Motor function and regulation of myosin X

Pathophysiology of human hearing loss associated with variants in myosins

Deleterious variants of more than one hundred genes are associated with hearing loss including MYO3A, MYO6, MYO7A and MYO15A and two conventional myosins MYH9 and MYH14. Variants of MYO7A also manifest as Usher syndrome associated with dysfunction of the retina and vestibule as well as hearing loss. While the functions of MYH9 and MYH14 in the inner ear are debated, MYO3A, MYO6, MYO7A and MYO15A are expressed in inner ear hair cells along with class-I myosin MYO1C and are essential for developing and maintaining functional stereocilia on the apical surface of hair cells. Stereocilia are large, cylindrical, actin-rich protrusions functioning as biological mechanosensors to detect sound, acceleration and posture. The rigidity of stereocilia is sustained by highly crosslinked unidirectionally-oriented F-actin, which also provides a scaffold for various proteins including unconventional myosins and their cargo. Typical myosin molecules consist of an ATPase head motor domain to transmit forces to F-actin, a neck containing IQ-motifs that bind regulatory light chains and a tail region with motifs recognizing partners. Instead of long coiled-coil domains characterizing conventional myosins, the tails of unconventional myosins have various motifs to anchor or transport proteins and phospholipids along the F-actin core of a stereocilium. For these myosins, decades of studies have elucidated their biochemical properties, interacting partners in hair cells and variants associated with hearing loss. However, less is known about how myosins traffic in a stereocilium using their motor function, and how each variant correlates with a clinical condition including the severity and onset of hearing loss, mode of inheritance and presence of symptoms other than hearing loss. Here, we cover the domain structures and functions of myosins associated with hearing loss together with advances, open questions about trafficking of myosins in stereocilia and correlations between hundreds of variants in myosins annotated in ClinVar and the corresponding deafness phenotypes.

Follicular fluid C3a-peptide promotes oocyte maturation through F-actin aggregation

BACKGROUND

Immature cumulus-oocyte complexes are retrieved to obtain mature oocytes by in vitro maturation (IVM), a laboratory tool in reproductive medicine to obtain mature oocytes. Unfortunately, the efficiency of IVM is not satisfactory. To circumvent this problem, we therefore intended to commence with the composition of ovarian follicular fluid (FF), an important microenvironment influencing oocyte growth. It is well known that FF has a critical role in oocyte development and maturation. However, the components in human FF remain largely unknown, particularly with regard to small molecular peptides.

RESULTS

In current study, the follicular fluid derived from human mature and immature follicles were harvested. The peptide profiles of FF were further investigated by using combined ultrafiltration and LC-MS/MS. The differential peptides were preliminary determined by performing differentially expressed analysis. Human and mouse oocyte culture were used to verify the influence of differential peptides on oocyte development. Constructing plasmids, cell transfecting, Co-IP, PLA etc. were used to reveal the detail molecular mechanism. The results from differentially expressed peptide as well as cultured human and mouse oocytes analyses showed that highly conserved C3a-peptide, a cleavage product of complement C3a, definitely affected oocytes development. Intriguingly, C3a-peptide possessed a novel function that promoted F-actin aggregation and spindle migration, raised the percentage of oocytes at the MII stage, without increasing the chromosome aneuploidy ratio, especially in poor-quality oocytes. These effects of C3a-peptide were attenuated by C3aR morpholino inhibition, suggesting that C3a-peptide affected oocytes development by collaborating with its classical receptor, C3aR. Specially, we found that C3aR co-localized to the spindle with β-tubulin to recruit F-actin toward the spindle and subcortical region of the oocytes through specific binding to MYO10, a key regulator for actin organization, spindle morphogenesis and positioning in oocytes.

CONCLUSIONS

Our results provide a new perspective for improving IVM culture systems by applying FF components and also provide molecular insights into the physiological function of C3a-peptide, its interaction with C3aR, and their roles in enabling meiotic division of oocytes.

Discovery of ultrafast myosin, its amino acid sequence, and structural features

Cytoplasmic streaming with extremely high velocity (∼70 μm s^-1) occurs in cells of the characean algae (Chara). Because cytoplasmic streaming is caused by myosin XI, it has been suggested that a myosin XI with a velocity of 70 μm s^-1, the fastest myosin measured so far, exists in Chara cells. However, the velocity of the previously cloned Chara corallina myosin XI (CcXI) was about 20 μm s^-1, one-third of the cytoplasmic streaming velocity in Chara Recently, the genome sequence of Chara braunii has been published, revealing that this alga has four myosin XI genes. We cloned these four myosin XI (CbXI-1, 2, 3, and 4) and measured their velocities. While the velocities of CbXI-3 and CbXI-4 motor domains (MDs) were similar to that of CcXI MD, the velocities of CbXI-1 and CbXI-2 MDs were 3.2 times and 2.8 times faster than that of CcXI MD, respectively. The velocity of chimeric CbXI-1, a functional, full-length CbXI-1 construct, was 60 μm s^-1 These results suggest that CbXI-1 and CbXI-2 would be the main contributors to cytoplasmic streaming in Chara cells and show that these myosins are ultrafast myosins with a velocity 10 times faster than fast skeletal muscle myosins in animals. We also report an atomic structure (2.8-Å resolution) of myosin XI using X-ray crystallography. Based on this crystal structure and the recently published cryo-electron microscopy structure of acto-myosin XI at low resolution (4.3-Å), it appears that the actin-binding region contributes to the fast movement of Chara myosin XI. Mutation experiments of actin-binding surface loops support this hypothesis.

Myosin-X is dispensable for spindle morphogenesis and positioning in the mouse oocyte

Off-center spindle positioning in mammalian oocytes enables asymmetric divisions in size, which are important for subsequent embryogenesis. The migration of the meiosis I spindle from the oocyte center to its cortex is mediated by F-actin. Specifically, an F-actin cage surrounds the microtubule spindle and applies forces to it. To better understand how F-actin transmits forces to the spindle, we studied a potential direct link between F-actin and microtubules. For this, we tested the implication of myosin-X, a known F-actin and microtubule binder involved in spindle morphogenesis and/or positioning in somatic cells, amphibian oocytes and embryos. Using a mouse strain conditionally invalidated for myosin-X in oocytes and by live-cell imaging, we show that myosin-X is not localized on the spindle, and is dispensable for spindle and F-actin assembly. It is not required for force transmission as spindle migration and chromosome alignment occur normally. More broadly, myosin-X is dispensable for oocyte developmental potential and female fertility. We therefore exclude a role for myosin-X in transmitting F-actin-mediated forces to the spindle, opening new perspectives regarding this mechanism in mouse oocytes, which differ from most mitotic cells.

Summary: Cortical spindle positioning in mammalian oocytes relies on the interplay between actin and the microtubule spindle. Myosin-X, an obvious candidate for linking these two cytoskeletal elements, is dispensable in mouse oocytes.

The myosin regulatory light chain Myl5 localizes to mitotic spindle poles and is required for proper cell division

Myosins are ATP-dependent actin-based molecular motors critical for diverse cellular processes like intracellular trafficking, cell motility, and cell invasion. During cell division, myosin MYO10 is important for proper mitotic spindle assembly, the anchoring of the spindle to the cortex, and positioning of the spindle to the cell mid-plane. However, myosins are regulated by myosin regulatory light chains (RLCs), and whether RLCs are important for cell division has remained unexplored. Here, we have determined that the previously uncharacterized myosin RLC Myl5 associates with the mitotic spindle and is required for cell division. We show that Myl5 localizes to the leading edge and filopodia during interphase and to mitotic spindle poles and spindle microtubules during early mitosis. Importantly, depletion of Myl5 led to defects in mitotic spindle assembly, chromosome congression, and chromosome segregation and to a slower transition through mitosis. Furthermore, Myl5 bound to MYO10 in vitro and co-localized with MYO10 at the spindle poles. These results suggest that Myl5 is important for cell division and that it may be performing its function through MYO10.

Cytoneme delivery of Sonic Hedgehog from ligand-producing cells requires Myosin 10 and a Dispatched-BOC/CDON co-receptor complex

Morphogens function in concentration-dependent manners to instruct cell fate during tissue patterning. The cytoneme morphogen transport model posits that specialized filopodia extend between morphogen-sending and responding cells to ensure that appropriate signaling thresholds are achieved. How morphogens are transported along and deployed from cytonemes, how quickly a cytoneme-delivered, receptor-dependent signal is initiated, and whether these processes are conserved across phyla are not known. Herein, we reveal that the actin motor Myosin 10 promotes vesicular transport of Sonic Hedgehog (SHH) morphogen in mouse cell cytonemes, and that SHH morphogen gradient organization is altered in neural tubes of Myo10^-/- mice. We demonstrate that cytoneme-mediated deposition of SHH onto receiving cells induces a rapid, receptor-dependent signal response that occurs within seconds of ligand delivery. This activity is dependent upon a novel Dispatched (DISP)-BOC/CDON co-receptor complex that functions in ligand-producing cells to promote cytoneme occurrence and facilitate ligand delivery for signal activation.

Roles of Myosin-Mediated Membrane Trafficking in TGF-β Signaling

Recent findings have revealed the role of membrane traffic in the signaling of transforming growth factor-β (TGF-β). These findings originate from the pivotal function of TGF-β in development, cell proliferation, tumor metastasis, and many other processes essential in malignancy. Actin and unconventional myosin have crucial roles in subcellular trafficking of receptors; research has also revealed a growing number of unconventional myosins that have crucial roles in TGF-β signaling. Unconventional myosins modulate the spatial organization of endocytic trafficking and tether membranes or transport them along the actin cytoskeletons. Current models do not fully explain how membrane traffic forms a bridge between TGF-β and the downstream effectors that produce its functional responsiveness, such as cell migration. In this review, we present a brief overview of the current knowledge of the TGF-β signaling pathway and the molecular components that comprise the core pathway as follows: ligands, receptors, and Smad mediators. Second, we highlight key role(s) of myosin motor-mediated protein trafficking and membrane domain segregation in the modulation of the TGF-β signaling pathway. Finally, we review future challenges and provide future prospects in this field.

Determination of Transglutaminase Activity in Plants

Transglutaminase (TGase:E.C. 2.3.2.13) catalyzes the acyl-transfer reaction between one or two primary amino groups of polyamines and protein-bound Gln residues giving rise to post-translational modifications. One increasing the positive charge on a proteins surface and the other results in the covalent crosslinking of proteins. Pioneering studies on TGase in plants started in the middle of the 1980's but the methodology designed for use with animal extracts was not directly applicable to plant extracts. Here we describe radioactive and colorimetric methods adapted to study plant TGase, as well as protocols to analyze the involvement of TGase and polyamines in the functionality of cytoskeletal proteins.

Myosin light chains: Teaching old dogs new tricks

The myosin holoenzyme is a multimeric protein complex consisting of heavy chains and light chains. Myosin light chains are calmodulin family members which are crucially involved in the mechanoenzymatic function of the myosin holoenzyme. This review examines the diversity of light chains within the myosin superfamily, discusses interactions between the light chain and the myosin heavy chain as well as regulatory and structural functions of the light chain as a subunit of the myosin holoenzyme. It covers aspects of the myosin light chain in the localization of the myosin holoenzyme, protein-protein interactions and light chain binding to non-myosin binding partners. Finally, this review challenges the dogma that myosin regulatory and essential light chain exclusively associate with conventional myosin heavy chains while unconventional myosin heavy chains usually associate with calmodulin.

Mouse myosin-19 is a plus-end-directed, high-duty ratio molecular motor

Class XIX myosin (Myo19) is a vertebrate-specific unconventional myosin, responsible for the transport of mitochondria. To characterize biochemical properties of Myo19, we prepared recombinant mouse Myo19-truncated constructs containing the motor domain and the IQ motifs using the baculovirus/Sf9 expression system. We identified regulatory light chain (RLC) of smooth muscle/non-muscle myosin-2 as the light chain of Myo19. The actin-activated ATPase activity and the actin-gliding velocity of Myo19-truncated constructs were about one-third and one-sixth as those of myosin-5a, respectively. The apparent affinity of Myo19 to actin was about the same as that of myosin-5a. The RLCs bound to Myo19 could be phosphorylated by myosin light chain kinase, but this phosphorylation had little effect on the actin-activated ATPase activity and the actin-gliding activity of Myo19-truncated constructs. Using dual fluorescence-labeled actin filaments, we determined that Myo19 is a plus-end-directed molecular motor. We found that, similar to that of the high-duty ratio myosin, such as myosin-5a, ADP release rate was comparable with the maximal actin-activated ATPase activity of Myo19, indicating that ADP release is a rate-limiting step for the ATPase cycle of acto-Myo19. ADP strongly inhibited the actin-activated ATPase activity and actin-gliding activity of Myo19-truncated constructs. Based on the above results, we concluded that Myo19 is a high-duty ratio molecular motor moving to the plus-end of the actin filament.

Molecular characterization and subcellular localization of Arabidopsis class VIII myosin, ATM1

Land plants possess myosin classes VIII and XI. Although some information is available on the molecular properties of class XI myosins, class VIII myosins are not characterized. Here, we report the first analysis of the enzymatic properties of class VIII myosin. The motor domain of Arabidopsis class VIII myosin, ATM1 (ATM1-MD), and the motor domain plus one IQ motif (ATM1-1IQ) were expressed in a baculovirus system and characterized. ATM1-MD and ATM1-1IQ had low actin-activated Mg(2+)-ATPase activity (Vmax = 4 s(-1)), although their affinities for actin were high (Kactin = 4 μM). The actin-sliding velocities of ATM1-MD and ATM1-1IQ were 0.02 and 0.089 μm/s, respectively, from which the value for full-length ATM1 is calculated to be ∼0.2 μm/s. The results of actin co-sedimentation assay showed that the duty ratio of ATM1 was ∼90%. ADP dissociation from the actin·ATM1 complex (acto-ATM1) was extremely slow, which accounts for the low actin-sliding velocity, low actin-activated ATPase activity, and high duty ratio. The rate of ADP dissociation from acto-ATM1 was markedly biphasic with fast and slow phase rates (5.1 and 0.41 s(-1), respectively). Physiological concentrations of free Mg(2+) modulated actin-sliding velocity and actin-activated ATPase activity by changing the rate of ADP dissociation from acto-ATM1. GFP-fused full-length ATM1 expressed in Arabidopsis was localized to plasmodesmata, plastids, newly formed cell walls, and actin filaments at the cell cortex. Our results suggest that ATM1 functions as a tension sensor/generator at the cell cortex and other structures in Arabidopsis.

DPP6 regulation of dendritic morphogenesis impacts hippocampal synaptic development

Dipeptidyl-peptidase 6 (DPP6) is an auxiliary subunit of Kv4-mediated A-type K⁺ channels that, in addition to enhancing channel surface expression, potently accelerates their kinetics. The DPP6 gene has been associated with a number of human CNS disorders including ASDs and schizophrenia. Here we employ knockdown and genetic deletion of DPP6 to reveal its importance for the formation and stability of dendritic filopodia during early neuronal development. We find that hippocampal neurons lacking DPP6 show a sparser dendritic branching pattern along with fewer spines throughout development and into adulthood. In electrophysiological and imaging experiments we show that these deficits lead to fewer functional synapses and occur independently of the potassium channel subunit Kv4.2. We report that the extracellular domain of DPP6 interacts with a filopodia-associated myosin as well as with fibronectin in the extracellular matrix. DPP6 therefore plays an unexpected but important role in cell-adhesion and motility, impacting hippocampal synaptic development and function.

A cancer-associated mutation in atypical protein kinase Cι occurs in a substrate-specific recruitment motif

Atypical protein kinase Cι (PKCι) has roles in cell growth, cellular polarity, and migration, and its abundance is frequently increased in cancer. We identified a protein interaction surface containing a dibasic motif (RIPR) that bound a distinct subset of PKCι substrates including lethal giant larvae 2 (LLGL2) and myosin X, but not other substrates such as Par3. Further characterization demonstrated that Arg471 in this motif was important for binding to LLGL2, whereas Arg474 was critical for interaction with myosin X, indicating that multiple complexes could be formed through this motif. A somatic mutation of the dibasic motif (R471C) was the most frequent mutation of PKCι in human cancer, and the intact dibasic motif was required for normal polarized epithelial morphogenesis in three-dimensional cysts. Thus, the R471C substitution is a change-of-function mutation acting at this substrate-specific recruitment site to selectively disrupt the polarizing activity of PKCι.

Myosin X and its motorless isoform differentially modulate dendritic spine development by regulating trafficking and retention of vasodilator-stimulated phosphoprotein

Myosin X (Myo10) is an unconventional myosin with two known isoforms: full-length (FL)-Myo10 that has motor activity, and a recently identified brain-expressed isoform, headless (Hdl)-Myo10, which lacks most of the motor domain. FL-Myo10 is involved in the regulation of filopodia formation in non-neuronal cells; however, the biological function of Hdl-Myo10 remains largely unknown. Here, we show that FL- and Hdl-Myo10 have important, but distinct, roles in the development of dendritic spines and synapses in hippocampal neurons. FL-Myo10 induces formation of dendritic filopodia and modulates filopodia dynamics by trafficking the actin-binding protein vasodilator-stimulated phosphoprotein (VASP) to the tips of filopodia. By contrast, Hdl-Myo10 acts on dendritic spines to enhance spine and synaptic density as well as spine head expansion by increasing the retention of VASP in spines. Thus, this study demonstrates a novel biological function for Hdl-Myo10 and an important new role for both Myo10 isoforms in the development of dendritic spines and synapses.

The role of actin and myosin in PtK2 spindle length changes induced by laser microbeam irradiations across the spindle

This study investigates spindle biomechanical properties to better understand how spindles function. In this report, laser microbeam cutting across mitotic spindles resulted in movement of spindle poles toward the spindle equator. The pole on the cut side moved first, the other pole moved later, resulting in a shorter but symmetric spindle. Intervening spindle microtubules bent and buckled during the equatorial movement of the poles. Because of this and because there were no detectable microtubules within the ablation zone, other cytoskeletal elements would seem to be involved in the equatorial movement of the poles. One possibility is actin and myosin since pharmacological poisoning of the actin-myosin system altered the equatorial movements of both irradiated and unirradiated poles. Immunofluorescence microscopy confirmed that actin, myosin and monophosphorylated myosin are associated with spindle fibers and showed that some actin and monophosphorylated myosin remained in the irradiated regions. Overall, our experiments suggest that actin, myosin and microtubules interact to control spindle length. We suggest that actin and myosin, possibly in conjunction with the spindle matrix, cause the irradiated pole to move toward the equator and that cross-talk between the two half spindles causes the unirradiated pole to move toward the equator until a balanced length is obtained.

Myosin-X: a MyTH-FERM myosin at the tips of filopodia

Myosin-X (Myo10) is an unconventional myosin with MyTH4-FERM domains that is best known for its striking localization to the tips of filopodia and its ability to induce filopodia. Although the head domain of Myo10 enables it to function as an actin-based motor, its tail contains binding sites for several molecules with central roles in cell biology, including phosphatidylinositol (3,4,5)-trisphosphate, microtubules and integrins. Myo10 also undergoes fascinating long-range movements within filopodia, which appear to represent a newly recognized system of transport. Myo10 is also unusual in that it is a myosin with important roles in the spindle, a microtubule-based structure. Exciting new studies have begun to reveal the structure and single-molecule properties of this intriguing myosin, as well as its mechanisms of regulation and induction of filopodia. At the cellular and organismal level, growing evidence demonstrates that Myo10 has crucial functions in numerous processes ranging from invadopodia formation to cell migration.

Kinetic analysis reveals differences in the binding mechanism of calmodulin and calmodulin-like protein to the IQ motifs of myosin-10

Myo10 is an unconventional myosin with important functions in filopodial motility, cell migration, and cell adhesion. The neck region of Myo10 contains three IQ motifs that bind calmodulin (CaM) or the tissue-restricted calmodulin-like protein (CLP) as light chains. However, little is known about the mechanism of light chain binding to the IQ motifs in Myo10. Binding of CaM and CLP to each IQ motif was assessed by nondenaturing gel electrophoresis and by stopped-flow experiments using fluorescence-labeled CaM and CLP. Although the binding kinetics are different in each case, there are similarities in the mechanism of binding of CaM and CLP to IQ1 and IQ2: for both IQ motifs Ca(2+) increased the binding affinity, mainly by increasing the rate of the forward steps. The general kinetic mechanism comprises a two-step process, which in some cases may involve the binding of a second IQ motif with lower affinity. For IQ3, however, the kinetics of CaM binding is very different from that of CLP. In both cases, binding in the absence of Ca(2+) is poor, and addition of Ca(2+) decreases the K(d) to below 10 nM. However, while the CaM binding kinetics are complex and best fitted by a multistep model, binding of CLP is fitted by a relatively simple two-step model. The results show that, in keeping with growing structural evidence, complexes between CaM or CaM-like myosin light chains and IQ motifs are highly diverse and depend on the specific sequence of the particular IQ motif as well as the light chain.

Melanin transfer in human skin cells is mediated by filopodia--a model for homotypic and heterotypic lysosome-related organelle transfer

Transfer of the melanocyte-specific and lysosome-related organelle, the melanosome, from melanocytes to keratinocytes is crucial for the protection of the skin against harmful ultraviolet radiation (UVR)--our main physiological cutaneous stressor. However, this commonplace event remains a most enigmatic process despite several early hypotheses. Recently, we and others have proposed a role for filopodia in melanin transfer, although conclusive experimental proof remained elusive. Using known filopodial markers (MyoX/Cdc42) and the filopodial disrupter, low-dose cytochalasin-B, we demonstrate here a requirement for filopodia in melanosome transfer from melanocytes to keratinocytes and also, unexpectedly, between keratinocytes. Melanin distribution throughout the skin represents the key phenotypic event in skin pigmentation. Melanocyte filopodia were also necessary for UVR-stimulated melanosome transfer, as this was also inhibited by MyoX knockdown and low-dose cytochalasin-B. Knockdown of keratinocyte MyoX protein, in its capacity as a phagocytosis effector, resulted in the inhibition of melanin uptake by keratinocytes. This indicates a central role for phagocytosis by keratinocytes of melanocyte filopodia. In summary, we propose a new model for the regulation of pigmentation in human skin cells under both constitutive and facultative (post-UVR) conditions, which we call the "filopodial-phagocytosis model." This model also provides a unique and highly accessible way to study lysosome-related organelle movement between mammalian cells.

Myosin-X induces filopodia by multiple elongation mechanism

Filopodia are actin-rich finger-like cytoplasmic projections extending from the leading edge of cells. Unconventional myosin-X is involved in the protrusion of filopodia. However, the underlying mechanism of myosin-X-induced filopodia formation is obscure. Here, we studied the movements of myosin-X during filopodia protrusion using a total internal reflection microscope to clarify the mechanism of myosin-X-induced filopodia formation. Myosin-X was recruited to the discrete site at the leading edge where it assembles with exponential kinetics before the filopodia extension. The myosin-X-induced filopodia showed repeated extension-retraction cycles with each extension of 2.4 microm, which was critical to produce long filopodia. Myosin-X, lacking the FERM domain, could move to the tip as does the wild type. However, it was transported toward the cell body during filopodia retraction, did not undergo multiple extension-retraction cycles, and failed to produce long filopodia. During the filopodia protrusion, the single molecules of full-length myosin-X moved within filopodia. The majority of the fluorescence spots showed two-step photobleaching, suggesting that the moving myosin-X is a dimer. Deletion of the FERM domain did not change the movement at the single molecule level with the same velocity of approximately 600 nm/s as wild-type, suggesting that the myosin-X in filopodia moves without interaction with the attached membrane via the FERM domain. Based upon these results, we have proposed a model of myosin-X-induced filopodia protrusion.

Single-molecule stepping and structural dynamics of myosin X

Myosin X is an unconventional myosin with puzzling motility properties. We studied the motility of dimerized myosin X using the single-molecule fluorescence techniques polTIRF, FIONA and Parallax to measure the rotation angles and three-dimensional position of the molecule during its walk. It was found that Myosin X steps processively in a hand-over-hand manner following a left-handed helical path along both single actin filaments and bundles. Its step size and velocity are smaller on actin bundles than individual filaments, suggesting myosin X often steps onto neighboring filaments in a bundle. The data suggest that a previously postulated single alpha-helical domain mechanically extends the lever arm, which has three IQ motifs, and either the neck-tail hinge or the tail is flexible. These structural features, in conjunction with the membrane- and microtubule-binding domains, enable myosin X to perform multiple functions on varied actin structures in cells.

The motor protein myosin-X transports VE-cadherin along filopodia to allow the formation of early endothelial cell-cell contacts

Vascular endothelium (VE), the monolayer of endothelial cells that lines the vascular tree, undergoes damage at the basis of some vascular diseases. Its integrity is maintained by VE-cadherin, an adhesive receptor localized at cell-cell junctions. Here, we show that VE-cadherin is also located at the tip and along filopodia in sparse or subconfluent endothelial cells. We observed that VE-cadherin navigates along intrafilopodial actin filaments. We found that the actin motor protein myosin-X is colocalized and moves synchronously with filopodial VE-cadherin. Immunoprecipitation and pulldown assays confirmed that myosin-X is directly associated with the VE-cadherin complex. Furthermore, expression of a dominant-negative mutant of myosin-X revealed that myosin-X is required for VE-cadherin export to cell edges and filopodia. These features indicate that myosin-X establishes a link between the actin cytoskeleton and VE-cadherin, thereby allowing VE-cadherin transportation along intrafilopodial actin cables. In conclusion, we propose that VE-cadherin trafficking along filopodia using myosin-X motor protein is a prerequisite for cell-cell junction formation. This mechanism may have functional consequences for endothelium repair in pathological settings.

Myosin VI: an innovative motor that challenged the swinging lever arm hypothesis

The swinging crossbridge hypothesis states that energy from ATP hydrolysis is transduced to mechanical movement of the myosin head while bound to actin. The light chain-binding region of myosin is thought to act as a lever arm that amplifies movements near the catalytic site. This model has been challenged by findings that myosin VI takes larger steps along actin filaments than early interpretations of its structure seem to allow. We now know that myosin VI does indeed operate by an unusual approximately 180 degrees lever arm swing and achieves its large step size using special structural features in its tail domain.

Myosin-X is required for cranial neural crest cell migration in Xenopus laevis

Myosin-X (MyoX) belongs to a large family of unconventional, nonmuscle, actin-dependent motor proteins. We show that MyoX is predominantly expressed in cranial neural crest (CNC) cells in embryos of Xenopus laevis and is required for head and jaw cartilage development. Knockdown of MyoX expression using antisense morpholino oligonucleotides resulted in retarded migration of CNC cells into the pharyngeal arches, leading to subsequent hypoplasia of cartilage and inhibited outgrowth of the CNC-derived trigeminal nerve. In vitro migration assays on fibronectin using explanted CNC cells showed significant inhibition of filopodia formation, cell attachment, spreading and migration, accompanied by disruption of the actin cytoskeleton. These data support the conclusion that MyoX has an essential function in CNC migration in the vertebrate embryo.

Insights into human beta-cardiac myosin function from single molecule and single cell studies

beta-Cardiac myosin is a mechanoenzyme that converts the energy from ATP hydrolysis into a mechanical force that drives contractility in muscle. Thirty percent of the point mutations that result in hypertrophic cardiomyopathy are localized to MYH7, the gene encoding human beta-cardiac myosin heavy chain (beta-MyHC). Force generation by myosins requires a tight and highly conserved allosteric coupling between its different protein domains. Hence, the effects of single point mutations on the force generation and kinetics of beta-cardiac myosin molecules cannot be predicted directly from their location within the protein structure. Great insight would be gained from understanding the link between the functional defect in the myosin protein and the clinical phenotypes of patients expressing them. Over the last decade, several single molecule techniques have been developed to understand in detail the chemomechanical cycle of different myosins. In this review, we highlight the single molecule techniques that can be used to assess the effect of point mutations on beta-cardiac myosin function. Recent bioengineering advances have enabled the micromanipulation of single cardiomyocyte cells to characterize their force-length dynamics. Here, we briefly review single cell micromanipulation as an approach to determine the effect of beta-MyHC mutations on cardiomyocyte function. Finally, we examine the technical challenges specific to studying beta-cardiac myosin function both using single molecule and single cell approaches.

Unconventional myosin traffic in cells reveals a selective actin cytoskeleton

Eukaryotic cells have a self-organizing cytoskeleton where motors transport cargoes along cytoskeletal tracks. To understand the sorting process, we developed a system to observe single-molecule motility in a cellular context. We followed myosin classes V, VI, and X on triton-extracted actin cytoskeletons from Drosophila S2, mammalian COS-7, and mammalian U2OS cells. We find that these cells vary considerably in their global traffic patterns. The S2 and U2OS cells have regions of actin that either enhance or inhibit specific myosin classes. U2OS cells allow for 1 motor class, myosin VI, to move along stress fiber bundles, while motility of myosin V and X are suppressed. Myosin X motors are recruited to filopodia and the lamellar edge in S2 cells, whereas myosin VI motility is excluded from the same regions. Furthermore, we also see different velocities of myosin V motors in central regions of S2 cells, suggesting regional control of motor motility by the actin cytoskeleton. We also find unexpected features of the actin cytoskeletal network, including a population of reversed filaments with the barbed-end toward the cell center. This myosin motor regulation demonstrates that native actin cytoskeletons are more than just a collection of filaments.

Myosin-10 and actin filaments are essential for mitotic spindle function

Mitotic spindles are microtubule-based structures responsible for chromosome partitioning during cell division. Although the roles of microtubules and microtubule-based motors in mitotic spindles are well established, whether or not actin filaments (F-actin) and F-actin-based motors (myosins) are required components of mitotic spindles has long been controversial. Based on the demonstration that myosin-10 (Myo10) is important for assembly of meiotic spindles, we assessed the role of this unconventional myosin, as well as F-actin, in mitotic spindles. We find that Myo10 localizes to mitotic spindle poles and is essential for proper spindle anchoring, normal spindle length, spindle pole integrity, and progression through metaphase. Furthermore, we show that F-actin localizes to mitotic spindles in dynamic cables that surround the spindle and extend between the spindle and the cortex. Remarkably, although proper anchoring depends on both F-actin and Myo10, the requirement for Myo10 in spindle pole integrity is F-actin independent, whereas F-actin and Myo10 actually play antagonistic roles in maintenance of spindle length.

Calmodulin-like protein enhances myosin-10 translation

Myosin-10 (Myo10) is involved in processes ranging from filopodial formation and extension to spindle orientation during cell division. Myo10 contains three IQ motifs that bind calmodulin and calmodulin-like protein (CLP) as light chains. We recently found that CLP expression up-regulates Myo10, leading to increased Myo10-dependent cell motility and filopodial extension [R.D. Bennett, et al., J. Biol. Chem. 282 (2007) 3205-3212]. CLP-dependent Myo10 up-regulation occurs without increase in Myo10 mRNA. We followed Myo10 degradation in vivo and in vitro and found that it was unaffected by CLP. Myo10 decayed rapidly with a half-life of approximately 2.5h. Using an in vitro transcription/translation system we determined that CLP increased Myo10 translation, resulting in a higher relative accumulation of Myo10 in the presence than in the absence of CLP. Our data suggest that CLP functions to increase translation of Myo10 possibly by acting as a chaperone for the emerging Myo10 heavy chain protein.

Myo10 in brain: developmental regulation, identification of a headless isoform and dynamics in neurons

Although Myo10 (myosin-X) is an unconventional myosin associated with filopodia, little is known about its isoforms and roles in the nervous system. We report here that, in addition to full-length Myo10, brain expresses a shorter form of Myo10 that lacks a myosin head domain. This ;headless' Myo10 is thus unable to function as a molecular motor, but is otherwise identical to full-length Myo10 and, like it, contains three pleckstrin homology (PH) domains, a myosin-tail homology 4 (MyTH4) domain, and a band-4.1/ezrin/radixin/moesin (FERM) domain. Immunoblotting demonstrates that both full-length and headless Myo10 exhibit dramatic developmental regulation in mouse brain. Immunofluorescence with an antibody that detects both isoforms demonstrates that Myo10 is expressed in neurons, such as Purkinje cells, as well as non-neuronal cells, such as astrocytes and ependymal cells. CAD cells, a neuronal cell line, express both full-length and headless Myo10, and this endogenous Myo10 is present in cell bodies, neurites, growth cones and the tips of filopodia. To investigate the dynamics of the two forms of Myo10 in neurons, CAD cells were transfected with GFP constructs corresponding to full-length or headless Myo10. Only full-length Myo10 localizes to filopodial tips and undergoes intrafilopodial motility, demonstrating that the motor domain is necessary for these activities. Live cell imaging also reveals that full-length Myo10 localizes to the tips of neuronal filopodia as they explore and interact with their surroundings, suggesting that this myosin has a role in neuronal actin dynamics.

Calmodulin-like protein increases filopodia-dependent cell motility via up-regulation of myosin-10

Human calmodulin-like protein (CLP) is an epithelial-specific protein that is expressed during cell differentiation but down-regulated in primary cancers and transformed cell lines. Using stably transfected and inducible HeLa cell lines, we found that CLP expression did not alter the proliferation rate and colony-forming potential of these cells. However, remarkable phenotypic changes were observed in CLP-expressing compared with control cells. Soft agar colonies of CLP-expressing cells had rough boundaries, with peripheral cells migrating away from the colony. Cells expressing CLP displayed a striking increase in the number and length of myosin-10-positive filopodia and showed increased mobility in a wound healing assay. This increase in wound healing capacity was prevented by small interference RNA-mediated down-regulation of myosin-10. Fluorescence microscopy and Western blotting revealed that CLP expression results in up-regulation of its target protein, myosin-10. This up-regulation occurs at the protein level by stabilization of myosin-10. Thus, CLP functions by increasing the stability of myosin-10, leading to enhanced myosin-10 function and a subsequent increase in filopodial dynamics and cell migration. In stratified epithelia, CLP may be required during terminal differentiation to increase myosin-10 function as cells migrate toward the upper layers and establish new adhesive contacts.

Myosin at work: motor adaptations for a variety of cellular functions

Cells have evolved multiple mechanisms to overcome the effects of entropy and diffusion to create a highly ordered environment. For cells to function properly, some components must be anchored to provide a framework or structure. Others must be rapidly transported over long distances to generate asymmetries in cell morphology and composition. To accomplish long-range transport, cells cannot rely on diffusion alone as many large organelles and macromolecular complexes are essentially immobilized by the dense meshwork of the cytosol. One strategy used by cells to overcome diffusion is to harness the free energy liberated by ATP hydrolysis through molecular motors. Myosins are a family of actin based molecular motors that have evolved a variety of ways to contribute to cellular organization through numerous modifications to the manner they convert that free energy into mechanical work.

Structural Evidence for Non-canonical Binding of Ca2+ to a Canonical EF-hand of a Conventional Myosin

We have previously identified a single inhibitory Ca2+-binding site in the first EF-hand of the essential light chain of Physarum conventional myosin (Farkas, L., Malnasi-Csizmadia, A., Nakamura, A., Kohama, K., and Nyitray, L. (2003) J. Biol. Chem. 278, 27399-27405). As a general rule, conformation of the EF-hand-containing domains in the calmodulin family is "closed" in the absence and "open" in the presence of bound cations; a notable exception is the unusual Ca2+-bound closed domain in the essential light chain of the Ca2+-activated scallop muscle myosin. Here we have reported the 1.8 A resolution structure of the regulatory domain (RD) of Physarum myosin II in which Ca2+ is bound to a canonical EF-hand that is also in a closed state. The 12th position of the EF-hand loop, which normally provides a bidentate ligand for Ca2+ in the open state, is too far in the structure to participate in coordination of the ion. The structure includes a second Ca2+ that only mediates crystal contacts. To reveal the mechanism behind the regulatory effect of Ca2+, we compared conformational flexibilities of the liganded and unliganded RD. Our working hypothesis, i.e. the modulatory effect of Ca2+ on conformational flexibility of RD, is in line with the observed suppression of hydrogen-deuterium exchange rate in the Ca2+-bound form, as well as with results of molecular dynamics calculations. Based on this evidence, we concluded that Ca2+-induced change in structural dynamics of RD is a major factor in Ca2+-mediated regulation of Physarum myosin II activity.

Myosin-X: a molecular motor at the cell's fingertips

Research in several areas, including unconventional myosins and deafness genes, has converged recently on a group of myosins whose tails contain myosin tail homology 4 (MyTH4) and band 4.1, ezrin, radixin, moesin (FERM) domains. Although these 'MyTH-FERM' myosins are not present in yeast and plants, they are present in slime molds, worms, flies and mammals, where they mediate interactions between the cytoskeleton and the plasma membrane. The most broadly distributed MyTH-FERM myosin in vertebrate cells appears to be myosin-X (Myo10). This myosin can act as a link to integrins and microtubules, stimulate the formation of filopodia and undergo a novel form of motility within filopodia.

Myosins: tails (and heads) of functional diversity

The myosin family of actin filament-based molecular motors consists of at least 20 structurally and functionally distinct classes. The human genome contains nearly 40 myosin genes, encoding 12 of these classes. Myosins have been implicated in a variety of intracellular functions, including cell migration and adhesion; intracellular transport and localization of organelles and macromolecules; signal transduction; and tumor suppression. In this review, recent insights into the remarkable diversity in the mechanochemical and functional properties associated with this family of molecular motors are discussed.

Myosin X is a high duty ratio motor

Myosin X is expressed in a variety of cell types and plays a role in cargo movement and filopodia extension, but its mechanoenzymatic characteristics are not fully understood. Here we analyzed the kinetic mechanism of the ATP hydrolysis cycle of acto-myosin X using a single-headed construct (M10IQ1). Myosin X was unique for the weak "strong actin binding state" (AMD) with a K(d) of 1.6 microm attributed to the large dissociation rate constant (2.1 s(-1)). V(max) and K(ATPase) of the actin-activated ATPase activity of M10IQ1 were 13.5 s(-1) and 17.4 mum, respectively. The ATP hydrolysis rate (>100 s(-1)) and the phosphate release rate from acto-myosin X (>100 s(-1)) were much faster than the entire ATPase cycle rate and, thus, not rate-limiting. The ADP off-rate from acto-myosin X was 23 s(-1), which was two times larger than the V(max). The P(i)-burst size was low (0.46 mol/mol), indicating that the equilibrium is significantly shifted toward the prehydrolysis intermediate. The steady-state ATPase rate can be explained by a combination of the unfavorable equilibrium constant of the hydrolysis step and the relatively slow ADP off-rate. The duty ratio calculated from our kinetic model, 0.6, was consistent with the duty ratio, 0.7, obtained from comparison of K(m ATPase) and K(m motility). Our results suggest that myosin X is a high duty ratio motor.

Mechanism of action of myosin X, a membrane-associated molecular motor

We have performed a detailed biochemical kinetic and spectroscopic study on a recombinant myosin X head construct to establish a quantitative model of the enzymatic mechanism of this membrane-bound myosin. Our model shows that during steady-state ATP hydrolysis, myosin X exhibits a duty ratio (i.e. the fraction of the cycle time spent strongly bound to actin) of around 16%, but most of the remaining myosin heads are also actin-attached even at moderate actin concentrations in the so-called "weak" actin-binding states. Contrary to the high duty ratio motors myosin V and VI, the ADP release rate constant from actomyosin X is around five times greater than the maximal steady-state ATPase activity, and the kinetic partitioning between different weak actin-binding states is a major contributor to the rate limitation of the enzymatic cycle. Two different ADP states of myosin X are populated in the absence of actin, one of which shows very similar kinetic properties to actomyosin.ADP. The nucleotide-free complex of myosin X with actin shows unique spectral and biochemical characteristics, indicating a special mode of actomyosin interaction.

Muscle fiber differentiation in fish embryos as shown by in situ hybridization of a large repertoire of muscle-specific transcripts

Skeletal muscles are composed of different fiber types, largely defined by differential expression of protein isoforms involved in myofibrillogenesis or metabolism. To learn more about the gene activations that underlie the differentiation and the diversification of embryonic fish myotomal fibers, we investigated the developmental expression of 25 muscle genes in trout embryos by in situ hybridization of muscle-specific transcripts. The earliest event of muscle differentiation, at approximately the 25-somite stage, was the expression of a variety of muscle-specific genes, including slow-twitch and fast-twitch muscle isoforms. The activation of these muscle genes started in the deep somitic domain, where the slow muscle precursors (the adaxial cells) were initially located, and progressively spread laterally throughout the width of the myotome. This mediolateral progression of gene expression was coordinated with the lateral migration of slow adaxial cells, which specifically expressed the slow myosin light chain 1 and the SLIM1/FHL1 genes. Subsequently, the fast and slow skeletal muscle isoforms precociously expressed in the course of the mediolateral wave of muscle gene activation became down-regulated in the superficial slow fibers and the deep fast fibers, respectively. Finally, several muscle-specific genes, including troponins, a slow myosin-binding protein C, tropomodulins, and parvalbumin started their transcription only in late embryos. Taken together, these findings show in fish embryos that a common myogenic program is triggered in a mediolateral progression in all muscle cells. The acquisition of the slow phenotype involves the additional activation of several slow-specific genes in migrating adaxial muscle cells. These events are followed by sequential gene activations and repressions in fast and slow muscle cells.

Imaging myosin 10 in cells

Cellular motors (kinesin, dynein and myosin) are ubiquitous. A major task in cell biology is to determine how they function in cells. Here we focus on myosin 10, an intrafilopodial motor, and show how imaging green fluorescent protein fused to myosin 10 or its tail domains can help us understand the function of this myosin.

A microtubule-binding myosin required for nuclear anchoring and spindle assembly

Proper spindle positioning and orientation are essential for asymmetric cell division and require microtubule-actin filament (F-actin) interactions in many systems. Such interactions are particularly important in meiosis, where they mediate nuclear anchoring, as well as meiotic spindle assembly and rotation, two processes required for asymmetric cell division. Myosin-10 proteins are phosphoinositide-binding, actin-based motors that contain carboxy-terminal MyTH4 and FERM domains of unknown function. Here we show that Xenopus laevis myosin-10 (Myo10) associates with microtubules in vitro and in vivo, and is concentrated at the point where the meiotic spindle contacts the F-actin-rich cortex. Microtubule association is mediated by the MyTH4-FERM domains, which bind directly to purified microtubules. Disruption of Myo10 function disrupts nuclear anchoring, spindle assembly and spindle-F-actin association. Thus, this myosin has a novel and critically important role during meiosis in integrating the F-actin and microtubule cytoskeletons.

Myosin X transports Mena/VASP to the tip of filopodia

Myosin X (M10) is a two-headed actin based motor expressed in a variety of cell types, that is thought to play a role in cargo movement in mammalian cells, but its cellular function is unknown. Here we found that M10 binds to Mena/VASP, which facilitates actin polymerization by competing with actin capping proteins. Immunocytochemistry revealed that endogenous M10 co-localized with Mena/VASP at the tip of filopodia. Consistently, both EGFP-M10 and RFP-VASP were found at the tip of filopodia. The result raises a hypothesis that M10 transports Mena/VASP towards the tip of filopodia. Supporting this idea, the amount of VASP at the tip of filopodia was proportional to that of M10. Furthermore, we directly visualized the movement of M10 and VASP in living HeLa cells under fluorescence microscope. EGFP-M10 and RFP-VASP move together from the root to the tip of the filopodia. Interestingly, the amount of M10 at the tip of filopodia was linearly related to the length of filopodia, consistent with the actin filament extending function of VASP. These results show that M10 is a specific motor carrying Mena/VASP from the root to the tip of the filopodia where extension of actin filament takes place.

Regulation of the Actin Cytoskeleton by PI(4,5)P2 and PI(3,4,5)P3

The actin cytoskeleton is fundamental for various motile and morphogenetic processes in cells. The structure and dynamics of the actin cytoskeleton are regulated by a wide array of actin-binding proteins, whose activities are controlled by various signal transduction pathways. Recent studies have shown that certain membrane phospholipids, especially PI(4,5)P2 and PI(3,4,5)P3, regulate actin filament assembly in cells and in cell extracts. PI(4,5)P2 appears to be a general regulator of actin polymerization at the plasma membrane or at membrane microdomains, whereas PI(3,4,5)P3 promotes the assembly of specialized actin filament structures in response to some growth factors. Biochemical studies have demonstrated that the activities of many proteins promoting actin assembly are upregulated by PI(4,5)P2, whereas proteins that inhibit actin assembly or promote filament disassembly are down-regulated by PI(4,5)P2. PI(3,4,5)P3 promotes its effects on the actin cytoskeleton mainly through activation of the Rho family of small GTPases. In addition to their effects on actin dynamics, both PI(4,5)P2 and PI(3,4,5)P3 promote the formation of specific actin filament structures through activation/inactivation of actin filament cross-linking proteins and proteins that mediate cytoskeleton-plasma membrane interactions.

Recombinant motor domain constructs of Chara corallina myosin display fast motility and high ATPase activity

The mechanism and structural features that are responsible for the fast motility of Chara corallina myosin (CCM) have not been elucidated, so far. The low yields of native CCM that can be purified to homogeneity were the major reason for this. Here, we describe the expression of recombinant CCM motor domains, which support the fast movement of actin filaments in an in vitro motility assay. A CCM motor domain without light chain binding site moved actin filaments at a velocity of 8.8 microm/s at 30 degrees C and a CCM motor domain with an artificial lever arm consisting of two alpha-actinin repeats moved actin filaments at 16.2 microm/s. Both constructs displayed high actin-activated ATPase activities ( approximately 500 Pi/s/head), which is indicative of a very fast hydrolysis step. Our results provide an excellent system to dissect the specific structural and functional features that distinguish the myosin responsible for fast cytoplasmic streaming.

Localization and characterization of the inhibitory Ca2+-binding site of Physarum polycephalum myosin II

A myosin II is thought to be the driving force of the fast cytoplasmic streaming in the plasmodium of Physarum polycephalum. This regulated myosin, unique among conventional myosins, is inhibited by direct Ca2+ binding. Here we report that Ca2+ binds to the first EF-hand of the essential light chain (ELC) subunit of Physarum myosin. Flow dialysis experiments of wild-type and mutant light chains and the regulatory domain revealed a single binding site that shows moderate specificity for Ca2+. The regulatory light chain, in contrast to regulatory light chains of higher eukaryotes, is unable to bind divalent cations. Although the Ca2+-binding loop of ELC has a canonical sequence, replacement of glutamic acid to alanine in the -z coordinating position only slightly decreased the Ca2+ affinity of the site, suggesting that the Ca2+ coordination is different from classical EF-hands; namely, the specific "closed-to-open" conformational transition does not occur in the ELC in response to Ca2+. Ca2+- and Mg2+-dependent conformational changes in the microenvironment of the binding site were detected by fluorescence experiments. Transient kinetic experiments showed that the displacement of Mg2+ by Ca2+ is faster than the change in direction of cytoplasmic streaming; therefore, we conclude that Ca2+ inhibition could operate in physiological conditions. By comparing the Physarum Ca2+ site with the well studied Ca2+ switch of scallop myosin, we surmise that despite the opposite effect of Ca2+ binding on the motor activity, the two conventional myosins could have a common structural basis for Ca2+ regulation.

Possible involvement of myosin-X in intercellular adhesion: importance of serial pleckstrin homology regions for intracellular localization

Subcellular fractionation experiments with mouse hepatocytes, combined with sodium dodecylsulfate (SDS)-polyacrylamide gel electrophoresis (PAGE)-immunoblot analysis using antibodies against two different tail regions of mouse myosin-X demonstrated a 240 kDa molecular mass to be associated with the plasma membrane-rich P2 fraction. The basolateral plasma membrane fraction, but not the brush border fraction, isolated from renal cortices also contained the 240 kDa form of myosin-X. In an attempt to assess relative contributions of possible functional domains in the tail of myosin-X to localization and function, cDNA corresponding to all three pleckstrin homology (PH) domains and different regions (PH1, 2 and 3, and the two subdomains of PH1: PHS1 and PHS2), as well as the myosin tail homology 4 domain (MyTH4) and the band4.1/ezrin/radixin/moesin-like domain (FERM) were separately inserted into the pEGFP vector and expressed in cultured COS-1 cells. As a result, two distinct regions responsible for localization were identified with regard to PH: one covers all three forms that tends to localize to regions of dynamic actin, such as membrane ruffles, lamellipodia and thick cortical actin bundles at the sites of cell-cell adhesion in a Rac- and Cdc42-dependent manner. The other covers PHS1 and PH2 that localizes to filopodia, filopodial puncta and the sites of intercellular adhesion in a Cdc42-dependent manner. Expression of green fluorescent protein (GFP)-MyTH4 fusion protein resulted in formation of phalloidin-positive granules, while GFP-FERM affected the actin cytoskeletal system in a distinctly different way. Taken altogether, the results lend support to the view that myosin-X is involved in cell-cell adhesion-associated signaling-linked membrane and/or cytoskeleton reorganization.

Myosin X is a downstream effector of PI(3)K during phagocytosis

Phagocytosis is a phosphatidylinositol-3-OH-kinase (PI(3)K)-dependent process in macrophages. We identified Myo10 (Myosin-X), an unconventional myosin with pleckstrin homology (PH) domains, as a potential downstream target of PI(3)K. Myo10 was recruited to phagocytic cups in a wortmannin-sensitive manner. Expression of a truncation construct of Myo10 (Myo10 tail) in a macrophage cell line or cytosolic loading of anti-Myo10 antibodies in bovine alveolar macrophages inhibited phagocytosis. In contrast, expression of a Myo10 tail construct containing a point mutation in one of its PH domains failed to inhibit phagocytosis. Expression of Myo10 tail inhibited spreading, but not adhesion, on IgG-coated substrates, consistent with a function for Myo10 in pseudopod extension. We propose that Myo10 provides a molecular link between PI(3)K and pseudopod extension during phagocytosis.