The yeast kinesin-related protein Smy1p exerts its effects on the class V myosin Myo2p via a physical interaction

The microtubular preprophase band recruits Myosin XI to the cortical division site to guide phragmoplast expansion during plant cytokinesis

In plant vegetative tissues, cell division employs a mitotic microtubule array called the preprophase band (PPB) that marks the cortical division site. This transient cytoskeletal array imprints the spatial information to be read by the cytokinetic phragmoplast at later stages of mitotic cell division. In Arabidopsis thaliana, we discovered that the PPB recruited the Myosin XI motor MYA1/Myo11F to the cortical division site, where it joined microtubule-associated proteins and motors to form a ring of prominent cytoskeletal assemblies that received the expanding phragmoplast. Such a myosin localization pattern at the cortical division site was dependent on the POK1/2 Kinesin-12 motors. This regulatory function of MYA1/Myo11F in phragmoplast guidance was dependent on intact actin filaments. The discovery of these cytoskeletal motor assemblies pinpoints a mechanism underlying how two dynamic cytoskeletal networks work in concert to govern PPB-dependent division plane orientation in flowering plants.

Cargo Recognition Mechanisms of Yeast Myo2 Revealed by AlphaFold2-Powered Protein Complex Prediction

Myo2, a yeast class V myosin, transports a broad range of organelles and plays important roles in various cellular processes, including cell division in budding yeast. Despite the fact that several structures of Myo2/cargo adaptor complexes have been determined, the understanding of the versatile cargo-binding modes of Myo2 is still very limited, given the large number of cargo adaptors identified for Myo2. Here, we used ColabFold, an AlphaFold2-powered and easy-to-use tool, to predict the complex structures of Myo2-GTD and its several cargo adaptors. After benchmarking the prediction strategy with three Myo2/cargo adaptor complexes that have been determined previously, we successfully predicted the atomic structures of Myo2-GTD in complex with another three cargo adaptors, Vac17, Kar9 and Pea2, which were confirmed by our biochemical characterizations. By systematically comparing the interaction details of the six complexes of Myo2 and its cargo adaptors, we summarized the cargo-binding modes on the three conserved sites of Myo2-GTD, providing an overall picture of the versatile cargo-recognition mechanisms of Myo2. In addition, our study demonstrates an efficient and effective solution to study protein-protein interactions in the future via the AlphaFold2-powered prediction.

Viscoelasticity, Like Forces, Plays a Role in Mechanotransduction

Viscoelasticity and its alteration in time and space has turned out to act as a key element in fundamental biological processes in living systems, such as morphogenesis and motility. Based on experimental and theoretical findings it can be proposed that viscoelasticity of cells, spheroids and tissues seems to be a collective characteristic that demands macromolecular, intracellular component and intercellular interactions. A major challenge is to couple the alterations in the macroscopic structural or material characteristics of cells, spheroids and tissues, such as cell and tissue phase transitions, to the microscopic interferences of their elements. Therefore, the biophysical technologies need to be improved, advanced and connected to classical biological assays. In this review, the viscoelastic nature of cytoskeletal, extracellular and cellular networks is presented and discussed. Viscoelasticity is conceptualized as a major contributor to cell migration and invasion and it is discussed whether it can serve as a biomarker for the cells' migratory capacity in several biological contexts. It can be hypothesized that the statistical mechanics of intra- and extracellular networks may be applied in the future as a powerful tool to explore quantitatively the biomechanical foundation of viscoelasticity over a broad range of time and length scales. Finally, the importance of the cellular viscoelasticity is illustrated in identifying and characterizing multiple disorders, such as cancer, tissue injuries, acute or chronic inflammations or fibrotic diseases.

Type V myosin focuses the polarisome and shapes the tip of yeast cells

The polarisome is a cortical proteinaceous microcompartment that organizes the growth of actin filaments and the fusion of secretory vesicles in yeasts and filamentous fungi. Polarisomes are compact, spotlike structures at the growing tips of their respective cells. The molecular forces that control the form and size of this microcompartment are not known. Here we identify a complex between the polarisome subunit Pea2 and the type V Myosin Myo2 that anchors Myo2 at the cortex of yeast cells. We discovered a point mutation in the cargo-binding domain of Myo2 that impairs the interaction with Pea2 and consequently the formation and focused localization of the polarisome. Cells carrying this mutation grow round instead of elongated buds. Further experiments and biophysical modeling suggest that the interactions between polarisome-bound Myo2 motors and dynamic actin filaments spatially focus the polarisome and sustain its compact shape.

Yeast Rgd3 is a phospho-regulated F-BAR-containing RhoGAP involved in the regulation of Rho3 distribution and cell morphology

Polarized growth requires the integration of polarity pathways with the delivery of exocytic vesicles for cell expansion and counterbalancing endocytic uptake. In budding yeast, the myosin-V Myo2 is aided by the kinesin-related protein Smy1 in carrying out the essential Sec4-dependent transport of secretory vesicles to sites of polarized growth. Overexpression suppressors of a conditional myo2 smy1 mutant identified a novel F-BAR (Fes/CIP4 homology-Bin-Amphiphysin-Rvs protein)-containing RhoGAP, Rgd3, that has activity primarily on Rho3, but also Cdc42. Internally tagged Rho3 is restricted to the plasma membrane in a gradient corresponding to cell polarity that is altered upon Rgd3 overexpression. Rgd3 itself is localized to dynamic polarized vesicles that, while distinct from constitutive secretory vesicles, are dependent on actin and Myo2 function. In vitro Rgd3 associates with liposomes in a PIP₂-enhanced manner. Further, the Rgd3 C-terminal region contains several phosphorylatable residues within a reported SH3-binding motif. An unphosphorylated mimetic construct is active and highly polarized, while the phospho-mimetic form is not. Rgd3 is capable of activating Myo2, dependent on its phospho state, and Rgd3 overexpression rescues aberrant Rho3 localization and cell morphologies seen at the restrictive temperature in the myo2 smy1 mutant. We propose a model where Rgd3 functions to modulate and maintain Rho3 polarity during growth.

The class V myosin interactome of the human pathogen Aspergillus fumigatus reveals novel interactions with COPII vesicle transport proteins

The human fungal pathogen Aspergillus fumigatus causes life-threatening invasive aspergillosis in immunocompromised individuals. Adaptation to the host environment is integral to survival of A. fumigatus and requires the coordination of short- and long-distance vesicular transport to move essential components throughout the fungus. We previously reported the importance of MyoE, the only class V myosin, for hyphal growth and virulence of A. fumigatus. Class V myosins are actin-based, cargo-carrying motor proteins that contain unique binding sites for specific cargo. Specific cargo carried by myosin V has not been identified in any fungus, and previous studies have only identified single components that interact with class V myosins. Here we utilized a mass spectrometry-based whole proteomic approach to identify MyoE interacting proteins in A. fumigatus for the first time. Several proteins previously shown to interact with myosin V through physical and genetic approaches were confirmed, validating our proteomic analysis. Importantly, we identified novel MyoE-interacting proteins, including members of the cytoskeleton network, cell wall synthesis, calcium signaling and a group of coat protein complex II (COPII) proteins involved in the endoplasmic reticulum (ER) to Golgi transport. Furthermore, we analyzed the localization patterns of the COPII proteins, UsoA (Uso1), SrgE (Sec31), and SrgF (Sec23), which suggested a potential role for MyoE in ER to Golgi trafficking.

Mechanotransduction: from the cell surface to the nucleus via RhoA

Cells respond and adapt to their physical environments and to the mechanical forces that they experience. The translation of physical forces into biochemical signalling pathways is known as mechanotransduction. In this review, we focus on two aspects of mechanotransduction. First, we consider how forces exerted on cell adhesion molecules at the cell surface regulate the RhoA signalling pathway by controlling the activities of guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). In the second part of the review, we discuss how the nucleus contributes to mechanotransduction as a physical structure connected to the cytoskeleton. We focus on recent studies that have either severed the connections between the nucleus and the cytoskeleton, or that have entirely removed the nucleus from cells. These actions reduce the levels of active RhoA, thereby altering the mechanical properties of cells and decreasing their ability to generate tension and respond to external mechanical forces.

This article is part of a discussion meeting issue ‘Forces in cancer: interdisciplinary approaches in tumour mechanobiology’.

Structural mechanism for versatile cargo recognition by the yeast class V myosin Myo2

Class V myosins are actin-dependent motors, which recognize numerous cellular cargos mainly via the C-terminal globular tail domain (GTD). Myo2, a yeast class V myosin, can transport a broad range of organelles. However, little is known about the capacity of Myo2-GTD to recognize such a diverse array of cargos specifically at the molecular level. Here, we solved crystal structures of Myo2-GTD (at 1.9-3.1 Å resolutions) in complex with three cargo adaptor proteins: Smy1 (for polarization of secretory vesicles), Inp2 (for peroxisome transport), and Mmr1 (for mitochondria transport). The structures of Smy1- and Inp2-bound Myo2-GTD, along with site-directed mutagenesis experiments, revealed a binding site in subdomain-I having a hydrophobic groove with high flexibility enabling Myo2-GTD to accommodate different protein sequences. The Myo2-GTD-Mmr1 complex structure confirmed and complemented a previously identified mitochondrion/vacuole-specific binding region. Moreover, differences between the conformations and locations of cargo-binding sites identified here for Myo2 and those reported for mammalian MyoVA (MyoVA) suggest that class V myosins potentially have co-evolved with their specific cargos. Our structural and biochemical analysis not only uncovers a molecular mechanism that explains the diverse cargo recognition by Myo2-GTD, but also provides structural information useful for future functional studies of class V myosins in cargo transport.

A "Driver Switchover" Mechanism of Influenza Virus Transport from Microfilaments to Microtubules

When infecting host cells, influenza virus must move on microfilaments (MFs) at the cell periphery and then move along microtubules (MTs) through the cytosol to reach the perinuclear region for genome release. But how viruses switch from the actin roadway to the microtubule highway remains obscure. To settle this issue, we systematically dissected the role of related motor proteins in the transport of influenza virus between cytoskeletal filaments in situ and in real-time using quantum dot (QD)-based single-virus tracking (SVT) and multicolor imaging. We found that the switch between MF- and MT-based retrograde motor proteins, myosin VI (myoVI) and dynein, was responsible for the seamless transport of viruses from MFs to MTs during their infection. After virus entry by endocytosis, both the two types of motor proteins are attached to virus-carrying vesicles. MyoVI drives the viruses on MFs with dynein on the virus-carrying vesicle hitchhiking. After role exchanges at actin-microtubule intersections, dynein drives the virus along MTs toward the perinuclear region with myoVI remaining on the vesicle moving together. Such a "driver switchover" mechanism has answered the long-pending question of how viruses switch from MFs to MTs for their infection. It will also facilitate in-depth understanding of endocytosis.

Kinesin-related Smy1 enhances the Rab-dependent association of myosin-V with secretory cargo

Smy1 is a kinesin-related protein that enhances the association of the Myo2 myosin-V motor with its receptor, the Rab Sec4, on secretory vesicles. This function requires Smy1’s head, coiled-coil, and tail domains and is specific for secretory vesicle transport but not for mitochondrial segregation by Myo2, which also uses a Rab protein, Ypt11.

The mechanisms by which molecular motors associate with specific cargo is a central problem in cell organization. The kinesin-like protein Smy1 of budding yeast was originally identified by the ability of elevated levels to suppress a conditional myosin-V mutation (myo2-66), but its function with Myo2 remained mysterious. Subsequently, Myo2 was found to provide an essential role in delivery of secretory vesicles for polarized growth and in the transport of mitochondria for segregation. By isolating and characterizing myo2 smy1 conditional mutants, we uncover the molecular function of Smy1 as a factor that enhances the association of Myo2 with its receptor, the Rab Sec4, on secretory vesicles. The tail of Smy1—which binds Myo2—its central dimerization domain, and its kinesin-like head domain are all necessary for this function. Consistent with this model, overexpression of full-length Smy1 enhances the number of Sec4 receptors and Myo2 motors per transporting secretory vesicle. Rab proteins Sec4 and Ypt11, receptors for essential transport of secretory vesicles and mitochondria, respectively, bind the same region on Myo2, yet Smy1 functions selectively in the transport of secretory vesicles. Thus a kinesin-related protein can function intimately with a myosin-V and its receptor in the transport of a specific cargo.

Myosins FaMyo2B and Famyo2 Affect Asexual and Sexual Development, Reduces Pathogenicity, and FaMyo2B Acts Jointly with the Myosin Passenger Protein FaSmy1 to Affect Resistance to Phenamacril in Fusarium asiaticum

We previously reported that mutations occurred in the gene myosin5 were responsible for resistance to the fungicide phenamacril in Fusarium graminearum. Here, we determined whether there is a functional link between phenamacril resistance and the myosin proteins FaMyo2B and Famyo2 in Fusarium asiaticum, which is the major causal agent of Fusarium head blight in China. We found that FaMyo2B acts jointly with FaSmy1 to affect resistance to phenamacril in F. asiaticum. We also found that FaMyo2B disruption mutant and Famyo2 deletion mutant were defective in hyphal branching, conidiation, and sexual reproduction. ΔFamyo2 also had an enhanced sensitivity to cell wall damaging agents and an abnormal distribution of septa and nuclei. In addition, the FaMyo2B and Famyo2 mutants had reduced pathogenicity on wheat coleoptiles and flowering wheat heads. Taken together, these results reveal that FaMyo2B and Famyo2 are required for several F. asiaticum developmental processes and activities, which help us better understand the resistance mechanism and find the most effective approach to control FHB.

Common formin-regulating sequences in Smy1 and Bud14 are required for the control of actin cable assembly in vivo

Formins comprise a large family of proteins with diverse roles in remodeling the actin cytoskeleton. However, the spatiotemporal mechanisms used by cells to control formin activities are only beginning to be understood. Here we dissected Smy1, which has dual roles in regulating formins and myosin. Using mutagenesis, we identified specific sequences in Smy1 critical for its in vitro inhibitory effects on the FH2 domain of the formin Bnr1. By integrating smy1 alleles targeting those sequences, we genetically uncoupled Smy1's functions in regulating formins and myosin. Quantitative imaging analysis further demonstrated that the ability of Smy1 to directly control Bnr1 activity is crucial in vivo for proper actin cable length, shape, and velocity and, in turn, efficient secretory vesicle transport. A Smy1-like sequence motif was also identified in a different Bnr1 regulator, Bud14, and found to be essential for Bud14 functions in regulating actin cable architecture and function in vivo. Together these observations reveal unanticipated mechanistic ties between two distinct formin regulators. Further, they emphasize the importance of tightly controlling formin activities in vivo to generate specialized geometries and dynamics of actin structures tailored to their physiological roles.

Systematic analysis of dimeric E3-RING interactions reveals increased combinatorial complexity in human ubiquitination networks

Ubiquitination controls the stability or function of many human proteins, thereby regulating a wide range of physiological processes. In most cases the combinatorial pattern of protein interactions that facilitate substrate recognition or modification remain unclear. Moreover, the efficiency of ubiquitination reactions can be altered by the formation of homo- and heterotypic E3-RING complexes. To establish the prevalence and nature of binary E3-RING/E3-RING interactions systematic yeast two-hybrid screens were performed to test 7269 potential interactions between 124 human E3-RING proteins. These studies identified 228 dimeric interactions between 100 E3-RINGs, of which 205 were novel. Complementary co-immunoprecipitation studies were performed to test predicted network interactions, showing a high correlation (64%) with primary yeast two-hybrid data. This data was integrated with known E3-RING interactions, tissue expression profiles and proteomic ubiquitination datasets to facilitate identification of subnetworks in which E3-RING dimerization events have the potential to alter network structure. These results reveal a widespread yet selective pattern of E3-RING dimerization events, which have the potential to confer further combinatorial complexity within human ubiquitination cascades.

Unconventional functions of microtubule motors

With the functional characterization of proteins advancing at fast pace, the notion that one protein performs different functions - often with no relation to each other - emerges as a novel principle of how cells work. Molecular motors are no exception to this new development. Here, we provide an account on recent findings revealing that microtubule motors are multifunctional proteins that regulate many cellular processes, in addition to their main function in transport. Some of these functions rely on their motor activity, but others are independent of it. Of the first category, we focus on the role of microtubule motors in organelle biogenesis, and in the remodeling of the cytoskeleton, especially through the regulation of microtubule dynamics. Of the second category, we discuss the function of microtubule motors as static anchors of the cargo at the destination, and their participation in regulating signaling cascades by modulating interactions between signaling proteins, including transcription factors. We also review atypical forms of transport, such as the cytoplasmic streaming in the oocyte, and the movement of cargo by microtubule fluctuations. Our goal is to provide an overview of these unexpected functions of microtubule motors, and to incite future research in this expanding field.

Phylogenetic analysis of the Kinesin superfamily from physcomitrella

Kinesins are an ancient superfamily of microtubule dependent motors. They participate in an extensive and diverse list of essential cellular functions, including mitosis, cytokinesis, cell polarization, cell elongation, flagellar development, and intracellular transport. Based on phylogenetic relationships, the kinesin superfamily has been subdivided into 14 families, which are represented in most eukaryotic phyla. The functions of these families are sometimes conserved between species, but important variations in function across species have been observed. Plants possess most kinesin families including a few plant specific families. With the availability of an ever increasing number of genome sequences from plants, it is important to document the complete complement of kinesins present in a given organism. This will help develop a molecular framework to explore the function of each family using genetics, biochemistry, and cell biology. The moss Physcomitrella patens has emerged as a powerful model organism to study gene function in plants, which makes it a key candidate to explore complex gene families, such as the kinesin superfamily. Here we report a detailed phylogenetic characterization of the 71 kinesins of the kinesin superfamily in Physcomitrella. We found a remarkable conservation of families and subfamily classes with Arabidopsis, which is important for future comparative analysis of function. Some of the families, such as kinesins 14s are composed of fewer members in moss, while other families, such as the kinesin 12s are greatly expanded. To improve the comparison between species, and to simplify communication between research groups, we propose a classification of subfamilies based on our phylogenetic analysis.

The myosin passenger protein Smy1 controls actin cable structure and dynamics by acting as a formin damper

Formins are a conserved family of proteins with robust effects in promoting actin nucleation and elongation. However, the mechanisms restraining formin activities in cells to generate actin networks with particular dynamics and architectures are not well understood. In S. cerevisiae, formins assemble actin cables, which serve as tracks for myosin-dependent intracellular transport. Here, we show that the kinesin-like myosin passenger-protein Smy1 interacts with the FH2 domain of the formin Bnr1 to decrease rates of actin filament elongation, which is distinct from the formin displacement activity of Bud14. In vivo analysis of smy1Δ mutants demonstrates that this "damper" mechanism is critical for maintaining proper actin cable architecture, dynamics, and function. We directly observe Smy1-3GFP being transported by myosin V and transiently pausing at the neck in a manner dependent on Bnr1. These observations suggest that Smy1 is part of a negative feedback mechanism that detects cable length and prevents overgrowth.

PI4P and Rab inputs collaborate in myosin-V-dependent transport of secretory compartments in yeast

Cell polarity involves transport of specific membranes and macromolecules at the right time to the right place. In budding yeast, secretory vesicles are transported by the myosin-V Myo2p to sites of cell growth. We show that phosphatidylinositol 4-phosphate (PI4P) is present in late secretory compartments and is critical for their association with, and transport by, Myo2p. Further, the trans-Golgi network Rab Ypt31/32p and secretory vesicle Rab Sec4p each bind directly, but distinctly, to Myo2p, and these interactions are also required for secretory compartment transport. Enhancing the interaction of Myo2p with PI4P bypasses the requirement for interaction with Ypt31/32p and Sec4p. Together with additional genetic data, the results indicate that Rab proteins and PI4P collaborate in the association of secretory compartments with Myo2p. Thus, we show that a coincidence detection mechanism coordinates inputs from PI4P and the appropriate Rab for secretory compartment transport.

Yeast homologues of lethal giant larvae and type V myosin cooperate in the regulation of Rab-dependent vesicle clustering and polarized exocytosis

Lgl family members play an important role in the regulation of cell polarity in eukaryotic cells. The yeast homologues Sro7 and Sro77 are thought to act downstream of the Rab GTPase Sec4 to promote soluble N-ethylmaleimide-sensitive factor adaptor protein receptor (SNARE) function in post-Golgi transport. In this article, we characterize the interaction between Sro7 and the type V myosin Myo2 and show that this interaction is important for two distinct aspects of Sro7 function. First, we show that this interaction plays a positive role in promoting the polarized localization of Sro7 to sites of active growth. Second, we find evidence that Myo2 negatively regulates Sro7 function in vesicle clustering. Mutants in either Myo2 or Sro7 that are defective for this interaction show hypersensitivity to Sro7 overexpression, which results in Sec4-dependent accumulation of large groups of vesicles in the cytoplasm. This suggests that Myo2 serves a dual function, to both recruit Sro7 to secretory vesicles and inhibit its Rab-dependent tethering activity until vesicles reach the plasma membrane. Thus Sro7 appears to coordinate the spatial and temporal nature of both Rab-dependent tethering and SNARE-dependent membrane fusion of exocytic vesicles with the plasma membrane.

Myosin Va participates in acrosomal formation and nuclear morphogenesis during spermatogenesis of Chinese mitten crab Eriocheir sinensis

Background

The Chinese mitten crab Eriocheir sinensis belongs to the Class Crustacea, Decapoda, Brachyura. The spermatozoon of this species is of aflagellated type, it has a spherical acrosome surrounded by the cup-shaped nucleus, which are unique to brachyurans. For the past several decades, studies on the spermatogenesis of the mitten crab mainly focus on the morphology. Compared with the extensive study of molecular mechanism of spermatogenesis in mammals, relatively less information is available in crustacean species. Myosin Va, a member of Class V myosin, has been implicated in acrosome biogenesis and vesicle transport during spermatogenesis in mammals. In the present study we demonstrate the expression and cellular localization of myosin Va during spermatogenesis in E. sinensis.

Methodology/Principal Findings

Western blot demonstrated that myosin Va is expressed during spermatogenesis. Immunocytochemical and ultrastructural analyses showed that myosin Va mainly localizes in the cytoplasm in spermatocytes. At the early stage of spermiogenesis, myosin Va binds to the endoplasmic reticulum vesicle (EV) and proacrosomal granule (PG). Subsequently, myosin Va localizes within the proacrosomal vesicle (PV) formed by PG and EV fusion and locates in the membrane complex (MC) at the mid spermatid stage. At the late spermatid stage, myosin Va is associated with the shaping nucleus and mitochondria. In mature spermatozoon, myosin Va predominates in acrosomal tubule (AT) and nucleus.

Conclusions/Significance

Our study demonstrates that myosin Va may be involved in acrosome biogenesis and nuclear morphogenesis during spermatogenesis in E. sinensis. Considering the distribution and molecular characteristics of myosin Va, we also propose a hypothesis of AT formation in this species. It is the first time to uncover the role of myosin Va in crustacean spermatogenesis.

Kif26b, a kinesin family gene, regulates adhesion of the embryonic kidney mesenchyme

The kidney develops through reciprocal interactions between two precursor tissues: the metanephric mesenchyme and the ureteric bud. We previously demonstrated that the zinc finger protein Sall1 is essential for ureteric bud attraction toward the mesenchyme. Here, we show that Kif26b, a kinesin family gene, is a downstream target of Sall1 and that disruption of this gene causes kidney agenesis because of impaired ureteric bud attraction. In the Kif26b-null metanephros, compact adhesion between mesenchymal cells adjacent to the ureteric buds and the polarized distribution of integrin alpha8 were impaired, resulting in failed maintenance of Gdnf, a critical ureteric bud attractant. Overexpression of Kif26b in vitro caused increased cell adhesion through interactions with nonmuscle myosin. Thus, Kif26b is essential for kidney development because it regulates the adhesion of mesenchymal cells in contact with ureteric buds.

Two kinesin-like proteins mediate actin-based chloroplast movement in Arabidopsis thaliana

Organelle movement is essential for efficient cellular function in eukaryotes. Chloroplast photorelocation movement is important for plant survival as well as for efficient photosynthesis. Chloroplast movement generally is actin dependent and mediated by blue light receptor phototropins. In Arabidopsis thaliana, phototropins mediate chloroplast movement by regulating short actin filaments on chloroplasts (cp-actin filaments), and the chloroplast outer envelope protein CHUP1 is necessary for cp-actin filament accumulation. However, other factors involved in cp-actin filament regulation during chloroplast movement remain to be determined. Here, we report that two kinesin-like proteins, KAC1 and KAC2, are essential for chloroplasts to move and anchor to the plasma membrane. A kac1 mutant showed severely impaired chloroplast accumulation and slow avoidance movement. A kac1kac2 double mutant completely lacked chloroplast photorelocation movement and showed detachment of chloroplasts from the plasma membrane. KAC motor domains are similar to those of the kinesin-14 subfamily (such as Ncd and Kar3) but do not have detectable microtubule-binding activity. The C-terminal domain of KAC1 could interact with F-actin in vitro. Instead of regulating microtubules, KAC proteins mediate chloroplast movement via cp-actin filaments. We conclude that plants have evolved a unique mechanism to regulate actin-based organelle movement using kinesin-like proteins.

A nonprocessive class V myosin drives cargo processively when a kinesin- related protein is a passenger

During secretory events, kinesin transports cargo along microtubules and then shifts control to myosin V for delivery on actin filaments to the cell membrane [1]. When kinesin and myosin V are present on the same cargo, kinesin interacts electrostatically with actin to enhance myosin V-based transport in vitro [2]. The relevance of this observation within the cell was questioned. In budding yeast, overexpression of a kinesin-family protein (Smy1p) suppressed a transport defect in a strain with a mutant class V myosin (Myo2p) [3]. We postulate that this is a cellular manifestation of the in vitro observation. We demonstrate that Smy1p binds electrostatically to actin bundles. Although a single Myo2p cannot transport cargo along actin bundles, addition of Smy1p causes the complex to undergo long-range, continuous movement. We propose that the kinesin-family protein acts as a tether that prevents cargo dissociation from actin, allowing the myosin to take many steps before dissociating. We demonstrate that both the tether and the motor reside on moving secretory vesicles in yeast cells, a necessary feature for this mechanism to apply in vivo. The presence of both kinesin and myosin on the same cargo may be a general mechanism to enhance cellular transport in yeast and higher organisms.

A computationally guided protein-interaction screen uncovers coiled-coil interactions involved in vesicular trafficking

Mapping protein-protein interactions at a domain or motif level can provide structural annotation of the interactome. The alpha-helical coiled coil is among the most common protein-interaction motifs, and proteins predicted to contain coiled coils participate in diverse biological processes. Here, we introduce a combined computational/experimental screening strategy that we used to uncover coiled-coil interactions among proteins involved in vesicular trafficking in Saccharomyces cerevisiae. A number of coiled-coil complexes have already been identified and reported to play important roles in this important biological process. We identify additional examples of coiled coils that can form physical associations. The computational strategy used to prioritize coiled-coil candidates for testing dramatically improved the efficiency of discovery in a large experimental screen. As assessed by comprehensive yeast two-hybrid assays, computational prefiltering retained 90% of positive interacting pairs and eliminated >60% of negatives from a set of interaction candidates. The coiled-coil-mediated interaction network elucidated using the combined computational/experimental approach comprises 80 coiled-coil associations between 58 protein pairs, among which 21 protein interactions have not been previously reported in interaction databases and 26 interactions were previously known at the protein level but have now been localized to the coiled-coil motif. The coiled-coil-mediated interactions were specific rather than promiscuous, and many interactions could be recapitulated in a green fluorescent protein complementation assay. Our method provides an efficient route to discovering new coiled-coil interactions and uncovers a number of associations that may have functional significance for vesicular trafficking.

PTC1 is required for vacuole inheritance and promotes the association of the myosin-V vacuole-specific receptor complex

Organelle inheritance occurs during cell division. In Saccharomyces cerevisiae, inheritance of the vacuole, and the distribution of mitochondria and cortical endoplasmic reticulum are regulated by Ptc1p, a type 2C protein phosphatase. Here we show that PTC1/VAC10 controls the distribution of additional cargoes moved by a myosin-V motor. These include peroxisomes, secretory vesicles, cargoes of Myo2p, and ASH1 mRNA, a cargo of Myo4p. We find that Ptc1p is required for the proper distribution of both Myo2p and Myo4p. Surprisingly, PTC1 is also required to maintain the steady-state levels of organelle-specific receptors, including Vac17p, Inp2p, and Mmr1p, which attach Myo2p to the vacuole, peroxisomes, and mitochondria, respectively. Furthermore, Vac17p fused to the cargo-binding domain of Myo2p suppressed the vacuole inheritance defect in ptc1Delta cells. These findings suggest that PTC1 promotes the association of myosin-V with its organelle-specific adaptor proteins. Moreover, these observations suggest that despite the existence of organelle-specific receptors, there is a higher order regulation that coordinates the movement of diverse cellular components.

Inter-dependence of dimerization and organelle binding in myosin XI

Cytoplasmic streaming is a ubiquitous process in plant cells that is thought to be driven by the active movement of myosin XI motor proteins along actin filaments. These myosin motors bind to organelles through their C-terminal globular tail domain, although recent studies have also suggested a role for the central coiled-coil region during organelle binding. Here we have investigated the relationship between these two protein domains of MYA1, an Arabidopsis myosin XI, in a series of in vivo experiments demonstrating that dimerization of the coiled-coil region stabilizes organelle binding of the globular tail. Surprisingly, yeast two-hybrid assays, bimolecular fluorescence complementation, Förster resonance energy transfer and in vitro pull-down experiments all demonstrated that dimerization of the 174-residue MYA1 coiled coils by themselves was unstable. Furthermore, only the first of the two major coiled-coil segments in MYA1 contributed significantly to dimer formation. Interestingly, dimerization of myosin tail constructs that included the organelle-binding globular tail was stable, although the globular tails by themselves did not interact. This suggests an inter-dependent relationship between dimerization and organelle binding in myosin XI, whereby each process synergistically stimulates the other.

Direct interaction between a myosin V motor and the Rab GTPases Ypt31/32 is required for polarized secretion

Rab GTPases recruit myosin motors to endocytic compartments, which in turn are required for their motility. However, no Ypt/Rab GTPase has been shown to regulate the motility of exocytic compartments. In yeast, the Ypt31/32 functional pair is required for the formation of trans-Golgi vesicles. The myosin V motor Myo2 attaches to these vesicles through its globular-tail domain (GTD) and mediates their polarized delivery to sites of cell growth. Here, we identify Myo2 as an effector of Ypt31/32 and show that the Ypt31/32-Myo2 interaction is required for polarized secretion. Using the yeast-two hybrid system and coprecipitation of recombinant proteins, we show that Ypt31/32 in their guanosine triphosphate (GTP)-bound form interact directly with Myo2-GTD. The physiological relevance of this interaction is shown by colocalization of the proteins, genetic interactions between their genes, and rescue of the lethality caused by a mutation in the Ypt31/32-binding site of Myo2-GTD through fusion with Ypt32. Furthermore, microscopic analyses show a defective Myo2 intracellular localization in ypt31Delta/32ts and in Ypt31/32-interaction-deficient myo2 mutant cells, as well as accumulation of unpolarized secretory vesicles in the latter mutant cells. Together, these results indicate that Ypt31/32 play roles in both the formation of trans-Golgi vesicles and their subsequent Myo2-dependent motility.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

A point mutation in the cargo-binding domain of myosin V affects its interaction with multiple cargoes

Class V myosins move diverse intracellular cargoes, which attach via interaction of cargo-specific proteins to the myosin V globular tail. The globular tail of the yeast myosin V, Myo2p, contains two structural and functional subdomains. Subdomain I binds to the vacuole-specific protein, Vac17p, while subdomain II likely binds to an as yet unidentified secretory vesicle-specific protein. All functions of Myo2p require the tight association of subdomains I and II, which suggests that binding of a cargo to one subdomain may inhibit cargo-binding to a second subdomain. Thus, two types of mutations are predicted to specifically affect a subset of Myo2p cargoes: first are mutations within a cargo-specific binding region; second are mutations that mimic the inhibited conformation of one of the subdomains. Here we analyze a point mutation in subdomain I, myo2-2(G1248D), which is likely to be this latter type of mutation. myo2-2 has no effect on secretory vesicle movement. The secretory vesicle binding site is in subdomain II. However, myo2-2 is impaired in several Myo2p-related functions. While subdomains I and II of myo2-2p tightly associate, there are measurable differences in the conformation of its globular tail. Based solely on the ability to restore vacuole inheritance, a set of intragenic suppressors of myo2-2 were identified. All suppressor mutations reside in subdomain I. Moreover, subdomain I and II interactions occurred in all suppressors, demonstrating the importance of subdomain I and II association for Myo2p function. Furthermore, 3 of the 10 suppressors globally restored all tested defects in myo2-2. This large proportion of global suppressors strongly suggests that myo2-2(G1248) causes a conformational change in subdomain I that simultaneously affects multiple cargoes.

Myosin V attachment to cargo requires the tight association of two functional subdomains

The myosin V carboxyl-terminal globular tail domain is essential for the attachment of myosin V to all known cargoes. Previously, the globular tail was viewed as a single, functional entity. Here, we show that the globular tail of the yeast myosin Va homologue, Myo2p, contains two structural subdomains that have distinct functions, namely, vacuole-specific and secretory vesicle–specific movement. Biochemical and genetic analyses demonstrate that subdomain I tightly associates with subdomain II, and that the interaction does not require additional proteins. Importantly, although neither subdomain alone is functional, simultaneous expression of the separate subdomains produces a functional complex in vivo. Our results suggest a model whereby intramolecular interactions between the globular tail subdomains help to coordinate the transport of multiple distinct cargoes by myosin V.

Mechanisms of polarized growth and organelle segregation in yeast

Cell polarity, as reflected by polarized growth and organelle segregation during cell division in yeast, appears to follow a simple hierarchy. On the basis of physical cues from previous cell cycles or stochastic processes, yeast cells select a site for bud emergence that also defines the axis of cell division. Once polarity is established, rho protein-based signal pathways set up a polarized cytoskeleton by activating localized formins to nucleate and assemble polarized actin cables. These serve as tracks for the transport of secretory vesicles, the segregation of the trans Golgi network, the vacuole, peroxisomes, endoplasmic reticulum, mRNAs for cell fate determination, and microtubules that orient the nucleus in preparation for mitosis, all by myosin-Vs encoded by the MYO2 and MYO4 genes. Most of the proteins participating in these processes in yeast are conserved throughout the kingdoms of life, so the emerging models are likely to be generally applicable. Indeed, several parallels to cellular organization in animals are evident.

dsu functions in a MYO5A-independent pathway to suppress the coat color of dilute mice

MYO5A is a major actin-based vesicle transport motor that binds to one of its cargos, the melanosome, by means of a RAB27A/MLPH receptor. When one of the members of this receptor-motor complex is mutated, the melanosomes clump in the perinuclear region of the melanocyte and are transferred unevenly to the developing hair, leading to a dilution of coat color. Mutation of a fourth gene, dilute suppressor (dsu), suppresses this coat color dilution. MYO5A is required for the peripheral accumulation of melanosomes in melanocytes, but its role in melanosome transfer to neighboring keratinocytes and the hair is unknown. Here, we show that MYO5A is nonessential for melanosome transfer, although pigment incorporation into the hair in MYO5A-deficient mice is uneven, probably due to the clumping of melanosomes that occurs in the perinuclear region of mutant melanocytes. We also show that dsu is caused by a loss-of-function mutation in a unique vertebrate-specific protein that appears to function in an MYO5A-independent pathway to alter pigment incorporation into the hair. Therefore, dsu identifies a unique protein involved in pigmentation of the mammalian hair.

Polarized distribution of intracellular components by class V myosins in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has three classes of myosins corresponding to three actin structures: class I myosin for endocytic actin structure, actin patches; class II myosin for contraction of the actomyosin contractile ring around the bud neck; and class V myosin for transport along a cable-like actin structure (actin cables), extending toward the growing cortex. Myo2p and Myo4p constitute respective class V myosins as the heavy chain and, like class V myosins in other organisms, function as actin-based motors for polarized distribution of organelles and intracellular molecules. Proper distribution of organelles is essential for autonomously replicating organelles that cannot be reproduced de novo, and is also quite important for other organelles to ensure their efficient segregation and proper positioning, even though they can be newly synthesized, such as those derived from endoplasmic reticulum. In the budding yeast, microtubule-based motors play limited roles in the distribution. Instead, the actin-based motor myosins, especially Myo2p, play a major role. Studies on Myo2p have revealed a wide variety of Myo2p cargo and Myo2p-interacting proteins and have established that Myo2p interacts with cargo and transfers it along actin cables. Moreover, recent findings suggest that Myo2p has another way to distribute cargo in that Myo2p conveys the attaching cargo along the actin track. Thus, the myosin have "dual paths" for distribution of a cargo. This dual path mechanism is proposed in the last section of this review.

A complete inventory of fungal kinesins in representative filamentous ascomycetes

Complete inventories of kinesins from three pathogenic filamentous ascomycetes, Botryotinia fuckeliana, Cochliobolus heterostrophus, and Gibberella moniliformis, are described. These protein sequences were compared with those of the filamentous saprophyte, Neurospora crassa and the two yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Data mining and phylogenetic analysis of the motor domain yielded a constant set of 10 kinesins in the filamentous fungal species, compared with a smaller set in S. cerevisiae and S. pombe. The filamentous fungal kinesins fell into nine subfamilies when compared with well-characterized kinesins from other eukaryotes. A few putative kinesins (one in B. fuckeliana and two in C. heterostrophus) could not be defined as functional, due to unorthodox organization and lack of experimental data. The broad representation of filamentous fungal kinesins across most of the known subfamilies and the ease of gene manipulation make fungi ideal models for functional and evolutionary investigation of these proteins.

Complex formation with Ypt11p, a rab-type small GTPase, is essential to facilitate the function of Myo2p, a class V myosin, in mitochondrial distribution in Saccharomyces cerevisiae

We identified Ypt11p, a rab-type small GTPase, by its functional and two-hybrid interaction with Myo2p, a class V myosin of the budding yeast Saccharomyces cerevisiae. The tail domain of Myo2p was coimmunoprecipitated with Ypt11p, suggesting that Ypt11p forms a complex with Myo2p at its tail domain in vivo. Mutational analysis of YPT11 suggests that Myo2p is a putative effector of Ypt11p. Deletion of YPT11 induced partial delay of mitochondrial transmission to the bud, and overexpression of YPT11 resulted in mitochondrial accumulation in the bud, indicating that Ypt11p acts positively on mitochondrial distribution toward the bud. We isolated two myo2 mutants, myo2-338 and myo2-573, which showed genetic interactions with YPT11. The myo2-573 mutation, identified by a synthetic lethal interaction with ypt11-null, induced a defect in mitochondrial distribution toward the bud, indicating that Myo2p plays a crucial role in polarized distribution of mitochondria. The myo2-338 mutation was identified as the mutation that abolished the effect of overexpressed YPT11, such as the Ypt11p-dependent accumulation of mitochondria in the bud, and the affinity of Myo2p for Ypt11p was reduced. These results indicate that complex formation of Ypt11p with Myo2p accelerates the function of Myo2p for mitochondrial distribution toward the bud.

Tensile stress stimulates microtubule outgrowth in living cells

Cell motility is driven by the sum of asymmetric traction forces exerted on the substrate through adhesion foci that interface with the actin cytoskeleton. Establishment of this asymmetry involves microtubules, which exert a destabilising effect on adhesion foci via targeting events. Here, we demonstrate the existence of a mechano-sensing mechanism that signals microtubule polymerisation and guidance of the microtubules towards adhesion sites under increased stress. Stress was applied either by manipulating the body of cells moving on glass with a microneedle or by stretching a flexible substrate that cells were migrating on. We propose a model for this mechano-sensing phenomenon whereby microtubule polymerisation is stimulated and guided through the interaction of a microtubule tip complex with actin filaments under tension.

Myosin-1c interacts with hair-cell receptors through its calmodulin-binding IQ domains

Myosin-1c plays an essential role in adaptation of hair-cell mechanoelectrical transduction. To mediate adaptation, myosin-1c must interact directly or indirectly with other components of the transduction apparatus, including the mechanically gated transduction channel. As a first step toward identifying myosin-1c receptors, we used recombinant myosin-1c fragments to identify specific binding sites in hair cells and to biochemically characterize their interaction with myosin-1c. Myosin-1c fragments bound to tips of hair-cell stereocilia, the location of transduction and adaptation. Surprisingly, this interaction did not depend on the C-terminal tail of myosin-1c, proposed previously to be the receptor-binding site of the molecule. Instead, the interaction of myosin-1c with stereociliary receptors depended on its calmodulin-binding IQ domains. This interaction was blocked by calmodulin, which probably bound to a previously unoccupied IQ domain of myosin-1c. The calcium-sensitive binding of calmodulin to myosin-1c may therefore modulate the interaction of the adaptation motor with other components of the transduction apparatus.

Expression of constructs of the neuronal isoform of myosin-Va interferes with the distribution of melanosomes and other vesicles in melanoma cells

Myosin-Va has been implicated in melanosome translocation, but the exact molecular mechanisms underlying this function are not known. In the dilute, S91 melanoma cells, melanosomes move to the cell periphery but do not accumulate in the tips of dendrites as occurs in wild-type B16 melanocytes; rather, they return and accumulate primarily at the pericentrosomal region in a microtubule-dependent manner. Expression of the full-length neuronal isoform of myosin-Va in S91 cells causes melanosomes to disperse, occupying a cellular area approximately twice that observed in non-transfected cells, suggesting a partial rescue of the dilute phenotype. Overexpression of the full tail domain in S91 cells is not sufficient to induce melanosome dispersion, rather it causes melanosomal clumping. Overexpression of the head and head-neck domains of myosin-Va in B16 cells does not alter the melanosome distribution. However, overexpression of the full tail domain in these cells induces melanosome aggregation and the appearance of tail-associated, aggregated particles or vesicular structures that exhibit variable degrees of staining for melanosomal and Golgi beta-COP markers, as well as colocalization with the endogenous myosin-Va. Altogether, the present data suggest that myosin-Va plays a role in regulating the direction of microtubule-dependent melanosome translocation, in addition to promoting the capture of melanosomes at the cell periphery as suggested by previous studies. These studies also reinforce the notion that myosin-V has a broader function in melanocytes by acting on vesicular targeting or intracellular protein trafficking.

Rabs grab motors: defining the connections between Rab GTPases and motor proteins

Rab GTPases and their effectors regulate membrane traffic by determining, along with cognate SNAREs, the specificity of transport vesicle docking and fusion steps. Recent studies have also implicated Rabs in the movement of these transport vesicles from their site of formation to their site of fusion, and several Rabs have been linked to specific microtubule- or actin-based motor proteins. Analyses of Rab and motor protein mutants, coupled with advanced imaging techniques, have led to the suggestion that certain Rabs function as essential components of the vesicle receptor for specific motor proteins.

Functions of unconventional myosins in the yeast Saccharomyces cerevisiae

Unconventional myosins in the budding yeast play essential roles in diverse cellular functions, including endocytosis, actin organization, and polarized distribution of organelles. Several lines of evidence suggest that novel proteins, interacting with the unconventional myosins, regulate their functions. In this review, we focus on the functions of unconventional myosins from the point of view of myosin-interacting proteins.

Myosin vb is associated with plasma membrane recycling systems

Myosin Va is associated with discrete vesicle populations in a number of cell types, but little is known of the function of myosin Vb. Yeast two-hybrid screening of a rabbit parietal cell cDNA library with dominant active Rab11a (Rab11aS20V) identified myosin Vb as an interacting protein for Rab11a, a marker for plasma membrane recycling systems. The isolated clone, corresponding to the carboxyl terminal 60 kDa of the myosin Vb tail, interacted with all members of the Rab11 family (Rab11a, Rab11b, and Rab25). GFP-myosin Vb and endogenous myosin Vb immunoreactivity codistributed with Rab11a in HeLa and Madin-Darby canine kidney (MDCK) cells. As with Rab11a in MDCK cells, the myosin Vb immunoreactivity was dispersed with nocodazole treatment and relocated to the apical corners of cells with taxol treatment. A green fluorescent protein (GFP)-myosin Vb tail chimera overexpressed in HeLa cells retarded transferrin recycling and caused accumulation of transferrin and the transferrin receptor in pericentrosomal vesicles. Expression of the myosin Vb tail chimera in polarized MDCK cells stably expressing the polymeric IgA receptor caused accumulation of basolaterally endocytosed polymeric IgA and the polymeric IgA receptor in the pericentrosomal region. The myosin Vb tail had no effects on transferrin trafficking in polarized MDCK cells. The GFP-myosin Va tail did not colocalize with Rab11a and had no effects on recycling system vesicle distribution in either HeLa or MDCK cells. The results indicate myosin Vb is associated with the plasma membrane recycling system in nonpolarized cells and the apical recycling system in polarized cells. The dominant negative effects of the myosin Vb tail chimera indicate that this unconventional myosin is required for transit out of plasma membrane recycling systems.

Force and focal adhesion assembly: a close relationship studied using elastic micropatterned substrates

Mechanical forces play a major role in the regulation of cell adhesion and cytoskeletal organization. In order to explore the molecular mechanism underlying this regulation, we have investigated the relationship between local force applied by the cell to the substrate and the assembly of focal adhesions. A novel approach was developed for real-time, high-resolution measurements of forces applied by cells at single adhesion sites. This method combines micropatterning of elastomer substrates and fluorescence imaging of focal adhesions in live cells expressing GFP-tagged vinculin. Local forces are correlated with the orientation, total fluorescence intensity and area of the focal adhesions, indicating a constant stress of 5.5 +/- 2 nNmicrom(-2). The dynamics of the force-dependent modulation of focal adhesions were characterized by blocking actomyosin contractility and were found to be on a time scale of seconds. The results put clear constraints on the possible molecular mechanisms for the mechanosensory response of focal adhesions to applied force.

A role for actin, Cdc1p, and Myo2p in the inheritance of late Golgi elements in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, Golgi elements are present in the bud very early in the cell cycle. We have analyzed this Golgi inheritance process using fluorescence microscopy and genetics. In rapidly growing cells, late Golgi elements show an actin-dependent concentration at sites of polarized growth. Late Golgi elements are apparently transported into the bud along actin cables and are also retained in the bud by a mechanism that may involve actin. A visual screen for mutants defective in the inheritance of late Golgi elements yielded multiple alleles of CDC1. Mutations in CDC1 severely depolarize the actin cytoskeleton, and these mutations prevent late Golgi elements from being retained in the bud. The efficient localization of late Golgi elements to the bud requires the type V myosin Myo2p, further suggesting that actin plays a role in Golgi inheritance. Surprisingly, early and late Golgi elements are inherited by different pathways, with early Golgi elements localizing to the bud in a Cdc1p- and Myo2p-independent manner. We propose that early Golgi elements arise from ER membranes that are present in the bud. These two pathways of Golgi inheritance in S. cerevisiae resemble Golgi inheritance pathways in vertebrate cells.

Rab27a enables myosin Va-dependent melanosome capture by recruiting the myosin to the organelle

The peripheral accumulation of melanosomes characteristic of wild-type mouse melanocytes is driven by a cooperative process involving long-range, bidirectional, microtubule-dependent movements coupled to capture and local movement in the actin-rich periphery by myosin Va, the product of the dilute locus. Genetic evidence suggests that Rab27a, the product of the ashen locus, functions with myosin Va in this process. Here we show that ashen melanocytes, like dilute melanocytes, exhibit normal dendritic morphology and melanosome biogenesis, an abnormal accumulation of end-stage melanosomes in the cell center, and rapid, bidirectional, microtubule-dependent melanosome movements between the cell center and the periphery. This phenotype suggests that ashen melanocytes, like dilute melanocytes, are defective in peripheral melanosome capture. Consistent with this, introduction into ashen melanocytes of cDNAs encoding wild-type and GTP-bound versions of Rab27a restores the peripheral accumulation of melanosomes in a microtubule-dependent manner. Conversely, introduction into wild-type melanocytes of the GDP-bound version of Rab27a generates an ashen/dilute phenotype. Rab27a colocalizes with end-stage melanosomes in wild-type cells, and is most concentrated in melanosome-rich dendritic tips, where it also colocalizes with myosin Va. Finally, neither endogenous myosin Va nor an expressed, GFP-tagged, myosin Va tail domain fusion protein colocalize with melanosomes in ashen melanocytes, in contrast to that seen previously in wild-type cells. These results argue that Rab27a serves to enable the myosinVa-dependent capture of melanosomes delivered to the periphery by bidirectional, microtubule-dependent transport, and that it does so by recruiting the myosin to the melanosome surface. We suggest that Rab27a, in its GTP-bound and melanosome-associated form, predominates in the periphery, and that it is this form that recruits the myosin, enabling capture. These results argue that Rab27a serves as a myosin Va ‘receptor’, and add to the growing evidence that Rab GTPases regulate vesicle motors as well as SNARE pairing.

Two type V myosins with non-overlapping functions in the fission yeast Schizosaccharomyces pombe: Myo52 is concerned with growth polarity and cytokinesis, Myo51 is a component of the cytokinetic actin ring

The fission yeast genome project has identified five myosin genes: one type I myosin, myo1(+), two type II myosins, myo2(+) and myp2(+), and two type V myosins, myo51(+) and myo52(+). Cells deleted for myo51(+) show normal morphology and growth rates whereas deletion of myo52(+) results in a partial loss of cell polarity, slow growth and cytokinetic defects. Combining both deletions in a single strain is phenotypically non-additive, myo52(delta) being epistatic to myo51(delta). Overproduction of Myo51 gives rise to elongated cells which fail to form functional septa whereas overproduction of Myo52 results in branched cells with aberrant septa that fail to cleave. Myo52 localises to the poles of growing cells but during cell division it relocalises to the cell equator as a bar that is bisected by the cytokinetic septum. Myo51 shows no obvious localisation during interphase but at cytokinesis it is associated with the contractile cytokinetic actin ring (CAR). Both myosins are dependent upon an intact actin cytoskeleton for localisation. Myo52 partially colocalises with the (alpha)-glucan synthase Mok1 at the cell tips and to a lesser extent at the septum. Mok1 is delocalised and upregulated in myo52(delta) and myo52(delta) cell walls are resistant to digestion by the cell wall degrading enzyme zymolyase. Thus myo52(+) appears to be involved in the local delivery or positioning of vesicles containing cell wall precursors at the cell tips and has a role in the maturation or cleavage of the septum. Myo51 has a non-essential role in cytokinesis as a component of the cytokinetic actin ring.

Identification and characterization of a novel microtubule-based motor associated with membranous organelles in tobacco pollen tubes

Pollen tube growth depends on the differential distribution of organelles and vesicles along the tube. The role of microtubules in organelle movement is uncertain, mainly because information at the molecular level is limited. In an effort to understand the molecular basis of microtubule-based movement, we isolated from tobacco pollen tubes polypeptides that cosediment with microtubules in an ATP-dependent manner. Major polypeptides released from microtubules by ATP (ATP-MAPs) had molecular masses of 90, 80, and 41 kD. Several findings indicate that the 90-kD ATP-MAP is a kinesin-related motor: binding of the polypeptide to microtubules was enhanced by the nonhydrolyzable ATP analog AMP-PNP; the 90-kD polypeptide reacted specifically with a peptide antibody directed against a highly conserved region in the motor domain of the kinesin superfamily; purified 90-kD ATP-MAP induced microtubules to glide in motility assays in vitro; and the 90-kD ATP-MAP cofractionated with microtubule-activated ATPase activity. Immunolocalization studies indicated that the 90-kD ATP-MAP binds to organelles associated with microtubules in the cortical region of the pollen tube. These findings suggest that the 90-kD ATP-MAP is a kinesin-related microtubule motor that moves organelles in the cortex of growing pollen tubes.

Myosin V orientates the mitotic spindle in yeast

Coordination of spindle orientation with the axis of cell division is an essential process in all eukaryotes. In addition to ensuring accurate chromosomal segregation, proper spindle orientation also establishes differential cell fates and proper morphogenesis. In both animal and yeast cells, this process is dependent on cytoplasmic microtubules interacting with the cortical actin-based cytoskeleton, although the motive force was unknown. Here we show that yeast Myo2, a myosin V that translocates along polarized actin cables into the bud, orientates the spindle early in the cell cycle by binding and polarizing the microtubule-associated protein Kar9 (refs 7-9). The tail domain of Myo2 that binds Kar9 also interacts with secretory vesicles and vacuolar elements, making it a pivotal component of yeast cell polarization.

Two distinct regions in a yeast myosin-V tail domain are required for the movement of different cargoes

The Saccharomyces cerevisiae myosin-V, Myo2p, is essential for polarized growth, most likely through transport of secretory vesicles to the developing bud. Myo2p is also required for vacuole movement, a process not essential for growth. The globular region of the myosin-V COOH-terminal tail domain is proposed to bind cargo. Through random mutagenesis of this globular tail, we isolated six new single point mutants defective in vacuole inheritance, but not polarized growth. These point mutations cluster to four amino acids in an 11-amino acid span, suggesting that this region is important for vacuole movement. In addition, through characterization of myo2-DeltaAflII, a deletion of amino acids 1,459-1,491, we identified a second region of the globular tail specifically required for polarized growth. Whereas this mutant does not support growth, it complements the vacuole inheritance defect in myo2-2 (G1248D) cells. Moreover, overexpression of the myo2-DeltaAflII globular tail interferes with vacuole movement, but not polarized growth. These data indicate that this second region is dispensable for vacuole movement. The identification of these distinct subdomains in the cargo-binding domain suggests how myosin-Vs can move multiple cargoes. Moreover, these studies suggest that the vacuole receptor for Myo2p differs from the receptor for the essential cargo.