Determination of human myosin III as a motor protein having a protein kinase activity

MYO3B promotes cancer progression in endometrial cancer by mediating the calcium ion-RhoA/ROCK1 signaling pathway

PURPOSE

This study aimed to investigate the effect of MYO3B on endometrial cancer (EC) proliferation and invasion.

METHODS

The expression of MYO3B in EC tissues and cells was analyzed using TCGA database, immunohistochemical staining, real-time PCR, and western blot (WB). Cell proliferation was detected by CCK8, Annexin V-APC/PI flow cytometry was used to detect apoptosis, intracellular calcium ion (Ca2+) was detected by flow cytometry with Fluo-4 AM fluorescent probe, cell migration by scratch assay, and cell invasion by Transwell assay, and the expression of proteins related to Ca2+ homeostasis and RhoA/ROCK1 signaling pathway was detected by WB and immunofluorescence staining.

RESULTS

The expression of MYO3B was an influential factor in EC recurrence, and the expression of MYO3B was significantly up-regulated in EC tissues and cells, but down-regulated in KLE cells, and MYO3B knockdown inhibited the proliferation, migration, and invasion ability of EC cells and promoted apoptosis, suggesting that MYO3B plays a tumor-promoting role in EC. Furthermore, MYO3B knockdown decreased Ca2+ concentration in EC cells and the RhoA/ROCK1 signaling pathway was inhibited, and the effect of MYO3B knockdown on RhoA/ROCK1 signaling was reversed by treatment with the Calmodulin agonist CALP-2, and the effects of MYO3B knockdown on cell proliferation, migration, and invasion were reversed after treatment with the RhoA agonist U-46,619.

CONCLUSION

MYO3B promotes the proliferation and migration of endometrial cancer cells via Ca2+-RhoA/ROCK1 signaling pathway. High expression of MYO3B may be a biomarker for EC metastasis.

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.

The actin cytoskeleton in hair bundle development and hearing loss

Inner ear hair cells assemble mechanosensitive hair bundles on their apical surface that transduce sounds and accelerations. Each hair bundle is comprised of ∼ 100 individual stereocilia that are arranged into rows of increasing height and width; their specific and precise architecture being necessary for mechanoelectrical transduction (MET). The actin cytoskeleton is fundamental to establishing this architecture, not only by forming the structural scaffold shaping each stereocilium, but also by composing rootlets and the cuticular plate that together provide a stable foundation supporting each stereocilium. In concert with the actin cytoskeleton, a large assortment of actin-binding proteins (ABPs) function to cross-link actin filaments into specific topologies, as well as control actin filament growth, severing, and capping. These processes are individually critical for sensory transduction and are all disrupted in hereditary forms of human hearing loss. In this review, we provide an overview of actin-based structures in the hair bundle and the molecules contributing to their assembly and functional properties. We also highlight recent advances in mechanisms driving stereocilia elongation and how these processes are tuned by MET.

Protrusion growth driven by myosin-generated force

Actin-based protrusions extend from the surface of all eukaryotic cells, where they support diverse activities essential for life. Models of protrusion growth hypothesize that actin filament assembly exerts force for pushing the plasma membrane outward. However, membrane-associated myosin motors are also abundant in protrusions, although their potential for contributing, growth-promoting force remains unexplored. Using an inducible system that docks myosin motor domains to membrane-binding modules with temporal control, we found that application of myosin-generated force to the membrane is sufficient for driving robust protrusion elongation in human, mouse, and pig cell culture models. Protrusion growth scaled with motor accumulation, required barbed-end-directed force, and was independent of cargo delivery or recruitment of canonical elongation factors. Application of growth-promoting force was also supported by structurally distinct myosin motors and membrane-binding modules. Thus, myosin-generated force can drive protrusion growth, and this mechanism is likely active in diverse biological contexts.

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 XVI in the Nervous System

The myosin family is a large inventory of actin-associated motor proteins that participate in a diverse array of cellular functions. Several myosin classes are expressed in neural cells and play important roles in neural functioning. A recently discovered member of the myosin superfamily, the vertebrate-specific myosin XVI (Myo16) class is expressed predominantly in neural tissues and appears to be involved in the development and proper functioning of the nervous system. Accordingly, the alterations of MYO16 has been linked to neurological disorders. Although the role of Myo16 as a generic actin-associated motor is still enigmatic, the N-, and C-terminal extensions that flank the motor domain seem to confer unique structural features and versatile interactions to the protein. Recent biochemical and physiological examinations portray Myo16 as a signal transduction element that integrates cell signaling pathways to actin cytoskeleton reorganization. This review discusses the current knowledge of the structure-function relation of Myo16. In light of its prevalent localization, the emphasis is laid on the neural aspects.

Characterization of a novel MYO3A missense mutation associated with a dominant form of late onset hearing loss

Whole-exome sequencing of samples from affected members of two unrelated families with late-onset non-syndromic hearing loss revealed a novel mutation (c.2090 T > G; NM_017433) in MYO3A. The mutation was confirmed in 36 affected individuals, showing autosomal dominant inheritance. The mutation alters a single residue (L697W or p.Leu697Trp) in the motor domain of the stereocilia protein MYO3A, leading to a reduction in ATPase activity, motility, and an increase in actin affinity. MYO3A-L697W showed reduced filopodial actin protrusion initiation in COS7 cells, and a predominant tipward accumulation at filopodia and stereocilia when coexpressed with wild-type MYO3A and espin-1, an actin-regulatory MYO3A cargo. The combined higher actin affinity and duty ratio of the mutant myosin cause increased retention time at stereocilia tips, resulting in the displacement of the wild-type MYO3A protein, which may impact cargo transport, stereocilia length, and mechanotransduction. The dominant negative effect of the altered myosin function explains the dominant inheritance of deafness.

Myosin-Driven Intracellular Transport

The delivery of intracellular material within cells is crucial for maintaining normal function. Myosins transport a wide variety of cargo, ranging from vesicles to ribonuclear protein particles (RNPs), in plants, fungi, and metazoa. The properties of a given myosin transporter are adapted to move on different actin filament tracks, either on the disordered actin networks at the cell cortex or along highly organized actin bundles to distribute their cargo in a localized manner or move it across long distances in the cell. Transport is controlled by selective recruitment of the myosin to its cargo that also plays a role in activation of the motor.

Myosins: Domain Organisation, Motor Properties, Physiological Roles and Cellular Functions

Myosins are cytoskeletal motor proteins that use energy derived from ATP hydrolysis to generate force and movement along actin filaments. Humans express 38 myosin genes belonging to 12 classes that participate in a diverse range of crucial activities, including muscle contraction, intracellular trafficking, cell division, motility, actin cytoskeletal organisation and cell signalling. Myosin malfunction has been implicated a variety of disorders including deafness, hypertrophic cardiomyopathy, Usher syndrome, Griscelli syndrome and cancer. In this chapter, we will first discuss the key structural and kinetic features that are conserved across the myosin family. Thereafter, we summarise for each member in turn its unique functional and structural adaptations, cellular roles and associated pathologies. Finally, we address the broad therapeutic potential for pharmacological interventions that target myosin family members.

Kinome-wide screening of HER2+ breast cancer cells for molecules that mediate cell proliferation or sensitize cells to trastuzumab therapy

Understanding the signaling differences that distinguish human HER2-amplified (HER2-positive (HER2+)) breast cancers from other breast cancer subtypes may help to identify protein drug targets for the specific treatment of HER2+ breast cancers. We performed two kinome-wide small interfering RNA (siRNA) screens on five HER2+ breast cancer cell lines, seven breast cancer cell lines in which HER2 was not amplified and two normal breast cell lines. To pinpoint the main kinases driving HER2 signaling, we performed a comprehensive siRNA screen that identified loss of the HER2/HER3 heterodimer as having the most prominent inhibitory effect on the growth of HER2+ breast cancer cells. In a second siRNA screen focused on identifying genes that could sensitize HER2+ cells to trastuzumab treatment, we found that loss of signaling members downstream of phosphatidylinositol 3 kinase (PI3K) potentiated the growth inhibitory effects of trastuzumab. Loss of HER2 and HER3, as well as proteins involved in mitogenic and environmental stress pathways inhibited the proliferation of HER2+ cells only in the absence of trastuzumab, suggesting that these pathways are inhibited by trastuzumab treatment. Loss of essential G2/M cell cycle mediators or proteins involved in vesicle organization exerted inhibitory effects on HER2+ cell growth that were unaffected by trastuzumab. Furthermore, the use of a sensitization index (SI) identified targeting the PI3K pathway to sensitize to trastuzumab treatment. Antagonism using the SI identified MYO3A, MYO3B and MPZL1 as antagonizers to trastuzumab treatment among HER2+ cell lines. Our results suggest that the dimerization partners of HER2 are important for determining the activation of downstream proliferation pathways. Understanding the complex layers of signaling triggered downstream of HER2 homodimers and heterodimers will facilitate the selection of better targets for combination therapies intended to treat HER2+ breast cancers.

Phosphorylation of the kinase domain regulates autophosphorylation of myosin IIIA and its translocation in microvilli

Motor activity of myosin III is regulated by autophosphorylation. To investigate the role of the kinase activity on the transporter function of myosin IIIA (Myo3A), we identified the phosphorylation sites of kinase domain (KD), which is responsible for the regulation of kinase activity and thus motor function. Using mass spectrometry, we identified six phosphorylation sites in the KD, which are highly conserved among class III myosins and Ste20-related misshapen (Msn) kinases. Two predominant sites, Thr¹⁸⁴ and Thr¹⁸⁸, in KD are important for phosphorylation of the KD as well as the motor domain, which regulates the affinity for actin. In the Caco2 cells, the full-length human Myo3A (hMyo3AFull) markedly enlarged the microvilli, although it did not show discrete localization within the microvilli. On the other hand, hMyo3AFull(T184A) and hMyo3AFull(T188A) both showed clear localization at the microvilli tips. Our results suggest that Myo3A induces large actin bundle formation to form microvilli, and phosphorylation of KD at Thr¹⁸⁴ and Thr¹⁸⁸ is critical for the kinase activity of Myo3A, and regulation of Myo3A translocation to the tip of microvilli. Retinal extracts potently dephosphorylate both KD and motor domain without IQ motifs (MDIQo), which was inhibited by okadaic acid (OA) with nanomolar range and by tautomycetin (TMC) with micromolar range. The results suggest that Myo3A phosphatase is protein phosphatase type 2A (PP2A). Supporting this result, recombinant PP2Ac potently dephosphorylates both KD and MDIQo. We propose that the phosphorylation–dephosphorylation mechanism plays an essential role in mediating the transport and actin bundle formation and stability functions of hMyo3A.

Myosin 3A kinase activity is regulated by phosphorylation of the kinase domain activation loop

Class III myosins are unique members of the myosin superfamily in that they contain both a motor and kinase domain. We have found that motor activity is decreased by autophosphorylation, although little is known about the regulation of the kinase domain. We demonstrate by mass spectrometry that Thr-178 and Thr-184 in the kinase domain activation loop and two threonines in the loop 2 region of the motor domain are autophosphorylated (Thr-908 and Thr-919). The kinase activity of MYO3A 2IQ with the phosphomimic (T184E) or phosphoblock (T184A) mutations demonstrates that kinase activity is reduced 30-fold as a result of the T184A mutation, although the Thr-178 site only had a minor impact on kinase activity. Interestingly, the actin-activated ATPase activity of MYO3A 2IQ is slightly reduced as a result of the T178A and T184A mutations suggesting coupling between motor and kinase domains. Full-length GFP-tagged T184A and T184E MYO3A constructs transfected into COS7 cells do not disrupt the ability of MYO3A to localize to filopodia structures. In addition, we demonstrate that T184E MYO3A reduces filopodia elongation in the presence of espin-1, whereas T184A enhances filopodia elongation in a similar fashion to kinase-dead MYO3A. Our results suggest that as MYO3A accumulates at the tips of actin protrusions, autophosphorylation of Thr-184 enhances kinase activity resulting in phosphorylation of the MYO3A motor and reducing motor activity. The differential regulation of the kinase and motor activities allows for MYO3A to precisely self-regulate its concentration in the actin bundle-based structures of cells.

PDA (prolonged depolarizing afterpotential)-defective mutants: the story of nina's and ina's--pinta and santa maria, too

Our objective is to present a comprehensive view of the PDA (prolonged depolarizing afterpotential)-defective Drosophila mutants, nina's and ina's, from the discussion of the PDA and the PDA-based mutant screening strategy to summaries of the knowledge gained through the studies of mutants generated using the strategy. The PDA is a component of the light-evoked photoreceptor potential that is generated when a substantial fraction of rhodopsin is photoconverted to its active form, metarhodopsin. The PDA-based mutant screening strategy was adopted to enhance the efficiency and efficacy of ERG (electroretinogram)-based screening for identifying phototransduction-defective mutants. Using this strategy, two classes of PDA-defective mutants were identified and isolated, nina and ina, each comprising multiple complementation groups. The nina mutants are characterized by allele-dependent reduction in the major rhodopsin, Rh1, whereas the ina mutants display defects in some aspects of functions related to the transduction channel, TRP (transient receptor potential). The signaling proteins that have been identified and elucidated through the studies of nina mutants include the Drosophila opsin protein (NINAE), the chaperone protein for nascent opsin (NINAA), and the multifunctional protein, NINAC, required in multiple steps of the Drosophila phototransduction cascade. Also identified by the nina mutants are some of the key enzymes involved in the biogenesis of the rhodopsin chromophore. As for the ina mutants, they led to the discovery of the scaffold protein, INAD, responsible for the nucleation of the supramolecular signaling complex. Also identified by the ina mutants is one of the key members of the signaling complex, INAC (ePKC), and two other proteins that are likely to be important, though their roles in the signaling cascade have not yet been fully elucidated. In most of these cases, the protein identified is the first member of its class to be so recognized.

Mouse class III myosins: kinase activity and phosphorylation sites

As class III unconventional myosins are motor proteins with an N-terminal kinase domain, it seems likely they play a role in both signaling and actin based transport. A growing body of evidence indicates that the motor functions of human class IIIA myosin, which has been implicated in progressive hearing loss, are modulated by intermolecular autophosphorylation. However, the phosphorylation sites have not been identified. We studied the kinase activity and phosphorylation sites of mouse class III myosins, mMyo3A and 3B, which are highly similar to their human orthologs. We demonstrate that the kinase domains of mMyo3A and 3B are active kinases, and that they have similar, if not identical, substrate specificities. We show that the kinase domains of these proteins autophosphorylate, and that they can phosphorylate sites within their myosin and tail domains. Using liquid chromatography-mass spectrometry, we identified phosphorylated sites in the kinase, myosin motor and tail domains of both mMyo3A and 3B. Most of the phosphorylated sites we identified and their consensus phosphorylation motifs are highly conserved among vertebrate class III myosins, including human class III myosins. Our findings are a major step toward understanding how the functions of class III myosins are regulated by phosphorylation.

Intermolecular autophosphorylation regulates myosin IIIa activity and localization in parallel actin bundles

Myosin IIIa (Myo3A) transports cargo to the distal end of actin protrusions and contains a kinase domain that is thought to autoregulate its activity. Because Myo3A tends to cluster at the tips of actin protrusions, we investigated whether intermolecular phosphorylation could regulate Myo3A biochemical activity, cellular localization, and cellular function. Inactivation of Myo3A 2IQ kinase domain with the point mutation K50R did not alter maximal ATPase activity, whereas phosphorylation of Myo3A 2IQ resulted in reduced maximal ATPase activity and actin affinity. The rate and degree of Myo3A 2IQ autophosphorylation was unchanged by the presence of actin but was found to be dependent upon Myo3A 2IQ concentration within the range of 0.1 to 1.2 μm, indicating intermolecular autophosphorylation. In cultured cells, we observed that the filopodial tip localization of Myo3A lacking the kinase domain decreased when co-expressed with kinase-active, full-length Myo3A. The cellular consequence of reduced Myo3A tip localization was decreased filopodial density along the cell periphery, identifying a novel cellular function for Myo3A in mediating the formation and stability of actin-based protrusions. Our results suggest that Myo3A motor activity is regulated through a mechanism involving concentration-dependent autophosphorylation. We suggest that this regulatory mechanism plays an essential role in mediating the transport and actin bundle formation/stability functions of Myo3A.

Effect of phosphorylation in the motor domain of human myosin IIIA on its ATP hydrolysis cycle

Previous findings suggested that the motor activity of human myosin IIIA (HM3A) is influenced by phosphorylation [Kambara, T., et al. (2006) J. Biol. Chem. 281, 37291-37301]; however, how phosphorylation controls the motor activity of HM3A is obscure. In this study, we clarify the kinetic basis of the effect of phosphorylation on the ATP hydrolysis cycle of the motor domain of HM3A (huM3AMD). The affinity of human myosin IIIA for filamentous actin in the presence of ATP is more than 100-fold decreased by phosphorylation, while the maximum rate of ATP turnover is virtually unchanged. The rate of release of ADP from acto-phosphorylated huM3AMD is 6-fold greater than the overall cycle rate, and thus not a rate-determining step. The rate constant of the ATP hydrolysis step of the actin-dissociated form is markedly increased by phosphorylation by 30-fold. The dissociation constant for dissociation of the ATP-bound form of huM3AMD from actin is greatly increased by phosphorylation, and this result agrees well with the significant increase in the K(actin) value of the steady-state ATPase reaction. The rate constant of the P(i) off step is greater than 60 s(-1), suggesting that this step does not limit the overall ATP hydrolysis cycle rate. Our kinetic model indicates that phosphorylation induces the dissociation of huM3AMD from actin during the ATP hydrolysis cycle, and this is due to the phosphorylation-dependent marked decrease in the affinity of huM3AMD.ATP for actin and the increase in the ATP hydrolysis rate of huM3AMD in the actin-dissociated state. These results suggest that the phosphorylation of myosin IIIA significantly lowers the duty ratio, which may influence the cargo transporting ability of the native form of myosin IIIA that contains the ATP-independent actin binding site in the tail.

The mammalian family of sterile 20p-like protein kinases

Twenty-eight kinases found in mammalian genomes share similarity to the budding yeast kinase Ste20p. This review article examines the biological function of these mammalian Ste20 kinases. Some of them have conserved the Ste20p function of transducing extracellular signals to mitogen-activated kinases. Others affect ion transport, cell cycle, cytoskeleton organization, and program cell death. A number of molecular details involved in the activation of the kinases are discussed including autophosphorylation, substrate recognition, autoinhibition, dimerization, and substrate binding.

Cloning and distribution of myosin 3B in the mouse retina: differential distribution in cone outer segments

Class III myosins are important for the function and survival of photoreceptors and ciliary hair cells. Although vertebrates possess two class III myosin genes, myo3A and myo3B, recent studies have focused on Myo3A because mutations in the human gene are implicated in progressive hearing loss. Myo3B may compensate for defects in Myo3A, yet little is known about its distribution and function. This study focuses on Myo3B expression in the mouse retina. We cloned two variants of myo3B from mouse retina and determined that they are expressed early in retinal development. In this study we show for the first time in a mammal that both Myo3B and Myo3A proteins are present in inner segments of all photoreceptors. Myo3B is also present in outer segments of S opsin-immunoreactive cones but not M opsin dominant cones. Myo3B is also detected in rare cells of the inner nuclear layer and some ganglion cells. Myo3B may have diverse roles in retinal neurons. In photoreceptor inner segments Myo3B is positioned appropriately to prevent photoreceptor loss of function caused by Myo3A defects.

Interplay between TRP channels and the cytoskeleton in health and disease

Transient receptor potential (TRP) channels are a family of cation channels that play a key role in ion homeostasis and cell volume regulation. In addition, TRP channels are considered universal integrators of sensory information required for taste, vision, hearing, touch, temperature, and the detection of mechanical force. Seminal investigations exploring the molecular mechanisms of phototransduction in Drosophila have demonstrated that TRP channels operate within macromolecular complexes closely associated with the cytoskeleton. More recent evidence shows that mammalian TRP channels similarly connect to the cytoskeleton to affect cytoskeletal organization and cell adhesion via ion-transport-dependent and -independent mechanisms. In this review, we discuss new insights into the interplay between TRP channels and the cytoskeleton and provide recent examples of such interactions in different physiological systems.

The kinase domain alters the kinetic properties of the myosin IIIA motor

Myosin IIIA is unique among myosin proteins in that it contains an N-terminal kinase domain capable of autophosphorylating sites on the motor domain. A construct of myosin IIIA lacking the kinase domain localizes more efficiently to the stereocilia tips and alters the morphology of the tips in inner ear hair cells. Therefore, we performed a kinetic analysis of myosin IIIA without the kinase domain (MIII DeltaK) and compared these results with our reported analysis of myosin IIIA containing the kinase domain (MIII). The steady-state kinetic properties of MIII DeltaK indicate that it has a 2-fold higher maximum actin-activated ATPase rate (kcat = 1.5 +/- 0.1 s-1) and a 5-fold tighter actin affinity (KATPase = 6.0 +/- 1.4 microM, and KActin = 1.4 +/- 0.4 microM) compared to MIII. The rate of ATP binding to the motor domain is enhanced in MIII DeltaK (K1k+2 approximately 0.10 +/- 0.01 microM-1.s-1) to a level similar to the rate of binding to MIII in the presence of actin. The rate of ATP hydrolysis in the absence of actin is slow and may be rate limiting. Actin-activated phosphate release is identical with and without the kinase domain. The transition between actomyosin.ADP states, which is rate limiting in MIII, is enhanced in MIII DeltaK. MIII DeltaK accumulates more efficiently at the tips of filopodia in HeLa cells. Our results suggest a model in which the activity and concentration of myosin IIIA localized to the tips of actin bundles mediates the morphology of the tips in sensory cells.

The actin-binding interface of a myosin III is phosphorylated in vivo in response to signals from a circadian clock

Class III unconventional myosins are critical for the normal function of auditory hair cells and the function and maintenance of photoreceptors; however, the roles of class III myosins in these sensory cells are unknown. Class III myosins are unique in that they have a kinase domain at their N-terminus; thus, they may have both signaling and motor functions. In the horseshoe crab Limulus polyphemus, enhanced phosphorylation of an abundant, photoreceptor specific class III myosin at night correlates with well-characterized circadian changes in photoreceptor structure and function. Thus, the Limulus visual system may be particularly useful for investigating the properties, modulation, and functions of a class III myosin. Previously, we showed that two sites within the actin interface of full-length Limulus myosin III expressed in baculovirus are substrates for both cyclic AMP-dependent protein kinase and autophosphorylation. In the current study, mass spectrometry was used to show that these same sites are phosphorylated in the endogenous protein extracted from Limulus lateral eye, and that enhanced phosphorylation at these sites occurs in vivo in response to natural circadian clock input to these eyes. These findings demonstrate in vivo changes in myosin III phosphorylation in response to a natural stimulus. This phosphorylation may modulate myosin III-actin interactions.

Phototransduction and retinal degeneration in Drosophila

Drosophila visual transduction is the fastest known G-protein-coupled signaling cascade and has therefore served as a genetically tractable animal model for characterizing rapid responses to sensory stimulation. Mutations in over 30 genes have been identified, which affect activation, adaptation, or termination of the photoresponse. Based on analyses of these genes, a model for phototransduction has emerged, which involves phosphoinoside signaling and culminates with opening of the TRP and TRPL cation channels. Many of the proteins that function in phototransduction are coupled to the PDZ containing scaffold protein INAD and form a supramolecular signaling complex, the signalplex. Arrestin, TRPL, and G alpha(q) undergo dynamic light-dependent trafficking, and these movements function in long-term adaptation. Other proteins play important roles either in the formation or maturation of rhodopsin, or in regeneration of phosphatidylinositol 4,5-bisphosphate (PIP2), which is required for the photoresponse. Mutation of nearly any gene that functions in the photoresponse results in retinal degeneration. The underlying bases of photoreceptor cell death are diverse and involve mechanisms such as excessive endocytosis of rhodopsin due to stable rhodopsin/arrestin complexes and abnormally low or high levels of Ca2+. Drosophila visual transduction appears to have particular relevance to the cascade in the intrinsically photosensitive retinal ganglion cells in mammals, as the photoresponse in these latter cells appears to operate through a remarkably similar mechanism.

Loop 2 of limulus myosin III is phosphorylated by protein kinase A and autophosphorylation

Little is known about the functions of class III unconventional myosins although, with an N-terminal kinase domain, they are potentially both signaling and motor proteins. Limulus myosin III is particularly interesting because it is a phosphoprotein abundant in photoreceptors that becomes more heavily phosphorylated at night by protein kinase A. This enhanced nighttime phosphorylation occurs in response to signals from an endogenous circadian clock and correlates with dramatic changes in photoreceptor structure and function. We seek to understand the role of Limulus myosin III and its phosphorylation in photoreceptors. Here we determined the sites that become phosphorylated in Limulus myosin III and investigated its kinase, actin binding, and myosin ATPase activities. We show that Limulus myosin III exhibits kinase activity and that a major site for both protein kinase A and autophosphorylation is located within loop 2 of the myosin domain, an important actin binding region. We also identify the phosphorylation of an additional protein kinase A and autophosphorylation site near loop 2, and a predicted phosphorylation site within loop 2. We show that the kinase domain of Limulus myosin III shares some pharmacological properties with protein kinase A, and that it is a potential opsin kinase. Finally, we demonstrate that Limulus myosin III binds actin but lacks ATPase activity. We conclude that Limulus myosin III is an actin-binding and signaling protein and speculate that interactions between actin and Limulus myosin III are regulated by both second messenger mediated phosphorylation and autophosphorylation of its myosin domain within and near loop 2.

Kinetic mechanism of human myosin IIIA

Myosin IIIA is specifically expressed in photoreceptors and cochlea and is important for the phototransduction and hearing processes. In addition, myosin IIIA contains a unique N-terminal kinase domain and C-terminal tail actin-binding motif. We examined the kinetic properties of baculovirus expressed human myosin IIIA containing the kinase, motor, and two IQ domains. The maximum actin-activated ATPase rate is relatively slow (k(cat) = 0.77 +/- 0.08 s(-1)), and high actin concentrations are required to fully activate the ATPase rate (K(ATPase) = 34 +/- 11 microm). However, actin co-sedimentation assays suggest that myosin III has a relatively high steady-state affinity for actin in the presence of ATP (K(actin) approximately 7 microm). The rate of ATP binding to the motor domain is quite slow both in the presence and absence of actin (K(1)k(+2) = 0.020 and 0.001 microm(-1).s(-1), respectively). The rate of actin-activated phosphate release is more than 100-fold faster (85 s(-1)) than the k(cat), whereas ADP release in the presence of actin follows a two-step mechanism (7.0 and 0.6 s(-1)). Thus, our data suggest a transition between two actomyosin-ADP states is the rate-limiting step in the actomyosin III ATPase cycle. Our data also suggest the myosin III motor spends a large fraction of its cycle in an actomyosin ADP state that has an intermediate affinity for actin (K(d) approximately 5 microm). The long lived actomyosin-ADP state may be important for the ability of myosin III to function as a cellular transporter and actin cross-linker in the actin bundles of sensory cells.

A new compartment at stereocilia tips defined by spatial and temporal patterns of myosin IIIa expression

Class III myosins are motor proteins that contain an N-terminal kinase domain and a C-terminal actin-binding domain. We show that myosin IIIa, which has been implicated in nonsyndromic progressive hearing loss, is localized at stereocilia tips. Myosin IIIa progressively accumulates during stereocilia maturation in a thimble-like pattern around the stereocilia tip, distinct from the cap-like localization of myosin XVa and the shaft localization of myosin Ic. Overexpression of deletion mutants for functional domains of green fluorescent protein (GFP)-myosin IIIa shows that the motor domain, but not the actin-binding tail domain, is required for stereocilia tip localization. Deletion of the kinase domain produces stereocilia elongation and bulging of the stereocilia tips. The thimble-like localization and the influence myosin IIIa has on stereocilia shape reveal a previously unrecognized molecular compartment at the distal end of stereocilia, the site of actin polymerization as well as operation of the mechanoelectrical transduction apparatus.

Human myosin III is a motor having an extremely high affinity for actin

Myosin IIIA is expressed in photoreceptor cells and thought to play a critical role in phototransduction processes, yet its function on a molecular basis is largely unknown. Here we clarified the kinetic mechanism of the ATPase cycle of human myosin IIIA. The steady-state ATPase activity was markedly activated approximately 10-fold with very low actin concentration. The rate of ADP off from actomyosin IIIA was 10 times greater than the overall cycling rate, thus not a rate-determining step. The rate constant of the ATP hydrolysis step of the actin-dissociated form was very slow, but the rate was markedly accelerated by actin binding. The dissociation constant of the ATP-bound form of myosin IIIA from actin is submicromolar, which agrees well with the low K(actin). These results indicate that ATP hydrolysis predominantly takes place in the actin-bound form for actomyosin IIIA ATPase reaction. The obtained K(actin) was much lower than the previously reported one, and we found that the autophosphorylation of myosin IIIA dramatically increased the K(actin), whereas the V(max) was unchanged. Our kinetic model indicates that both the actin-attached hydrolysis and the P(i) release steps determine the overall cycle rate of the dephosphorylated form. Although the stable steady-state intermediates of actomyosin IIIA ATPase reaction are not typical strong actin-binding intermediates, the affinity of the stable intermediates for actin is much higher than conventional weak actin binding forms. The present results suggest that myosin IIIA can spend a majority of its ATP hydrolysis cycling time on actin.

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.

A unique mechanism for the processive movement of single-headed myosin-IX

It has been puzzled that in spite of its single-headed structure, myosin-IX shows the typical character of processive motor in multi-molecule in vitro motility assay, because this cannot be explained by hand-over-hand mechanism of the two-headed processive myosins. Here, we show direct evidence of the processive movement of myosin-IX using two different single molecule techniques. Using optical trap nanometry, we found that myosin-IX takes several large ( approximately 20nm) steps before detaching from an actin filament. Furthermore, we directly visualized the single myosin-IX molecules moving on actin filaments for several hundred nanometers without dissociating from actin filament. Since myosin-IX processively moves without anchoring the neck domain, the result suggests that the neck tilting is not involved for the processive movement of myosin-IX. We propose that the myosin-IX head moves processively along an actin filament like an inchworm via a unique long and positively charged insertion in the loop 2 region of the head.

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.

TRP channels in Drosophila photoreceptor cells

TRP cation channels are conserved throughout animal phylogeny and include many members that function in sensory physiology. The founding TRP is required for Drosophila phototransduction and has served as a paradigm for unravelling the roles and macromolecular organizations of TRP channels in native tissues. Two other TRPC channels, TRPL and TRPgamma, are expressed in photoreceptor cells and form heteromultimers with TRP and with each other. TRP is a member of a supramolecular signalling complex, the signalplex, which includes the PDZ scaffold protein, INAD, and two other core members that remain bound and depend on INAD for localization. Other INAD binding proteins are proposed to interact dynamically with INAD, one of which, TRPL, undergoes light-dependent translocation in photoreceptor cells. Surprisingly, TRP has non-channel functions, including an anchoring role necessary for retaining INAD in the rhabdomeres. Loss of TRP function or constitutive TRP activity results in retinal degeneration, which can be suppressed by disruption or overexpression of the Na+/Ca2+ exchanger, CalX, respectively. Given that hypoxia-induced constitutive activity of some mammalian TRPs leads to neuronal cell death, interventions that increase Na+/Ca2+ exchanger or decrease TRP function have the potential to reduce the severity of cell death due to ischaemia.

The N-terminal domain of MYO18A has an ATP-insensitive actin-binding site

Myosin XVIII is the recently identified 18th class of myosins, and its members are composed of a unique N-terminal domain, a motor domain with an unusual sequence around the ATPase site, one IQ motif, a segmented coiled-coil region for dimerization, and a C-terminal globular tail. To gain insight into the functions of this unique myosin, we characterized its human homologue, MYO18A, focusing on the functional roles of the characteristic N-terminal domain that contains a PDZ module known to mediate protein-protein interaction. GFP-tagged full-length and C-terminally truncated MYO18A molecules that were expressed in HeLa cells exhibited colocalization with actin filaments. Chemical cross-linking of these molecules showed that they form stable dimers as expected from their putative coiled-coil tails. Cosedimentation of the various types of truncated MYO18A constructs with actin filaments indicated the presence of an ATP-insensitive actin-binding site in the N-terminal domain. Further studies on truncated constructs of the N-terminal domain indicated that this actin-binding site is located outside the PDZ module, but within the middle region of this domain, which does not show any homology with the known actin-binding motifs. These results imply that this dimeric myosin might stably cross-link actin filaments by two ATP-insensitive actin-binding sites at the N-terminal domains for higher-order organization of the actin cytoskeleton.

Myosin 3A transgene expression produces abnormal actin filament bundles in transgenic Xenopus laevis rod photoreceptors

Myo3A, a class III myosin, localizes to the distal (plus) ends of inner segment actin filament bundles that form the core of microvillus-like calycal processes encircling the base of the photoreceptor outer segment. To investigate Myo3A localization and function, we expressed green fluorescent protein-tagged bass Myo3A and related constructs in transgenic Xenopus rods using a modified opsin promoter. Tagged intact Myo3A localized to rod calycal processes, as previously reported for native bass Myo3A. Transgenic rods developed abnormally large calycal processes and subsequently degenerated. Modified Myo3A expression constructs demonstrated that calycal process localization required an active motor domain and the tail domain. Expressed tail domain alone localized to actin bundles along the entire inner segment length, rather than to the distal end. This tail domain localization required the conserved C-terminal domain (3THDII) previously shown to possess an actin-binding motif. Our findings suggest that Myo3A plays a role in the morphogenesis and maintenance of calycal processes of vertebrate photoreceptors.

Light-dependent subcellular translocation of Gqα in Drosophila photoreceptors is facilitated by the photoreceptor-specific myosin III NINAC

We examine the light-dependent subcellular translocation of the visual Gqα protein between the signaling compartment, the rhabdomere and the cell body in Drosophila photoreceptors. We characterize the translocation of Gqα and provide the first evidence implicating the involvement of the photoreceptor-specific myosin III NINAC in Gqα transport. Translocation of Gqα from the rhabdomere to the cell body is rapid, taking less than 5 minutes. Higher light intensities increased the quantity of Gqα translocated out of the rhabdomeres from 20% to 75%, consistent with a mechanism for light adaptation. We demonstrate that translocation of Gqα requires rhodopsin, but none of the known downstream phototransduction components, suggesting that the signaling pathway triggering translocation occurs upstream of Gqα. Finally, we show that ninaC mutants display a significantly reduced rate of Gqα transport from the cell body to the rhabdomere, suggesting that NINAC might function as a light-dependent plus-end motor involved in the transport of Gqα.

Light-dependent translocation of visual arrestin regulated by the NINAC myosin III

The rhodopsin regulatory protein, visual arrestin, undergoes light-dependent trafficking in mammalian and Drosophila photoreceptor cells, though the mechanisms underlying these movements are poorly understood. In Drosophila, the movement of the visual arrestin, Arr2, functions in long-term adaptation and is dependent on interaction with phosphoinositides (PIs). However, the basis for the requirement for PIs for light-dependent shuttling was unclear. Here, we demonstrated that the dynamic trafficking of Arr2 into the phototransducing compartment, the rhabdomere, required the eye-enriched myosin III, NINAC. We showed that defects in ninaC resulted in a long-term adaptation phenotype similar to that which occurred in arr2 mutants. The interaction between Arr2 and NINAC was PI dependent and NINAC bound directly to PIs. These data demonstrate that the light-dependent translocation of Arr2 into the rhabdomeres requires PI-mediated interactions between Arr2 and the NINAC myosin III.

Localization of a class III myosin to filopodia tips in transfected HeLa cells requires an actin-binding site in its tail domain

Bass Myo3A, a class III myosin, was expressed in HeLa cells as a GFP fusion in order to study its cellular localization. GFP-Myo3A localized to the cytoplasm and to the tips of F-actin bundles in filopodia, a localization that is consistent with the observed concentration toward the distal ends of F-actin bundles in photoreceptor cells. A mutation in the motor active site resulted in a loss of filopodia localization, suggesting that Myo3A motor activity is required for filopodial tip localization. Deletion analyses showed that the NH2-terminal kinase domain is not required but the CO2H-terminal 22 amino acids of the Myo3A tail are required for filopodial localization. Expression of this tail fragment alone produced fluorescence associated with F-actin throughout the cytoplasm and filopodia and a recombinant tail fragment bound to F-actin in vitro. An actin-binding motif was identified within this tail fragment, and a mutation within this motif abolished both filopodia localization by Myo3A and F-actin binding by the tail fragment alone. Calmodulin localized to filopodial tips when coexpressed with Myo3A but not in the absence of Myo3A, an observation consistent with the previous proposal that class III myosins bind calmodulin and thereby localize it in certain cell types.