin-associated proteins Wiskott-Aldrich Syndrome Protein (WASP) (known as Las17p in yeast) and WASP-Interacting Protein (WIP) (known as Vrp1p in yeast). Both yeast and human WASP bind to and stimulate the Arp2/3 complex which in turn nucleates assembly of actin monomers into filaments at polarized sites at the cortex. WASP-WIP interaction in yeast and humans are important for Arp2/3 complex stimulation in vitro. It has been proposed that these interactions are also important for polarized actin assembly in vivo. However, the redundancy of actin-associated proteins has made it difficult to test this hypothesis. We have identified two point mutations (L80T and H94L) in yeast WASP that in combination abolish WASP-WIP interaction in yeast. We also identify an N-terminal fragment of Las17p (N-Las17p1-368) able to interact with Vrp1p but not Arp2/3. Using these mutant and truncated forms of yeast WASP we provide novel evidence that WASP interaction with WIP is more important than interaction with Arp2/3 for polarized actin assembly and endocytosis in yeast.

Actin and endocytosis: mechanisms and phylogeny

The regulated assembly of actin filament networks is a crucial part of endocytosis, with crucial temporal and spatial relationships between proteins of the endocytic and actin assembly machinery. Of particular importance has been a wealth of studies in budding and fission yeast. Cell biology approaches, combined with molecular genetics, have begun to uncover the complexity of the regulation of actin dynamics during the endocytic process. In a wide range of organisms, clathrin-mediated endocytosis appears to be linked to Arp2/3-mediated actin assembly. The conservation of the components, across a wide range eukaryotic species, suggests that the partnership between endocytosis and actin may be evolutionarily ancient.

Actin in the endocytic pathway: from yeast to mammals

Genetic analysis of endocytosis in yeast early pointed to the essential role of actin in the uptake step. Efforts to identify the machinery involved demonstrated the important contribution of Arp2/3 and the myosins-I. Analysis of the process using live-cell fluorescence microscopy and electron microscopy have recently contributed to refine molecular models explaining clathrin and actin-dependent endocytic uptake. Increasing evidence now also indicates that actin plays important roles in post-internalization events along the endocytic pathway in yeast, including transport of vesicles, motility of endosomes and vacuole fusion. This review describes the present knowledge state on the roles of actin in endocytosis in yeast and points to similarities and differences with analogous processes in mammals.

Distinct acto/myosin-I structures associate with endocytic profiles at the plasma membrane

Endocytosis in yeast requires actin and clathrin. Live cell imaging has previously shown that massive actin polymerization occurs concomitant with a slow 200-nm inward movement of the endocytic coat (Kaksonen, M., Y. Sun, and D.G. Drubin. 2003. Cell. 115:475-487). However, the nature of the primary endocytic profile in yeast and how clathrin and actin cooperate to generate an endocytic vesicle is unknown. In this study, we analyze the distribution of nine different proteins involved in endocytic uptake along plasma membrane invaginations using immunoelectron microscopy. We find that the primary endocytic profiles are tubular invaginations of up to 50 nm in diameter and 180 nm in length, which accumulate the endocytic coat components at the tip. Interestingly, significant actin labeling is only observed on invaginations longer than 50 nm, suggesting that initial membrane bending occurs before initiation of the slow inward movement. We also find that in the longest profiles, actin and the myosin-I Myo5p form two distinct structures that might be implicated in vesicle fission.

Distinct roles for Arp2/3 regulators in actin assembly and endocytosis

The Arp2/3 complex is essential for actin assembly and motility in many cell processes, and a large number of proteins have been found to bind and regulate it in vitro. A critical challenge is to understand the actions of these proteins in cells, especially in settings where multiple regulators are present. In a systematic study of the sequential multicomponent actin assembly processes that accompany endocytosis in yeast, we examined and compared the roles of WASp, two type-I myosins, and two other Arp2/3 activators, along with that of coronin, which is a proposed inhibitor of Arp2/3. Quantitative analysis of high-speed fluorescence imaging revealed individual functions for the regulators, manifested in part by novel phenotypes. We conclude that Arp2/3 regulators have distinct and overlapping roles in the processes of actin assembly that drive endocytosis in yeast. The formation of the endocytic actin patch, the creation of the endocytic vesicle, and the movement of the vesicle into the cytoplasm display distinct dependencies on different Arp2/3 regulators. Knowledge of these roles provides insight into the in vivo relevance of the dendritic nucleation model for actin assembly.

A branched network of growing actin filaments, pushing against a membrane, provides the force for certain cellular movements. The Arp2/3 complex plays a central role in this process by generating new filaments and branch points. A number of proteins bind to and, in some cases, regulate Arp2/3. It is important to determine, in the cell, the precise roles of each of the many Arp2/3 regulators in generating actin networks during a complex, multistep, cellular movement. In yeast, endocytosis occurs at the plasma membrane in association with the assembly and movement of cortical actin patches, which contain six Arp2/3 regulators. We have used the actin patch as a model system to determine the specific roles of these regulators during patch assembly and movement. We used high-speed video microscopy, coupled with computer-aided particle tracking, to monitor the movement of fluorescently labeled actin patches in cells with one or more mutations of the Arp2/3 regulators. The sensitivity of this technique allowed us to identify previously unappreciated functions for Arp2/3 regulators and to assign each of the regulators a specific role during actin patch assembly and movement. Our results demonstrate that Arp2/3 regulatory proteins play overlapping roles at certain stages of actin patch movement, but distinct roles at other stages. In addition, our results provide new insight into how the assembly of an actin filament networks powers the movement of endocytic vesicles away from the membrane.

Branched networks of actin filaments, nucleated by the Arp2/3 complex, power many cellular movements. Quantitative analysis of actin patch motility in budding yeast reveals distinct and overlapping roles for Arp2/3 regulators in endocytosis.

Proteins involved in sterol synthesis interact with Ste20 and regulate cell polarity

The Saccharomyces cerevisiae p21-activated kinase (PAK) Ste20 regulates various aspects of cell polarity during vegetative growth, mating and filamentous growth. To gain further insight into the mechanisms of Ste20 action, we screened for interactors of Ste20 using the split-ubiquitin system. Among the identified proteins were Erg4, Cbr1 and Ncp1, which are all involved in sterol biosynthesis. The interaction between Ste20 and Erg4, as well as between Ste20 and Cbr1, was confirmed by pull-down experiments. Deletion of either ERG4 or NCP1 resulted in various polarity defects, indicating a role for these proteins in bud site selection, apical bud growth, cell wall assembly, mating and invasive growth. Interestingly, Erg4 was required for the polarized localization of Ste20 during mating. Lack of CBR1 produced no detectable phenotype, whereas the deletion of CBR1 in the absence of NCP1 was lethal. Using a conditional lethal mutant we demonstrate that both proteins have overlapping functions in bud morphology.

Verprolin function in endocytosis and actin organization

Vrp1p (verprolin, End5p) is the yeast ortholog of human Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP). Vrp1p localizes to the cortical actin cytoskeleton, is necessary for its polarization to sites of growth and is also essential for endocytosis. At elevated temperature, Vrp1p becomes essential for growth. A C-terminal Vrp1p fragment (C-Vrp1p) retains the ability to localize to the cortical actin cytoskeleton and function in actin-cytoskeleton polarization, endocytosis and growth. Here, we demonstrate that two submodules in C-Vrp1p are required for actin-cytoskeleton polarization: a novel C-terminal actin-binding submodule (CABS) that contains a novel G-actin-binding domain, which we call a verprolin homology 2 C-terminal (VH2-C) domain; and a second submodule comprising the Las17p-binding domain (LBD) that binds Las17p (yeast WASP). The LBD localizes C-Vrp1p to membranes and the cortical actin cytoskeleton. Intriguingly, the LBD is sufficient to restore endocytosis and growth at elevated temperature to Vrp1p-deficient cells. The CABS also restores these functions, but only if modified by a lipid anchor to provide membrane association. Our findings highlight the role of Las17p binding for Vrp1p membrane association, suggest general membrane association may be more important than specific targeting to the cortical actin cytoskeleton for Vrp1p function in endocytosis and cell growth, and suggest that Vrp1p binding to individual effectors may alter their physiological activity.

Interaction of the endocytic scaffold protein Pan1 with the type I myosins contributes to the late stages of endocytosis

The yeast endocytic scaffold Pan1 contains an uncharacterized proline-rich domain (PRD) at its carboxy (C)-terminus. We report that the pan1-20 temperature-sensitive allele has a disrupted PRD due to a frame-shift mutation in the open reading frame of the domain. To reveal redundantly masked functions of the PRD, synthetic genetic array screens with a pan1DeltaPRD strain found genetic interactions with alleles of ACT1, LAS17 and a deletion of SLA1. Through a yeast two-hybrid screen, the Src homology 3 domains of the type I myosins, Myo3 and Myo5, were identified as binding partners for the C-terminus of Pan1. In vitro and in vivo assays validated this interaction. The relative timing of recruitment of Pan1-green fluorescent protein (GFP) and Myo3/5-red fluorescent protein (RFP) at nascent endocytic sites was revealed by two-color real-time fluorescence microscopy; the type I myosins join Pan1 at cortical patches at a late stage of internalization, preceding the inward movement of Pan1 and its disassembly. In cells lacking the Pan1 PRD, we observed an increased lifetime of Myo5-GFP at the cortex. Finally, Pan1 PRD enhanced the actin polymerization activity of Myo5-Vrp1 complexes in vitro. We propose that Pan1 and the type I myosins interactions promote an actin activity important at a late stage in endocytic internalization.

Actin binding and proline rich motifs of CR16 play redundant role in growth of vrp1Delta cells

CR16, (Glucocorticoid-regulated) belongs to the verprolin family of proteins which are characterized by the presence of a V domain (verprolin) at the N-terminal. Expression of CR16 suppressed the growth and endocytosis defect of vrp1Delta strain without correcting the actin patch polarization defect. The V domain of CR16 is critical for suppression of the growth defect of vrp1Delta strain but not for localisation to cortical actin patches. Mutations in the actin binding motif alone did not abolish the activity of CR16 but the mutations in combination with deletion of N-terminal proline rich motif abolished the ability of CR16 to suppress the growth defect. This suggests that the V domain of CR16 has two functionally redundant motifs and either one of these motifs is sufficient for suppressing the growth defect of vrp1Delta strain. This is in contrast to the observation that both WIP and WIRE require the actin binding motif for their activity.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.