Vesicle-associated brain myosin-V can be activated to catalyze actin-based transport

Myosin Va transports dense core secretory vesicles in pancreatic MIN6 beta-cells

The role of unconventional myosins in neuroendocrine cells is not fully understood, with involvement suggested in the movement of both secretory vesicles and mitochondria. Here, we demonstrate colocalization of myosin Va (MyoVa) with insulin in pancreatic beta-cells and show that MyoVa copurifies with insulin in density gradients and with the vesicle marker phogrin-enhanced green fluorescent protein upon fluorescence-activated sorting of vesicles. By contrast, MyoVa immunoreactivity was poorly colocalized with mitochondrial or other markers. Demonstrating an important role for MyoVa in the recruitment of secretory vesicles to the cell surface, a reduction of MyoVa protein levels achieved by RNA interference caused a significant decrease in glucose- or depolarization-stimulated insulin secretion. Similarly, expression of the dominant-negative-acting globular tail domain of MyoVa decreased by approximately 50% the number of vesicles docked at the plasma membrane and by 87% the number of depolarization-stimulated exocytotic events detected by total internal reflection fluorescence microscopy. We conclude that MyoVa-driven movements of vesicles along the cortical actin network are essential for the terminal stages of regulated exocytosis in beta-cells.

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.

Motility of myosin V regulated by the dissociation of single calmodulin

Myosin V is a calmodulin-binding motor protein. The dissociation of single calmodulin molecules from individual myosin V molecules at 1 microM Ca(2+) correlates with a reduction in sliding velocity in an in vitro motility assay. The dissociation of two calmodulin molecules at 5 microM Ca(2+) correlates with a detachment of actin filaments from myosin V. To mimic the regulation of myosin V motility by Ca(2+) in a cell, caged Ca(2+) coupled with a UV flash system was used to produce Ca(2+) transients. During the Ca(2+) transient, myosin V goes through the functional cycle of reduced sliding velocity, actin detachment and reattachment followed by the recovery of the sliding velocity. These results indicate that myosin V motility is regulated by Ca(2+) through a reduction in actin-binding affinity resulting from the dissociation of single calmodulin molecules.

Myosin Va is required for normal photoreceptor synaptic activity

Myosin Va is an actin-based motor molecule, one of a large family of unconventional myosins. In humans, mutations in MYO5A cause Griscelli syndrome type 1 and Elejalde syndrome, diseases characterized by pigmentation defects and the prepubescent onset of severe neurological deficits that ultimately lead to a shortened lifespan. Mutations in the Myo5a gene in mouse cause the dilute series of mouse mutants, demonstrating that myosin Va is involved in pigmentation and neural function. Although the reason for the pigmentation abnormalities is well understood, the role of myosin Va in neural function is not. Myosin Va has been found in synaptic terminals in the retina and brain. We report here new physiological evidence for a role of myosin Va in synaptic function. Photoreceptor synapses in neurologically affected myosin Va mutant mice have both anatomical and physiological abnormalities. Thus, myosin Va is required for normal photoreceptor signalling, suggesting that it might function in central nervous system synapses in general, with aberrant synaptic activity potentially underlying the neurological defects observed in dilute lethal mice and patients with Griscelli syndrome type 1 and Elejalde syndrome.

Intracellular actin-based transport: how far you go depends on how often you switch

Intracellular molecular motor-driven transport is essential for such diverse processes as mitosis, neuronal function, and mitochondrial transport. Whereas there have been in vitro studies of how motors function at the single-molecule level, and in vivo studies of the structure of filamentary networks, studies of how the motors effectively use the networks for transportation have been lacking. We investigate how the combined system of myosin-V motors plus actin filaments is used to transport pigment granules in Xenopus melanophores. Experimentally, we characterize both the actin filament network, and how this transport is altered in response to external signals. We then develop a theoretical formalism to explain these changes. We show that cells regulate transport by controlling how often granules switch from one filament to another, rather than by altering individual motor activity at the single-molecule level, or by relying on structural changes in the network.

Myosin Va and microtubule-based motors are required for fast axonal retrograde transport of tetanus toxin in motor neurons

Using a novel assay based on the sorting and transport of a fluorescent fragment of tetanus toxin, we have investigated the cytoskeletal and motor requirements of axonal retrograde transport in living mammalian motor neurons. This essential process ensures the movement of neurotrophins and organelles from the periphery to the cell body and is crucial for neuronal survival. Unlike what is observed in sympathetic neurons, fast retrograde transport in motor neurons requires not only intact microtubules, but also actin microfilaments. Here, we show that the movement of tetanus toxin-containing carriers relies on the nonredundant activities of dynein as well as kinesin family members. Quantitative kinetic analysis indicates a role for dynein as the main motor of these carriers. Moreover, this approach suggests the involvement of myosin(s) in retrograde movement. Immunofluorescence screening with isoform-specific myosin antibodies reveals colocalization of tetanus toxin-containing retrograde carriers with myosin Va. Motor neurons from homozygous myosin Va null mice showed slower retrograde transport compared with wild-type cells, establishing a unique role for myosin Va in this process. On the basis of our findings, we propose that coordination of myosin Va and microtubule-dependent motors is required for fast axonal retrograde transport in motor neurons.

Myosin-dependent transport in neurons

Axonal transport in neurons has been shown to be microtubule dependent, driven by the molecular motor proteins kinesin and dynein. However, organelles undergoing fast transport can often pause or rapidly change directions without apparent dissociation from their transport tracks. Cytoskeletal polymers such as neurofilaments and microtubules have also been shown to make infrequent but rapid movements in axons indicating that their transport is likely to involve molecular motors. In addition, neurons have multiple compartments that are devoid of microtubules where transport of organelles is still seen to occur. These areas are rich in other cytoskeletal polymers such as actin filaments. Transported organelles have been shown to associate with multiple motor proteins including myosins. This suggests that nonmicrotubule-based transport may be myosin driven. In this review we will focus our attention on myosin motors known to be present in neurons and evaluate the evidence that they contribute to transport or other functions in the different compartments of the neuron.

Short-range axonal/dendritic transport by myosin-V: A model for vesicle delivery to the synapse

Myosin-V is a versatile motor involved in short-range axonal/dendritic transport of vesicles in the actin-rich cortex and synaptic regions of nerve cells. It binds to several different kinds of neuronal vesicles by its globular tail domain but the mechanism by which it is recruited to these vesicles is not known. In this study, we used an in vitro motility assay derived from axoplasm of the squid giant axon to study the effects of the globular tail domain on the transport of neuronal vesicles. We found that the globular tail fragment of myosin-V inhibited actin-based vesicle transport by displacing native myosin-V and binding to vesicles. The globular tail domain pulled down kinesin, a known binding partner of myosin-V, in affinity isolation experiments. These data confirmed earlier evidence that kinesin and myosin-V interact to form a hetero-motor complex. The formation of a kinesin/myosin-V hetero-motor complex on vesicles is thought to facilitate the coordination of long-range movement on microtubules and short-range movement on actin filaments. The direct interaction of motors from both filament systems may represent the mechanism by which the transition of vesicles from microtubules to actin filaments is regulated. These results are the first demonstration that the recombinant tail of myosin-V inhibits vesicle transport in an in vitro motility assay. Future experiments are designed to determine the functional significance of the interaction between myosin-V and kinesin and to identify other proteins that bind to the globular tail domain of myosin-V.

Immunohistochemical localization of myosin Va in the adult rat brain

Brain myosin Va (MVa) is a molecular motor associated with plastic changes during development. MVa has previously been detected in the cell body and in dendrites of neuronal cells in culture, in cells of the guinea-pig cochlea, as well as in cerebellar cells. Adult Wistar rats (n=14), 250-300 g, were perfused with standard methods for immunohistochemistry, using a polyclonal, affinity-purified rabbit antibody against MVa tail domain. Anti-MVa antibody specifically stained neuronal nuclei from forebrain to cerebellar regions, and more intensely sensory nuclei. Differences in MVa immunoreactivity were detected between brain nuclei, ranging from very intense to weak staining. The analysis of MVa and glial fibrillary acidic protein staining in adjacent brain sections demonstrated a clear-cut neuronal labeling rather than an astroglial staining. The studies presented here represent a comprehensive map of MVa regional distribution in the CNS of the adult rat and may contribute to the basic understanding of its role in brain function and plasticity, particularly in relationship to phenomena that involve molecular motors, such as neurite outgrowth, organelle transport and neurotransmitter-vesicle cycling. It is important to highlight that this is a pioneer immunohistochemical study on the distribution of MVa on the whole brain of adult rats, a first step toward the understanding of its function in the CNS.

Regulated conformation of myosin V

We have found that myosin V, an important actin-based vesicle transporter, has a folded conformation that is coupled to inhibition of its enzymatic activity in the absence of cargo and Ca(2+). In the absence of Ca(2+) where the actin-activated MgATPase activity is low, purified brain myosin V sediments in the analytical ultracentrifuge at 14 S as opposed to 11 S in the presence of Ca(2+) where the activity is high. At high ionic strength it sediments at 10 S independent of Ca(2+), and its regulation is poor. These data are consistent with myosin V having a compact, inactive conformation in the absence of Ca(2+) and an extended conformation in the presence of Ca(2+) or high ionic strength. Electron microscopy reveals that in the absence of Ca(2+) the heads and tail are both folded to give a triangular shape, very different from the extended appearance of myosin V at high ionic strength. A recombinant myosin V heavy meromyosin fragment that is missing the distal portion of the tail domain is not regulated by calcium and has only a small change in sedimentation coefficient, which is in the opposite direction to that seen with intact myosin V. Electron microscopy shows that its heads are extended even in the absence of calcium. These data suggest that interaction between the motor and cargo binding domains may be a general mechanism for shutting down motor protein activity and thereby regulating the active movement of vesicles in cells.

The GTP-binding protein RhoA mediates Na,K-ATPase exocytosis in alveolar epithelial cells

The purpose of this study was to define the role of the Rho family of small GTPases in the beta-adrenergic regulation of the Na,K-ATPase in alveolar epithelial cells (AEC). The beta-adrenergic receptor agonist isoproterenol (ISO) increased the Na,K-ATPase protein abundance at the plasma membrane and activated RhoA in a time-dependent manner. AEC pretreated with mevastatin, a specific inhibitor of prenylation, or transfected with the dominant negative RhoAN19, prevented ISO-mediated Na,K-ATPase exocytosis to the plasma membrane. The ISO-mediated activation of RhoA in AEC occurred via beta2-adrenergic receptors and involved Gs-PKA as demonstrated by incubation with the protein kinase A (PKA)-specific inhibitors H89 and PKI (peptide specific inhibitor), and Gi, as incubation with pertussis toxin or cells transfected with a minigene vector for Gi inhibited the ISO-mediated RhoA activation. However, cells transfected with minigene vectors for G12 and G13 did not prevent RhoA activation by ISO. Finally, the ISO-mediated Na,K-ATPase exocytosis was regulated by the Rho-associated kinase (ROCK), as preincubation with the specific inhibitor Y-27632 or transfection with dominant negative ROCK, prevented the increase in Na,K-ATPase at the plasma membrane. Accordingly, ISO regulates Na,K-ATPase exocytosis in AEC via the activation of beta2-adrenergic receptor, Gs, PKA, Gi, RhoA, and ROCK.

Myosin-Va proteolysis by Ca2+/calpain in depolarized nerve endings from rat brain

Myosin-Va is a molecular motor that may participate in synaptic vesicle cycling. Calpain cleaves myosin-Va in vitro at methionine 1141 in the tail domain. We show that intracellular proteolysis of myosin-Va occurs in rat cortical synaptosomes depolarized in the presence of calcium, evidenced by the formation of an 80 k polypeptide that co-migrates in SDS-PAGE with the 80 k fragment produced by the in vitro proteolysis of myosin-Va by calpain. Anti-myosin-Va antibody recognized this polypeptide in Western blots and immunoprecipitated it from synaptosome extracts. Calpastatin, a calpain-specific inhibitor, or leupeptin, a general cysteine protease inhibitor, suppressed or blocked formation of the 80 k polypeptide depending on membrane permeability. We conclude that myosin-Va undergoes intracellular proteolysis by endogenous calpain, when synaptosomes are depolarized in the presence of calcium, at the same cleavage site previously identified in vitro, thus, making it a target for calcium signaling during synaptic activation.

Myosin Va facilitates the distribution of secretory granules in the F-actin rich cortex of PC12 cells

Neuroendocrine secretory granules, the storage organelles for neuropeptides and hormones, are formed at the trans-Golgi network, stored inside the cell and exocytosed upon stimulation. Previously, we have reported that newly formed secretory granules of PC12 cells are transported in a microtubule-dependent manner from the trans-Golgi network to the F-actin-rich cell cortex, where they undergo short directed movements and exhibit a homogeneous distribution. Here we provide morphological and biochemical evidence that myosin Va is associated with secretory granules. Expression of a dominant-negative tail domain of myosin Va in PC12 cells led to an extensive clustering of secretory granules close to the cell periphery, a loss of their cortical restriction and a strong reduction in their motility in the actin cortex. Based on this data we propose a model that implies a dual transport system for secretory granules: after microtubule-dependent delivery to the cell periphery, secretory granules exhibit a myosin Va-dependent transport leading to their restriction and even dispersal in the F-actin-rich cortex of PC12 cells.

Myosins II and V in chromaffin cells: myosin V is a chromaffin vesicle molecular motor involved in secretion

The presence of myosin II and V in chromaffin cells and their subcellular distribution is described. Myosin II and V distribution in sucrose density gradients showed only a strong correlation between the distribution of myosin V and secretory vesicle markers. Confocal microscopy images demonstrated colocalization of myosin V with dopamine beta-hydroxylase, a chromaffin vesicle marker, whereas myosin II was present mainly in the cell cortex. Cell depolarization induced, in a Ca2+ and time-dependent manner, the dissociation of myosin V from chromaffin vesicles suggesting that this association was not permanent but determined by secretory cycle requirements. Myosin II was also found in the crude granule fraction, however, its distribution was not affected by cell depolarization. Myosin V head antibodies were able to inhibit secretion whereas myosin II antibodies had no inhibitory effect. The pattern of inhibition indicated that these treatments interfered with the transport of vesicles from the reserve to the release-ready compartment, suggesting the involvement of myosin V and not myosin II in this transport process. The results described here suggest that myosin V is a molecular motor involved in chromaffin vesicle secretion. However, these results do not discard an indirect role for myosin II in secretion through its interaction with F-actin networks.

Actin has a molecular scaffolding, not propulsive, role in presynaptic function

We used actin tagged with enhanced green fluorescent protein (EGFP-actin) to characterize the distribution and dynamics of actin in living presynaptic terminals in rat CNS neurons. Actin was preferentially concentrated around--and appeared to surround--the presynaptic vesicle cluster. In resting terminals, approximately 30% of actin was found to be in a polymerized but dynamic state, with a remodeling time scale of approximately 20 s. During electrical activity, actin was further polymerized and recruited from nearby axonal regions to the regions surrounding vesicles. Treatment of terminals with the actin monomer-sequestering agent latrunculin-A completely dispersed the actin network and abolished activity-dependent actin dynamics. We used a variety of methods to examine the role of actin in the presynaptic vesicle cycle. These data rule out a propulsive role for actin, either in maintaining the vesicle cluster or in guiding vesicle recycling. Instead, we propose that actin acts as a scaffolding system for regulatory molecules in the nerve terminal.

Myosin-V, a versatile motor for short-range vesicle transport

Myosin-V is a versatile motor involved in short-range transport of vesicles in the actin-rich cortex of the cell. It binds to several different kinds of vesicles, and the mechanism by which it interacts with the vesicle surface is being unraveled, primarily in melanocytes. Members of the Rab family of G-proteins are required for the recruitment of myosin-V to vesicles. Rab27a and its rabphilin-like effector protein, Melanophilin, recruit myosin-Va to melanosomes and appear to serve as the membrane receptor. Myosin-V is also involved in fast axonal/dendritic transport and, interestingly, it forms a complex with kinesin, a microtubule-based motor. This kinesin/myosin-V heteromotor complex allows long-range movement of vesicles within axons and dendrites on microtubules and short-range movement in the dendritic spines and axon terminals on actin filaments. The direct interaction of motors from both filament systems may represent the mechanism by which the transition of vesicles from microtubules to actin filaments is regulated.

Identification of mRNA/protein (mRNP) complexes containing Puralpha, mStaufen, fragile X protein, and myosin Va and their association with rough endoplasmic reticulum equipped with a kinesin motor

Puralpha, which is involved in diverse aspects of cellular functions, is strongly expressed in neuronal cytoplasm. Previously, we have reported that this protein controls BC1 RNA expression and its subsequent distribution within dendrites and that Puralpha is associated with polyribosomes. Here, we report that, following treatment with EDTA, Puralpha was released from polyribosomes in mRNA/protein complexes (mRNPs), which also contained mStaufen, Fragile X Mental Retardation Protein (FMRP), myosin Va, and other proteins with unknown functions. As the coimmunoprecipitation of these proteins by an anti-Puralpha antibody was abolished by RNase treatment, Puralpha may assist mRNP assembly in an RNA-dependent manner and be involved in targeting mRNPs to polyribosomes in cooperation with other RNA-binding proteins. The immunoprecipitation of mStaufen- and FMRP-containing mRNPs provided additional evidence that the anti-Puralpha detected structurally or functionally related mRNA subsets, which are distributed in the somatodendritic compartment. Furthermore, mRNPs appear to reside on rough endoplasmic reticulum equipped with a kinesin motor. Based on our present findings, we propose that this rough endoplasmic reticulum structure may form the molecular machinery that mediates and regulates multistep transport of polyribosomes along microtubules and actin filaments, as well as localized translation in the somatodendritic compartment.

Molecular motors involved in chromaffin cell secretion

Neurosecretory cells, including chromaffin cells, possess a mesh of filamentous actin underneath the plasma membrane. It has been proposed that filamentous actin network separates the secretory vesicles into two compartments: the reserve pool and the release-ready vesicle pool. Disassembly of chromaffin cell cortical filamentous actin in response to stimulation allows the movement of vesicles from the reserve pool into the release-ready vesicle pool. Electron microscopy of cytoskeletons revealed the presence of polygonal areas almost devoid of actin filaments in stimulated cells. The percentage of stimulated cells showing disrupted cytoskeleton correlates well with the increase in secretion in these cells. Fine filaments also remain in these areas of disassembly, and these reacted with actin antibodies, as demonstrated by immunogold staining. In addition, the movement of vesicles between pools requires Ca(2+) and ATP, a condition for activation of a molecular motor. Confocal microscopy images demonstrated colocalization of myosin Va with dopamine-beta-hydroxylase. Cell depolarization induced the dissociation of myosin Va from chromaffin vesicles. 2,3-Butadione-2-monoxime (BDM), an inhibitor of myosin ATPase, inhibited secretion, suggesting a blockage for chromaffin vesicle transport between the reserve pool and the release-ready vesicle pool. On the other hand, myosin II subcellular distribution was not affected by cell depolarization. Confocal microscopy images show myosin II to be localized in the cell cortex and in some perinuclear structures. Chromaffin vesicles were not stained by myosin II antibody.

Myosin Va is locally synthesized following nerve injury

The presence of Myosin Va (an actin-based molecular motor) in the peripheral nervous system was examined and its subcellular distribution within the axons of the sciatic nerve was demonstrated via immunocytochemistry. Myosin Va (M-Va) in the nerve was detected by using SDS-PAGE and Western blot techniques with a polyclonal antibody specifically raised against the M-Va globular tail domain. In addition, purification of M-Va from the rat sciatic nerve prior to immunoblotting yielded a M-Va standard band. Likewise, optical immunocytochemical procedures revealed the presence of M-Va, particularly in the cortical axoplasmic territory, but also in the Schwann cell soma. The above experiments were carried out both on intact as well as on severed sciatic nerves with similar results. The proximal stumps of severed sciatic nerves (from 0 to 72 h after injury) were labelled in vivo with (35)S-methionine. SDS-PAGE autoradiography of the immunoabsorbed M-Va from the radiolabelled homogenized nerve tissue showed a significant increment of the radioactive intensity of M-Va heavy chain band through time. Moreover, a significant increment of transcripts coding for M-Va heavy chain was detected through time using RT-PCR after nerve injury and compared to intact nerves. This data suggest that M-Va is up-regulated in a time-dependent manner. The latter suggests a possible involvement of M-Va in nerve regeneration processes.

Human myosin-Vc is a novel class V myosin expressed in epithelial cells

Class V myosins are one of the most ancient and widely distributed groups of the myosin superfamily and are hypothesized to function as motors for actin-dependent organelle transport. We report the discovery and initial characterization of a novel member of this family, human myosin-Vc (Myo5c). The Myo5c protein sequence shares ∼50% overall identity with the two other class V myosins in vertebrates, myosin-Va (Myo5a) and myosin-Vb (Myo5b). Systematic analysis of the mRNA and protein distribution of these myosins indicates that Myo5a is most abundant in brain, whereas Myo5b and Myo5c are expressed chiefly in non-neuronal tissues. Myo5c is particularly abundant in epithelial and glandular tissues including pancreas, prostate, mammary,stomach, colon and lung. Immunolocalization in colon and exocrine pancreas indicates that Myo5c is expressed chiefly in epithelial cells. A dominant negative approach using a GFP-Myo5c tail construct in HeLa cells reveals that the Myo5c tail selectively colocalizes with and perturbs a membrane compartment containing the transferrin receptor and rab8. Transferrin also accumulates in this compartment, suggesting that Myo5c is involved in transferrin trafficking. As a class V myosin of epithelial cells, Myo5c is likely to power actin-based membrane trafficking in many physiologically crucial tissues of the human body.

Biochemical and morphological characterization of an intracellular membrane compartment containing AMPA receptors

AMPA receptors cycle rapidly in and out of the postsynaptic membrane, while NMDA receptors are relatively immobile. Changing the distribution of AMPA receptors between intracellular and surface synaptic pools is an important means of controlling synaptic strength. However, little is known about the intracellular membrane compartments of neurons that contain AMPA receptors. Here we describe biochemical and morphological characteristics of an intracellular pool of AMPA receptors in rat brain. By velocity gradient centrifugation of microsomal light membranes from rat brain, we identified a membrane fraction enriched for AMPA receptor subunits GluR2/3 but lacking NMDA receptors. This membrane compartment sedimented more slowly than synaptosomes but faster than synaptic vesicles and cofractionated with GRIP, PICK-1 and syntaxin-13. Morphological examination of this fraction revealed round and tubular vesicles ranging from approximately 50 to 300 nm in diameter. Immunocytochemistry of cultured hippocampal neurons showed that a significant portion of AMPA receptors colocalized with syntaxin-13 (a SNARE protein associated with tubulovesicular recycling endosomes) and with transferrin receptors. Taken together, these results suggest that a pool of intracellular GluR2/3 resides in a syntaxin 13-positive tubulovesicular membrane compartment, which might serve as a reservoir for the dendritic recycling of AMPA receptors.

Process formation results from the imbalance between motor-mediated forces

Several reports have suggested that neurite outgrowth is mediated by opposing forces generated on microtubules and microfilaments but the molecular basis underlying these forces have not been determined. Here, we show that in non-neuronal cell lines, the inhibition of actomyosin activity by acidic calponin promotes the formation of processes. This effect is blocked by inhibition of the motor activity of cytoplasmic dynein. Therefore, neurite formation is due to an imbalance between tensile and compressive forces mediated by myosins and dyneins, respectively. We propose a mechanism that involves the motor-mediated forces in a tight regulation of the process formation.

High affinity binding of brain myosin-Va to F-actin induced by calcium in the presence of ATP

Brain myosin-Va consists of two heavy chains, each containing a neck domain with six tandem IQ motifs that bind four to five calmodulins and one to two essential light chains. Previous studies demonstrated that myosin-Va exhibits an unusually high affinity for F-actin in the presence of ATP and that its MgATPase activity is stimulated by micromolar Ca(2+) in a highly cooperative manner. We demonstrate here that Ca(2+) also induces myosin-Va binding to and cosedimentation with F-actin in the presence of ATP in a similar cooperative manner and calcium concentration range as that observed for the ATPase activity. Neither hydrolysis of ATP nor buildup of ADP was required for Ca(2+)-induced cosedimentation. The Ca(2+)-induced binding was inhibited by low temperature or by 0.6 m NaCl, but not by 1% Triton X-100. Tight binding between myosin-Va and pyrene-labeled F-actin in the presence of ATP and Ca(2+) was also detected by quenching of the pyrene fluorescence. Negatively stained preparations of actomyosin-Va under Ca(2+)-induced binding conditions showed tightly packed F-actin bundles cross-linked by myosin-Va. Our data demonstrate that high affinity binding of myosin-Va and F-actin in the presence of ATP or 5'-O-(thiotriphosphate) is induced by micromolar concentrations of Ca(2+). Since Ca(2+) regulates both the actin binding properties and actin-activated ATPase of myosin-Va over the same concentration range, we suggest that the calcium signal may regulate the mechanism of processivity of myosin Va.

Real-time visualization of processive myosin 5a-mediated vesicle movement in living astrocytes

Recycling endosomes in astrocytes show hormone-regulated, actin fiber-dependent delivery to the endosomal sorting pool. Recycling vesicle trafficking was followed in real time using a fusion protein composed of green fluorescent protein coupled to the 29-kDa subunit of the short-lived, membrane-bound enzyme type 2 deiodinase. Primary endosomes budded from the plasma membrane and oscillated near the cell periphery for 1-4 min. The addition of thyroid hormone triggered the processive, centripetal movement of the recycling vesicle in linear bursts at velocities of up to 200 nm/s. Vesicle migration was hormone-specific and blocked by inhibitors of actin polymerization and myosin ATPase. Domain mapping confirmed that the hormone-dependent vesicle-binding domain was located at the C terminus of the motor. In addition, the interruption of normal dimerization of native myosin 5a monomers inactivated vesicle transport, indicating that single-headed myosin 5a motors do not transport cargo in situ. This is the first demonstration of processive hormone-dependent myosin 5a movement in living cells.

Identification of two type V myosins in fission yeast, one of which functions in polarized cell growth and moves rapidly in the cell

We characterized the novel Schizosaccharomyces pombe genes myo4(+) and myo5(+), both of which encode myosin-V heavy chains. Disruption of myo4 caused a defect in cell growth and led to an abnormal accumulation of secretory vesicles throughout the cytoplasm. The mutant cells were rounder than normal, although the sites for cell polarization were still established. Elongation of the cell ends and completion of septation required more time than in wild-type cells, indicating that Myo4 functions in polarized growth both at the cell ends and during septation. Consistent with this conclusion, Myo4 was localized around the growing cell ends, the medial F-actin ring, and the septum as a cluster of dot structures. In living cells, the dots of green fluorescent protein-tagged Myo4 moved rapidly around these regions. The localization and movement of Myo4 were dependent on both F-actin cables and its motor activity but seemed to be independent of microtubules. Moreover, the motor activity of Myo4 was essential for its function. These results suggest that Myo4 is involved in polarized cell growth by moving with a secretory vesicle along the F-actin cables around the sites for polarization. In contrast, the phenotype of myo5 null cells was indistinguishable from that of wild-type cells. This and other data suggest that Myo5 has a role distinct from that of Myo4.

Dynamics of integrin clustering at focal contacts of endothelial cells studied by multimode imaging microscopy

Human umbilical vein endothelial cells were stained with FITC-labeled anti-β1 integrin antibody and plated on a glass cover slip to elucidate the mechanism of integrin clustering during focal contact formation. The process of integrin clustering was observed by time-lapse total-internal-reflection fluorescence microscopy, which can selectively visualize the labeled integrins at the basal surface of living cells. The clustering of integrins at focal contacts started at 1 hour after plating and individual clusters kept growing for ∼6 hours. Most integrin clusters (∼80%) elongated towards the cell center or along the cell margin at a rate of 0.29±0.24 μm minute−1. Photobleaching and recovery experiments with evanescent illumination revealed that the integrins at the extending tip of the clusters were supplied from the intracellular space. Simultaneous time-lapse imaging of exocytosis of integrin-containing vesicles and elongating focal contacts showed that most exocytosis occurred at or near the focal contacts followed by their elongation. Double staining of F-actins and integrins demonstrated that stress fibers were located near the integrin clusters and that intracellular punctate integrins were associated with these stress fibers. These results suggest that the clustering of integrins is mediated by actin-fiber-dependent translocation of integrins to the extending tip of focal contacts.

Myosin Va bound to phagosomes binds to F-actin and delays microtubule-dependent motility

We established a light microscopy-based assay that reconstitutes the binding of phagosomes purified from mouse macrophages to preassembled F-actin in vitro. Both endogenous myosin Va from mouse macrophages and exogenous myosin Va from chicken brain stimulated the phagosome-F-actin interaction. Myosin Va association with phagosomes correlated with their ability to bind F-actin in an ATP-regulated manner and antibodies to myosin Va specifically blocked the ATP-sensitive phagosome binding to F-actin. The uptake and retrograde transport of phagosomes from the periphery to the center of cells in bone marrow macrophages was observed in both normal mice and mice homozygous for the dilute-lethal spontaneous mutation (myosin Va null). However, in dilute-lethal macrophages the accumulation of phagosomes in the perinuclear region occurred twofold faster than in normal macrophages. Motion analysis revealed saltatory phagosome movement with temporarily reversed direction in normal macrophages, whereas almost no reversals in direction were observed in dilute-lethal macrophages. These observations demonstrate that myosin Va mediates phagosome binding to F-actin, resulting in a delay in microtubule-dependent retrograde phagosome movement toward the cell center. We propose an "antagonistic/cooperative mechanism" to explain the saltatory phagosome movement toward the cell center in normal macrophages.

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.

Myosin learns to walk

Recent experiments, drawing upon single-molecule, solution kinetic and structural techniques, have clarified our mechanistic understanding of class V myosins. The findings of the past two years can be summarized as follows: (1) Myosin V is a highly efficient processive motor, surpassing even conventional kinesin in the distance that individual molecules can traverse. (2) The kinetic scheme underlying ATP turnover resembles those of myosins I and II but with rate constants tuned to favor strong binding to actin. ADP release precedes dissociation from actin and is rate-limiting in the cycle. (3) Myosin V walks in strides averaging ∼36 nm, the long pitch pseudo-repeat of the actin helix, each step coupled to a single ATP hydrolysis. Such a unitary displacement, the largest molecular step size measured to date, is required for a processive myosin motor to follow a linear trajectory along a helical actin track.

Regulation of molecular motor proteins

Motor proteins in the kinesin, dynein, and myosin superfamilies are tightly regulated to perform multiple functions in the cell requiring force generation. Although motor proteins within families are diverse in sequence and structure, there are general mechanisms by which they are regulated. We first discuss the regulation of the subset of kinesin family members for which such information exists, and then address general mechanisms of kinesin family regulation. We review what is known about the regulation of axonemal and cytoplasmic dyneins. Recent work on cytoplasmic dynein has revealed the existence of multiple isoforms for each dynein chain, making the study of dynein regulation more complicated than previously realized. Finally, we discuss the regulation of myosins known to be involved in membrane trafficking. Myosins and kinesins may be evolutionarily related, and there are common themes of regulation between these two classes of motors.

Hippocampal synaptic transmission and plasticity are preserved in myosin Va mutant mice

Recent studies have identified myosin Va as an organelle motor that may have important functions in neurons. Abundantly expressed at the hippocampal postsynaptic density, it interacts with protein complexes involved in synaptic plasticity. It is also located in presynaptic terminals and may function to recruit vesicles in the reserve pool to the active zone. Dilute-lethal mice are spontaneous myosin Va mutants and have severe neurological symptoms. We studied hippocampal physiology at CA3-CA1 excitatory synapses in dilute-lethal mutant mice to test the hypothesis that myosin Va plays a role in pre- or postsynaptic elements of synaptic transmission. In all assays performed, the mutant synapses appeared to be functioning normally, both pre- and postsynaptically. These data suggest that myosin Va is not essential for the synaptic release machinery, postsynaptic receptor composition, or plasticity at this synapse, but does not exclude significant roles for myosin Va in other cell types nor potential compensation by other myosin V isoforms.

Regulation and expression of metazoan unconventional myosins

Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3-16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplification. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

Squid p196, a new member of the myosin-V class of motor proteins, is associated with motile axoplasmic organelles

Axoplasmic organelles obtained from the squid giant axon move on actin filaments at an average velocity of 1 microm/s [Nature 356 (1992) 722]. The unconventional myosins, in particular, the myosin-V class of motor proteins, represent the most likely candidates to have a role in this motility. Experiments were performed to determine whether a member of the myosin-V class of unconventional myosins is present in axoplasm and optic lobes. Western blots of axoplasm probed with an affinity purified antibody to chicken brain myosin-V (CBM-V) showed cross-reactivity with a protein of Mr 196 kD (p196) which was subsequently purified from squid optic lobes using a modification of a protocol for the purification of CBM-V [Methods Enzymol. 298 (1998) 3; Cell 75 (1993) 215]. Western blots of CBM-V probed with an alpha-p196 polyclonal IgG showed cross-reactivity with CBM-V. Purified p196 has been found to be a calmodulin (CaM) binding protein that possesses calcium-stimulated actin-activated ATPase activity. Equilibrium density fractionation of motile axoplasmic organelle preparations has revealed that p196 cosedimented with the peak organelle fraction into Percoll gradients in the presence of cytochalasin B and ATP. Based on this evidence, we conclude that the p196 present in axoplasm and purified from optic lobes is a squid homolog of CBM-V and functions as a motor for fast transport of membranous organelles on actin filaments in neurons.

Models of motor-assisted transport of intracellular particles

One-dimensional models are presented for the macroscopic intracellular transport of vesicles and organelles by molecular motors on a network of aligned intracellular filaments. A motor-coated vesicle or organelle is described as a diffusing particle binding intermittently to filaments, when it is transported at the motor velocity. Two models are treated in detail: 1) a unidirectional model, where only one kind of motor is operative and all filaments have the same polarity; and 2) a bidirectional model, in which filaments of both polarities exist (for example, a randomly polarized actin network for myosin motors) and/or particles have plus-end and minus-end motors operating on unipolar filaments (kinesin and dynein on microtubules). The unidirectional model provides net particle transport in the absence of a concentration gradient. A symmetric bidirectional model, with equal mixtures of filament polarities or plus-end and minus-end motors of the same characteristics, provides rapid transport down a concentration gradient and enhanced dispersion of particles from a point source by motor-assisted diffusion. Both models are studied in detail as a function of the diffusion constant and motor velocity of bound particles, and their rates of binding to and detachment from filaments. These models can form the basis of more realistic models for particle transport in axons, melanophores, and the dendritic arms of melanocytes, in which networks of actin filaments and microtubules coexist and motors for both types of filament are implicated.

The HPV16 E5 oncogene inhibits endocytic trafficking

The small hydrophobic E5 protein of Human Papillomavirus type 16 (HPV16) binds to the 16-kDa subunit of the V-H+-ATPase. This binding has been suggested to interfere with acidification of late endocytic structures. We here used video microscopy, ratio imaging and confocal microscopy of living C127 fibroblasts to study the effects of E5. Various endocytic markers including the pH-sensitive probe DM-NERF coupled to dextran, TransFluoSpheres and TRITC-concanavalin A, were applied. In E5-transfected cells, none of these markers colocalized with the membrane permeable probe LysoTracker Red, which accumulates in acidic, late endocytic structures, or with a green fluorescent version of the small GTPase Rab7 labeling late endocytic structures. Importantly, however, late endocytic structures accumulating LysoTracker were still present in the E5-transfected cells. It is therefore concluded that HPV16 E5 perturbs trafficking from early to late endocytic structures rather than acidification.

Myosin V plays an essential role in the thyroid hormone-dependent endocytosis of type II iodothyronine 5'-deiodinase

In astrocytes, thyroxine modulates type II iodothyronine 5'-deiodinase levels by initiating the binding of the endosomes containing the enzyme to microfilaments, followed by actin-based endocytosis. Myosin V is a molecular motor thought to participate in vesicle trafficking in the brain. In this report, we developed an in vitro actin-binding assay to characterize the thyroid hormone-dependent binding of endocytotic vesicles to microfilaments. Thyroxine and reverse triiodothyronine (EC(50) levels approximately 1 nm) were >100-fold more potent than 3,5,3'-triiodothyronine in initiating vesicle binding to actin fibers in vitro. Thyroxine-dependent vesicle binding was calcium-, magnesium-, and ATP-dependent, suggesting the participation of one or more myosin motors, presumably myosin V. Addition of the myosin V globular tail, lacking the actin-binding head, specifically blocked thyroid hormone-dependent vesicle binding, and direct binding of the myosin V tail to enzyme-containing endosomes was thyroxine-dependent. Progressive NH(2)-terminal deletion of the myosin V tail and domain-specific antibody inhibition studies revealed that the thyroxine-dependent vesicle-tethering domain was localized to the last 21 amino acids of the COOH terminus. These data show that myosin V is responsible for thyroid hormone-dependent binding of primary endosomes to the microfilaments and suggest that this motor mediates the actin-based endocytosis of the type II iodothyronine deiodinase.

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.

Direct observation of processive movement by individual myosin V molecules

Myosin V is an unconventional myosin thought to move processively along actin filaments. To have hard evidence for the high processivity, we sought to observe directly the movement by individual native chick brain myosin V (BMV) molecules with fluorescent calmodulin. Single BMV molecules did exhibit highly processive movement along actin filaments fixed to a coverslip. BMV continued to move up to the barbed end of its actin track, and did not readily detach from action. The barbed end, therefore, got brighter with time, because of a constant stream of BMV traffic. The maximum speed of the processive movement was 1 microm/s, and the maximum actin-activated ATPase rate was 2.4 s(-1). These values apparently imply that BMV travels a great distance, 400 nm, per an ATPase cycle.

Role of actin cortex in the subplasmalemmal transport of secretory granules in PC-12 cells

In neuroendocrine PC-12 cells, evanescent-field fluorescence microscopy was used to track motions of green fluorescent protein (GFP)-labeled actin or GFP-labeled secretory granules in a thin layer of cytoplasm where cells adhered to glass. The layer contained abundant filamentous actin (F-actin) locally condensed into stress fibers. More than 90% of the granules imaged lay within the F-actin layer. One-third of the granules did not move detectably, while two-thirds moved randomly; the average diffusion coefficient was 23 x 10(-4) microm(2)/s. A small minority (<3%) moved rapidly and in a directed fashion over distances more than a micron. Staining of F-actin suggests that such movement occurred along actin bundles. The seemingly random movement of most other granules was not due to diffusion since it was diminished by the myosin inhibitor butanedione monoxime, and blocked by chelating intracellular Mg(2+) and replacing ATP with AMP-PNP. Mobility was blocked also when F-actin was stabilized with phalloidin, and was diminished when the actin cortex was degraded with latrunculin B. We conclude that the movement of granules requires metabolic energy, and that it is mediated as well as limited by the actin cortex. Opposing actions of the actin cortex on mobility may explain why its degradation has variable effects on secretion.

The actin cytoskeleton and neurotransmitter release: an overview

Here we review evidence that actin and its binding partners are involved in the release of neurotransmitters at synapses. The spatial and temporal characteristics of neurotransmitter release are determined by the distribution of synaptic vesicles at the active zones, presynaptic sites of secretion. Synaptic vesicles accumulate near active zones in a readily releasable pool that is docked at the plasma membrane and ready to fuse in response to calcium entry and a secondary, reserve pool that is in the interior of the presynaptic terminal. A network of actin filaments associated with synaptic vesicles might play an important role in maintaining synaptic vesicles within the reserve pool. Actin and myosin also have been implicated in the translocation of vesicles from the reserve pool to the presynaptic plasma membrane. Refilling of the readily releasable vesicle pool during intense stimulation of neurotransmitter release also implicates synapsins as reversible links between synaptic vesicles and actin filaments. The diversity of actin binding partners in nerve terminals suggests that actin might have presynaptic functions beyond synaptic vesicle tethering or movement. Because most of these actin-binding proteins are regulated by calcium, actin might be a pivotal participant in calcium signaling inside presynaptic nerve terminals. However, there is no evidence that actin participates in fusion of synaptic vesicles.

Functions of unconventional myosins

To date, fourteen classes of unconventional myosins have been identified. Recent reports have implicated a number of these myosins in organelle transport, and in the formation, maintenance and/or dynamics of actin-rich structures involved in a variety of cellular processes including endocytosis, cell migration, and sensory transduction. Characterizations of organelle dynamics in pigment cells and neurons have further defined the contributions made by unconventional myosins and microtubule motors to the transport and distribution of organelles. Several studies have provided evidence of complexes through which cooperative organelle transport may be coordinated. Finally, the myosin superfamily has been shown to contain at least one processive motor and one backwards motor.

Characterization of myosin V binding to brain vesicles

Myosin II and V are important for the generation and segregation of subcellular compartments. We observed that vesicular myosin II and V were associated with the protein scaffolding of a common subset of vesicles by density sedimentation, electron microscopy, and immunofluorescence. Solubilization of either myosin II or V was caused by polyphosphates with the following efficacy at 10 mM: for myosin II ATP-Mg(2+) = ATP = AMP-PNP (5'-adenylyl imidodiphosphate) > pyrophosphate = tripolyphosphate > tetrapolyphosphate = ADP > cAMP = Mg(2+); and for myosin V pyrophosphate = tripolyphosphate > ATP-Mg(2+) = ATP = AMP-PNP > ADP = tetrapolyphosphate > cAMP = Mg(2+). Consequently, we suggest solubilization was not an effect of phosphorylation, hydrolysis, or disassociation of myosin from actin filaments. Scatchard analysis of myosin V binding to stripped dense vesicles showed saturable binding with a K(m) of 10 nM. Analysis of native vesicles indicates that these sites are fully occupied. Together, these data show there are over 100 myosin Vs/vesicle (100-nm radius). We propose that polyphosphate anions bind to myosin II and V and induce a conformational change that disrupts binding to a receptor.

Unconventional myosins: anchors in the membrane traffic relay

The family of unconventional myosins is ever growing and the functions attributed to them seem to expand in parallel. These actin-based motor proteins have been implicated in processes as seemingly diverse as endocytosis and exocytosis, the transport of organelles, in spermatogenesis and in neurosensory functions such as hearing and sight. A common myosin function may underlie them all--the regulation of intracellular membrane traffic.

Cooperation between microtubule- and actin-based motor proteins

Organelle transport has been proposed to proceed in two steps: long-range transport along microtubules and local delivery via actin filaments. This model is supported by recent studies of pigment transport in several cell types and transport in neurons, and in several cases, class V myosin has been implicated as the actin-based motor. Mutations in mice (dilute) and yeast (myo2) have also implicated this class of myosin in organelle transport, and genetic interactions in yeast have indicated that a kinesin-related protein (Smy1p) plays a supporting role. This link between members of two different motor superfamilies has now taken a surprising turn: There is evidence for a physical interaction between class V myosins and kinesin or Smy1p in both mice and yeast.

Kinetic characterization of a monomeric unconventional myosin V construct

An expressed, monomeric murine myosin V construct composed of the motor domain and two calmodulin-binding IQ motifs (MD(2IQ)) was used to assess the regulatory and kinetic properties of this unconventional myosin. In EGTA, the actin-activated ATPase activity of MD(2IQ) was 7.4 +/- 1.6 s(-1) with a K(app) of approximately 1 microM (37 degrees C), and the velocity of actin movement was approximately 0.3 micrometer/s (30 degrees C). Calcium inhibited both of these activities, but the addition of calmodulin restored the values to approximately 70% of control, indicating that calmodulin dissociation caused inhibition. In contrast to myosin II, MD(2IQ) is highly associated with actin at physiological ionic strength in the presence of ATP, but the motor is in a weakly bound conformation based on the pyrene-actin signal. The rate of dissociation of acto-MD(2IQ) by ATP is fast (>850 s(-1)), and ATP hydrolysis occurs at approximately 200 s(-1). The affinity of acto-MD(2IQ) for ADP is somewhat higher than that of smooth S1, and ADP dissociates more slowly. Actin does not cause a large increase in the rate of ADP release, nor does the presence of ADP appreciably alter the affinity of MD(2IQ) for actin. These kinetic data suggest that monomeric myosin V is not processive.

Myosin Va movements in normal and dilute-lethal axons provide support for a dual filament motor complex

To investigate the role that myosin Va plays in axonal transport of organelles, myosin Va-associated organelle movements were monitored in living neurons using microinjected fluorescently labeled antibodies to myosin Va or expression of a green fluorescent protein-myosin Va tail construct. Myosin Va-associated organelles made rapid bi-directional movements in both normal and dilute-lethal (myosin Va null) neurites. In normal neurons, depolymerization of microtubules by nocodazole slowed, but did not stop movement. In contrast, depolymerization of microtubules in dilute-lethal neurons stopped movement. Myosin Va or synaptic vesicle protein 2 (SV2), which partially colocalizes with myosin Va on organelles, did not accumulate in dilute-lethal neuronal cell bodies because of an anterograde bias associated with organelle transport. However, SV2 showed peripheral accumulations in axon regions of dilute-lethal neurons rich in tyrosinated tubulin. This suggests that myosin Va-associated organelles become stranded in regions rich in dynamic microtubule endings. Consistent with these observations, presynaptic terminals of cerebellar granule cells in dilute-lethal mice showed increased cross-sectional area, and had greater numbers of both synaptic and larger SV2 positive vesicles. Together, these results indicate that myosin Va binds to organelles that are transported in axons along microtubules. This is consistent with both actin- and microtubule-based motors being present on these organelles. Although myosin V activity is not necessary for long-range transport in axons, myosin Va activity is necessary for local movement or processing of organelles in regions, such as presynaptic terminals that lack microtubules.

Myosin-V is a processive actin-based motor

Class-V myosins, one of 15 known classes of actin-based molecular motors, have been implicated in several forms of organelle transport, perhaps working with microtubule-based motors such as kinesin. Such movements may require a motor with mechanochemical properties distinct from those of myosin-II, which operates in large ensembles to drive high-speed motility as in muscle contraction. Based on its function and biochemistry, it has been suggested that myosin-V may be a processive motor like kinesin. Processivity means that the motor undergoes multiple catalytic cycles and coupled mechanical advances for each diffusional encounter with its track. This allows single motors to support movement of an organelle along its track. Here we provide direct evidence that myosin-V is indeed a processive actin-based motor that can move in large steps approximating the 36-nm pseudo-repeat of the actin filament.

How many is enough? Exploring the myosin repertoire in the model eukaryote Dictyostelium discoideum

The cytoplasm of eukaryotic cells is a very complex milieu and unraveling how its unique cytoarchitecture is achieved and maintained is a central theme in modern cell biology. It is crucial to understand how organelles and macro-complexes of RNA and/or proteins are transported to and/or maintained at their specific cellular locations. The importance of filamentous-actin-directed myosin-powered cargo transport was only recently realized, and after an initial explosion in the identification of new molecules, the field is now concentrating on their functional dissection. Direct connections of myosins to a variety of cellular tasks are now slowly emerging, such as in cytokinesis, phagocytosis, endocytosis, polarized secretion and exocytosis, axonal transport, etc. Unconventional myosins have been identified in a wide variety of organisms, making the presence of actin and myosins a hallmark of eukaryotism. The genome of S. cerevisiae encodes only five myosins, whereas a mammalian cell has the capacity to express between two and three dozen myosins. Why is it so crucial to arrive at this final census? The main questions that we would like to discuss are the following. How many distinct myosin-powered functions are carried out in a typical higher eukaryote? Or, in other words, what is the minimal set of myosins essential to accomplish the multitude of tasks related to motility and intracellular dynamics in a multicellular organism? And also, as a corollary, what is the degree of functional redundancy inside a given myosin class? In that respect, the choice of a model organism suitable for such an investigation is more crucial than ever. Here we argue that Dictyostelium discoideum is affirming its position as an ideal system of intermediate complexity to study myosin-powered trafficking and is or will soon become the second eukaryote for which complete knowledge of the whole repertoire of myosins is available.