A novel mammalian myosin I from rat with an SH3 domain localizes to Con A-inducible, F-actin-rich structures at cell-cell contacts

Myosins and Disease

Myosins constitute a superfamily of actin-based molecular motor proteins that mediates a variety of cellular activities including muscle contraction, cell migration, intracellular transport, the formation of membrane projections, cell adhesion, and cell signaling. The 12 myosin classes that are expressed in humans share sequence similarities especially in the N-terminal motor domain; however, their enzymatic activities, regulation, ability to dimerize, binding partners, and cellular functions differ. It is becoming increasingly apparent that defects in myosins are associated with diseases including cardiomyopathies, colitis, glomerulosclerosis, neurological defects, cancer, blindness, and deafness. Here, we review the current state of knowledge regarding myosins and disease.

Class I myosins: Highly versatile proteins with specific functions in the immune system

Connections established between cytoskeleton and plasma membrane are essential in cellular processes such as cell migration, vesicular trafficking, and cytokinesis. Class I myosins are motor proteins linking the actin-cytoskeleton with membrane phospholipids. Previous studies have implicated these molecules in cell functions including endocytosis, exocytosis, release of extracellular vesicles and the regulation of cell shape and membrane elasticity. In immune cells, those proteins also are involved in the formation and maintenance of immunological synapse-related signaling. Thus, these proteins are master regulators of actin cytoskeleton dynamics in different scenarios. Although the localization of class I myosins has been described in vertebrates, their functions, regulation, and mechanical properties are not very well understood. In this review, we focused on and summarized the current understanding of class I myosins in vertebrates with particular emphasis in leukocytes.

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.

Myosin-I molecular motors at a glance

Myosin-I molecular motors are proposed to play various cellular roles related to membrane dynamics and trafficking. In this Cell Science at a Glance article and the accompanying poster, we review and illustrate the proposed cellular functions of metazoan myosin-I molecular motors by examining the structural, biochemical, mechanical and cell biological evidence for their proposed molecular roles. We highlight evidence for the roles of myosin-I isoforms in regulating membrane tension and actin architecture, powering plasma membrane and organelle deformation, participating in membrane trafficking, and functioning as a tension-sensitive dock or tether. Collectively, myosin-I motors have been implicated in increasingly complex cellular phenomena, yet how a single isoform accomplishes multiple types of molecular functions is still an active area of investigation. To fully understand the underlying physiology, it is now essential to piece together different approaches of biological investigation. This article will appeal to investigators who study immunology, metabolic diseases, endosomal trafficking, cell motility, cancer and kidney disease, and to those who are interested in how cellular membranes are coupled to the underlying actin cytoskeleton in a variety of different applications.

Crucial role for the LSP1-myosin1e bimolecular complex in the regulation of Fcγ receptor-driven phagocytosis

The actin cytoskeleton is fundamental for the innate immune process of phagocytosis. This study shows that LSP1 plays a pivotal role in the regulation of actin cytoskeleton remodeling during Fcγ receptor–mediated phagocytosis and that its interactions with myosin1e and actin are crucial for the efficiency of this actin-driven process.

Actin cytoskeleton remodeling is fundamental for Fcγ receptor–driven phagocytosis. In this study, we find that the leukocyte-specific protein 1 (LSP1) localizes to nascent phagocytic cups during Fcγ receptor–mediated phagocytosis, where it displays the same spatial and temporal distribution as the actin cytoskeleton. Down-regulation of LSP1 severely reduces the phagocytic activity of macrophages, clearly demonstrating a crucial role for this protein in Fcγ receptor–mediated phagocytosis. We also find that LSP1 binds to the class I molecular motor myosin1e. LSP1 interacts with the SH3 domain of myosin1e, and the localization and dynamics of both proteins in nascent phagocytic cups mirror those of actin. Furthermore, inhibition of LSP1–myosin1e and LSP1–actin interactions profoundly impairs pseudopodial formation around opsonized targets and their subsequent internalization. Thus the LSP1–myosin1e bimolecular complex plays a pivotal role in the regulation of actin cytoskeleton remodeling during Fcγ receptor–driven phagocytosis.

Myosin 1e is a component of the glomerular slit diaphragm complex that regulates actin reorganization during cell-cell contact formation in podocytes

Glomerular visceral epithelial cells, also known as podocytes, are critical to both normal kidney function and the development of kidney disease. Podocyte actin cytoskeleton and their highly specialized cell-cell junctions (also called slit diaphragm complexes) play key roles in controlling glomerular filtration. Myosin 1e (myo1e) is an actin-based molecular motor that is expressed in renal glomeruli. Disruption of the Myo1e gene in mice and humans promotes podocyte injury and results in the loss of the integrity of the glomerular filtration barrier. Here, we have used biochemical and microscopic approaches to determine whether myo1e is associated with the slit diaphragm complexes in glomerular podocytes. Myo1e was consistently enriched in the slit diaphragm fraction during subcellular fractionation of renal glomeruli and colocalized with the slit diaphragm markers in mouse kidney. Live cell imaging studies showed that myo1e was recruited to the newly formed cell-cell junctions in cultured podocytes, where it colocalized with the actin filament cables aligned with the nascent contacts. Myo1e-null podocytes expressing FSGS-associated myo1e mutant (A159P) did not efficiently assemble actin cables along new cell-cell junctions. We have mapped domains in myo1e that were critical for its localization to cell-cell junctions and determined that the SH3 domain of myo1e tail interacts with ZO-1, a component of the slit diaphragm complex and tight junctions. These findings suggest that myo1e represents a component of the slit diaphragm complex and may contribute to regulating junctional integrity in kidney podocytes.

Non-muscle myosins and the podocyte

Proteinuria is often accompanied by a pathological change in the glomerulus that is refereed as effacement of the podocyte foot processes. The highly dynamic podocyte foot processes contain an actin-based contractile apparatus comparable to that of pericytes, which needs to be precisely and temporally controlled to withstand high pressure in the capillaries and to maintain intact glomerular filtration properties. This review outlines the most recent concepts on the function of the podocyte contractile apparatus with a focus on the role of non-muscle myosins as they have been highlighted by studies in monogenic hereditary proteinuric diseases.

Myo1e binds anionic phospholipids with high affinity

Myo1e is a single-headed motor protein that has been shown to play roles in clathrin-mediated endocytosis in HeLa cells and podocyte function in the kidney. The myo1e C-terminal tail domain includes a basic region that is required for localization to clathrin-coated vesicles and contains a putative pleckstrin-homology (PH) domain that has been shown to play a role in phospholipid binding in other myosin-I proteins. We used sedimentation assays, stopped-flow fluorescence, and fluorescence microscopy to determine the membrane binding affinities, kinetics, and in vivo localization of fluorescently labeled recombinant myo1e-tail constructs. We found that the myo1e tail binds tightly to large unilamellar vesicles (LUVs) containing physiological concentrations of the anionic phospholipids phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) or phosphatidylserine. The rate of myo1e attachment to LUVs nears the diffusion limit while the calculated rate of detachment from LUVs is slow (k(diss) ≤ 0.4 s(-1)). Mutation of conserved residues in the myo1e PH domain has little effect on lipid binding in vitro or membrane localization in vivo. Soluble inositol phosphate headgroups, such as inositol 1,4,5-trisphosphate, can compete with PtdIns(4,5)P(2) for binding, but the apparent affinity for the soluble inositol phosphate is substantially lower than that for PtdIns(4,5)P(2). These results suggest that myo1e binds lipids through nonspecific electrostatic interactions rather than a stereospecific protein-phosphoinositide interaction.

Myosin motor proteins in the cell biology of axons and other neuronal compartments

Most neurons of both the central and peripheral nervous systems express multiple members of the myosin superfamily that include nonmuscle myosin II, and a number of classes of unconventional myosins. Several classes of unconventional myosins found in neurons have been shown to play important roles in transport processes. A general picture of the myosin-dependent transport processes in neurons is beginning to emerge, although much more work still needs to be done to fully define these roles and establish the importance of myosin for axonal transport. Myosins appear to contribute to three types of transport processes in neurons; recycling of receptors or other membrane components, dynamic tethering of vesicular components, and transport or tethering of protein translational machinery including mRNA. Defects in one or more of these functions have potential to contribute to disease processes.

A role for myosin 1e in cortical granule exocytosis in Xenopus oocytes

Xenopus oocytes undergo dynamic structural changes during maturation and fertilization. Among these, cortical granule exocytosis and compensatory endocytosis provide effective models to study membrane trafficking. This study documents an important role for myosin 1e in cortical granule exocytosis. Myosin 1e is expressed at the earliest stage that cortical granule exocytosis can be detected in oocytes. Prior to exocytosis, myosin 1e relocates to the surface of cortical granules. Overexpression of myosin 1e augments the kinetics of cortical granule exocytosis, whereas tail-derived fragments of myosin 1e inhibit this secretory event (but not constitutive exocytosis). Finally, intracellular injection of myosin 1e antibody inhibits cortical granule exocytosis. Further experiments identified cysteine string proteins as interacting partners for myosin 1e. As constituents of the membrane of cortical granules, cysteine string proteins are also essential for cortical granule exocytosis. Future investigation of the link between myosin 1e and cysteine string proteins should help to clarify basic mechanisms of regulated exocytosis.

Modulation of cell adhesion and motility in the immune system by Myo1f

Although class I myosins are known to play a wide range of roles, the physiological function of long-tailed class I myosins in vertebrates remains elusive. We demonstrated that one of these proteins, Myo1f, is expressed predominantly in the mammalian immune system. Cells from Myo1f-deficient mice exhibited abnormally increased adhesion and reduced motility, resulting from augmented exocytosis of beta2 integrin-containing granules. Also, the cortical actin that co-localizes with Myo1f was reduced in Myo1f-deficient cells. In vivo, Myo1f-deficient mice showed increased susceptibility to infection by Listeria monocytogenes and an impaired neutrophil response. Thus, Myo1f directs immune cell motility and innate host defense against infection.

Myo1c binds phosphoinositides through a putative pleckstrin homology domain

Myo1c is a member of the myosin superfamily that binds phosphatidylinositol-4,5-bisphosphate (PIP(2)), links the actin cytoskeleton to cellular membranes and plays roles in mechano-signal transduction and membrane trafficking. We located and characterized two distinct membrane binding sites within the regulatory and tail domains of this myosin. By sequence, secondary structure, and ab initio computational analyses, we identified a phosphoinositide binding site in the tail to be a putative pleckstrin homology (PH) domain. Point mutations of residues known to be essential for polyphosphoinositide binding in previously characterized PH domains inhibit myo1c binding to PIP(2) in vitro, disrupt in vivo membrane binding, and disrupt cellular localization. The extended sequence of this binding site is conserved within other myosin-I isoforms, suggesting they contain this putative PH domain. We also characterized a previously identified membrane binding site within the IQ motifs in the regulatory domain. This region is not phosphoinositide specific, but it binds anionic phospholipids in a calcium-dependent manner. However, this site is not essential for in vivo membrane binding.

Differential expression and molecular characterisation of Lmo7, Myo1e, Sash1, and Mcoln2 genes in Btk-defective B-cells

PURPOSE

Bruton's tyrosine kinase is crucial for B-lymphocyte development. By the use of gene expression profiling, we have identified four expressed sequence tags among 38 potential Btk target genes, which have now been characterised.

METHODS

Bioinformatics tools including data mining of additional unpublished gene expression profiles, sequence verification of PCR products and qualitative RT-PCR were used. Stimulations targeting the B-cell receptor and the protein kinase C were used to activate whole B-cell splenocytes.

RESULTS

Target genes were characterised as Lim domain only 7 (Lmo7); Myosin1e (Myo1e); SAM and SH3 domain containing 1 (Sash1); and Mucolipin2 (Mcoln2). Expression was found in cell lines of different origin and developmental stages as well as in whole B-cell splenocytes and Transitional type 1 (T1) splenic B-cells from wild type and Btk-defective mice, respectively. By the use of semi-quantitative RT-PCR we found Sash1 not to be expressed in the investigated haematopoietic cell lines, while transcripts were found in whole splenic B-cells from both wild type and Btk-defective mice, whereas Lmo7, Myo1e, and Mcoln2 were expressed in both B-cell lines and primary B-lymphocytes. Except for Lmo7, the transcript level was similarly affected by stimulation in control and Btk-defective cells.

Axonal isoforms of myosin-I

We have examined spinal motor neurons in Sprague-Dawley rats to further characterize a mechanoenzyme, myosin-Igamma (myr4), which is found in high concentration during axon tract formation in neonates. We raised an antibody to myr4 and made riboprobes for in situ hybridization. Myr4 mRNA was abundant in spinal cord motor neurons (particularly during axon regrowth). Nerves undergoing Wallerian degeneration (from a crush 7 days earlier) showed anti-myr4 labeling of the axolemma and SER--after microtubules, neurofilaments, and F-actin had already been degraded--which is consistent with a described lipid-binding domain in the tail region of myosin-Is. Newly synthesized myr4 was carried in axons by the slow component (SC) of axonal transport at 1-8 mm/day, whereas, none was carried by the fast component (FC). We conclude that SC delivers myr4 to the cytoplasmic surfaces of stationary axonal membranes (SER and axolemma). This positioning would anchor the tail domain of myr4 and leave the catalytic head domain free to interact with F-actin.

Relating biochemistry and function in the myosin superfamily

All characterized myosins share a common ATPase mechanism. However, detailed kinetic analyses suggest that modulation of the rate and equilibrium constants that define the ATPase cycle confers specific properties to these motor proteins, suiting them to specific physiological tasks. Understanding the kinetic mechanisms allows potential cellular functions of the different myosin classes and isoforms to be better defined.

Unconventional myosins, actin dynamics and endocytosis: a ménage à trois?

Ever since the discovery of class I myosins, the first nonmuscle myosins, about 30 years ago, the history of unconventional myosins has been linked to the organization and working of actin filaments. It slowly emerged from studies of class I myosins in lower eukaryotes that they are involved in mechanisms of endocytosis. Most interestingly, a flurry of recent findings assign a more active role to class I myosins in regulating the spatial and temporal organization of actin filament nucleation and elongation. The results highlight the multiple links between class I myosins and the major actin nucleator, the Arp2/3 complex, and its newly described activators. Two additional types of unconventional myosins, myosinIX, and Dictyostelium discoideum MyoM, have recently been tied to the signaling pathways controlling actin cytoskeleton remodeling. The present review surveys the links between these three classes of molecular motors and the complex cellular processes of endocytosis and actin dynamics, and concentrates on a working model accounting for the function of class I myosins via recruitment of the machinery responsible for actin nucleation and elongation.

The kinetic mechanism of Myo1e (human myosin-IC)

Myo1e is the widely expressed subclass-1 member of the myosin-I family. We performed a kinetic analysis of a truncated myo1e that consists of the motor and the single IQ motif with a bound calmodulin. We determined the rates and equilibrium constants for the key steps in the ATPase cycle. The maximum actin activated ATPase rate (V(max)) and the actin concentration at half-maximum of V(max) (K(ATPase)) of myo1e are similar to those of the native protein. The K(ATPase) is low (approximately 1 microm), however the affinity of myo1e for actin in the presence of ATP is very weak. A weak actin affinity and a rapid rate of phosphate release result in a pathway under in vitro assay conditions in which phosphate is released while myo1e is dissociated from actin. Actin activation of the ATPase activity and the low K(ATPase) are the result of actin activation of ADP release. We propose that myo1e is tuned to function in regions of high concentrations of cross-linked actin filaments. Additionally, we found that ADP release from actomyo1e is > 10-fold faster than other vertebrate myosin-I isoforms. We propose that subclass-1 myosin-Is are tuned for rapid sliding, whereas subclass-2 isoforms are tuned for tension maintenance or stress sensing.

Myr 8, a novel unconventional myosin expressed during brain development associates with the protein phosphatase catalytic subunits 1alpha and 1gamma1

Directed neuronal, astroglial, and oligodendroglial cell migrations comprise a prominent feature of mammalian brain development. Because molecular motor proteins have been implicated in a wide spectrum of processes associated with cell motility, we initiated studies to define the pool of myosins in migrating cerebellar granule neurons and type-1 neocortical astrocytes. Our analyses identified two isoforms of a novel unconventional myosin, which we have cloned, sequenced, and designated myr 8a and 8b (eighth unconventional myosin from rat). Phylogenetic analysis indicates that myr 8 myosins comprise a new class of myosins, which we have designated class XVI. The head domain contains a large N-terminal extension composed of multiple ankyrin repeats, which are implicated in mediating an association with the protein phosphatase 1 (PP1) catalytic subunits 1alpha and 1gamma. The motor domain is followed by a single putative light-chain binding domain. The tail domain of myr 8a is comparatively short with a net positive charge, whereas the tail domain of myr 8b is extended, bears an overall neutral charge, and reveals several stretches of poly-proline residues. Neither the myr 8a nor the myr 8b sequence reveals alpha-helical coiled-coil motifs, suggesting that these myosins exist as monomers. Both immunoblot and Northern blot analyses indicate that myr 8b is the predominant isoform expressed in brain, principally at developmental time periods. The structural features and restricted expression patterns suggest that members of this novel class of unconventional myosins comprise a mechanism to target selectively the protein phosphatase 1 catalytic subunits 1alpha and/or 1gamma in developing brain.

Myr 8, a novel unconventional myosin expressed during brain development associates with the protein phosphatase catalytic subunits 1alpha and 1gamma1

Directed neuronal, astroglial, and oligodendroglial cell migrations comprise a prominent feature of mammalian brain development. Because molecular motor proteins have been implicated in a wide spectrum of processes associated with cell motility, we initiated studies to define the pool of myosins in migrating cerebellar granule neurons and type-1 neocortical astrocytes. Our analyses identified two isoforms of a novel unconventional myosin, which we have cloned, sequenced, and designated myr 8a and 8b (eighth unconventional myosin from rat). Phylogenetic analysis indicates that myr 8 myosins comprise a new class of myosins, which we have designated class XVI. The head domain contains a large N-terminal extension composed of multiple ankyrin repeats, which are implicated in mediating an association with the protein phosphatase 1 (PP1) catalytic subunits 1alpha and 1gamma. The motor domain is followed by a single putative light-chain binding domain. The tail domain of myr 8a is comparatively short with a net positive charge, whereas the tail domain of myr 8b is extended, bears an overall neutral charge, and reveals several stretches of poly-proline residues. Neither the myr 8a nor the myr 8b sequence reveals alpha-helical coiled-coil motifs, suggesting that these myosins exist as monomers. Both immunoblot and Northern blot analyses indicate that myr 8b is the predominant isoform expressed in brain, principally at developmental time periods. The structural features and restricted expression patterns suggest that members of this novel class of unconventional myosins comprise a mechanism to target selectively the protein phosphatase 1 catalytic subunits 1alpha and/or 1gamma in developing brain.

Motor domain-dependent localization of myo1b (myr-1)

Myosin-I is the single-headed, membrane binding member of the myosin superfamily that plays a role in membrane dynamics and transport [1-6]. Its molecular functions and its mechanism of regulation are not known. In mammalian cells, myosin-I is excluded from specific microfilament populations, indicating that its localization is tightly regulated. Identifying the mechanism of this localization, and the specific actin populations with which myosin-I interacts, is crucial to understanding the molecular functions of this motor. eGFP chimeras of myo1b [7] were imaged in live and fixed NRK cells. Ratio-imaging microscopy shows that myo1b-eGFP concentrates within dynamic areas of the actin cytoskeleton, most notably in membrane ruffles. Myo1b-eGFP does not associate with stable actin bundles or stress fibers. Truncation mutants consisting of the motor or tail domains show a partially overlapping cytoplasmic localization with full-length myo1b, but do not concentrate in membrane ruffles. A chimera consisting of the light chain and tail domains of myo1b and the motor domain from nonmuscle myosin-IIb (nmMIIb) concentrates on actin filaments in ruffles as well as to stress fibers. In vitro motility assays show that the exclusion of myo1b from certain actin filament populations is due to the regulation of the actomyosin interaction by tropomyosin. Therefore, we conclude that tropomyosin and spatially regulated actin polymerization play important roles in regulating the function and localization of myo1b.

Identification and functional analysis of the gene for type I myosin in fission yeast

BACKGROUND

Type I myosin is highly conserved among eukaryotes, and apparently plays important roles in a number of cellular processes. In the budding yeast, two myosin I species have been identified and their role in F-actin assembly has been inferred.

RESULTS

We cloned the fission yeast myo1 gene, which apparently encoded a myosin I protein. Disruption of myo1 was not lethal, but it caused growth retardation at high and low temperatures, sensitivity to a high concentration of KCl, and aberrance in cell morphology associated with an abnormal distribution of F-actin patches. An abnormal deposition of cell wall materials was also seen. Homothallic myo1Delta cells could mate, but heterothallic myo1Delta cells were poor in conjugation. Myo1p was necessary for the encapsulation of spores. The tail domain of Myo1p was pivotal for its function. Calmodulin could bind to Myo1p through the IQ domain at the neck.

CONCLUSIONS

Myo1p appears to control the redistribution of F-actin patches during the cell cycle. Loss of Myo1p function is likely to slow down the actin assembly/disassembly process, which results in a failure of the actin cycle to catch up with other events in both the mitotic and meiotic cell cycles, including extension of the conjugation tubes.

Regulation and expression of metazoan unconventional myosins

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

Recruitment of a specific amoeboid myosin I isoform to the plasma membrane in chemotactic Dictyostelium cells

The Dictyostelium class I myosins, MyoA, -B, -C, and -D, participate in plasma membrane-based cellular processes such as pseudopod extension and macropinocytosis. Given the existence of a high affinity membrane-binding site in the C-terminal tail domain of these motor proteins and their localized site of action at the cortical membrane-cytoskeleton, it was of interest to determine whether each myosin I was directly associated with the plasma membrane. The membrane association of a myosin I heavy chain kinase that regulates the activity of one of the class I myosins, MyoD was also examined. Cellular fractionation experiments revealed that the majority of the Dicyostelium MyoA, -B, -C and -D heavy chains and the kinase are cytosolic. However, a small, but significant, fraction (appr. 7. -15%) of each myosin I and the kinase was associated with the plasma membrane. The level of plasma membrane-associated MyoB, but neither that of MyoC nor MyoD, increases up to 2-fold in highly motile, streaming cells. These results indicate that Dictyostelium specifically recruits myoB to the plasma membrane during directed cell migration, consistent with its known role in pseudopod formation.

Vezatin, a novel transmembrane protein, bridges myosin VIIA to the cadherin-catenins complex

Defects in myosin VIIA are responsible for deafness in the human and mouse. The role of this unconventional myosin in the sensory hair cells of the inner ear is not yet understood. Here we show that the C-terminal FERM domain of myosin VIIA binds to a novel transmembrane protein, vezatin, which we identified by a yeast two-hybrid screen. Vezatin is a ubiquitous protein of adherens cell-cell junctions, where it interacts with both myosin VIIA and the cadherin-catenins complex. Its recruitment to adherens junctions implicates the C-terminal region of alpha-catenin. Taken together, these data suggest that myosin VIIA, anchored by vezatin to the cadherin-catenins complex, creates a tension force between adherens junctions and the actin cytoskeleton that is expected to strengthen cell-cell adhesion. In the inner ear sensory hair cells vezatin is, in addition, concentrated at another membrane-membrane interaction site, namely at the fibrillar links interconnecting the bases of adjacent stereocilia. In myosin VIIA-defective mutants, inactivity of the vezatin-myosin VIIA complex at both sites could account for splaying out of the hair cell stereocilia.

Human brush border myosin-I and myosin-Ic expression in human intestine and Caco-2BBe cells

The human intestinal cell line, Caco-2BBe, has been established as an excellent model system for analysis of the enterocyte cytoskeleton including that of the actin rich apical brush border. To facilitate its use for functional analysis of a major component of the brush border, brush border myosin-I, human cDNAs encoding the heavy chain of this class I myosin were isolated and sequenced. The identity of this myosin as human brush border myosin-I was verified based on similarity with other vertebrate sequences, as well as its expression profile at both the RNA and protein levels. Localization of the protein in human intestine along the crypt-villus axis closely resembles that previously determined for brush border myosin-I in chicken, and is quite distinct from that of myosin-Ic, another myosin-I expressed in human intestine and Caco-2BBe cells. In immature cells of the crypt, brush border myosin-I staining is low, and there is significant cytosolic and basolateral localization, while villus cells stain much more intensely, and the protein is primarily localized to the brush border. Localization of myosin-Ic is essentially the inverse of brush border myosin-I in that crypt cells exhibit higher levels of staining, while villus cells have very low levels of myosin-Ic. The expression of both myosins-I was also examined during cell-contact induced differentiation of Caco-2BBe cells where expression and changes in localization closely resemble those that accompany differentiation of enterocyte in vivo.

Both calmodulin and the unconventional myosin Myr4 regulate membrane trafficking along the recycling pathway of MDCK cells

In epithelial cells, endocytosed transferrin and its receptor, which cycle basolaterally, have been shown to transit through recycling endosomes which can also be accessed by markers internalized from the apical surface. In this work, we have used an in vitro assay to follow transfer of an endocytosed marker from apical or basolateral early endosomes to recycling endosomes labeled with transferrin. We show that calmodulin (CaM) function is necessary for transfer and identified myr4, a member of the unconventional myosin superfamily known to use CaM as a light chain, as a possible target protein for CaM. Since myr4 is believed to act as an actin-based mechanoenzyme, we tested the role of polymerized actin in the assay. Our data show that conditions which either prevent actin polymerization or induce the breakdown of existing filaments strongly inhibit interactions between recycling endosomes and either set of early endosomes. Altogether, our data indicate that trafficking at early steps of the endocytic pathway in Madin-Darby Canine Kidney cells depends on the actin-based mechanoenzyme myr4, its light chain CaM, and polymerized actin.

Regulation of the enzymatic and motor activities of myosin I

Myosins I were the first unconventional myosins to be purified and they remain the best characterized. They have been implicated in various motile processes, including organelle translocation, ion channel gating and cytoskeletal reorganization but their exact cellular functions are still unclear. All members of the myosin I family, from yeast to man, have three structural domains: a catalytic head domain that binds ATP and actin; a tail domain believed to be involved in targeting the myosins to specific subcellular locations and a junction or neck domain that connects them and interacts with light chains. In this review we discuss how each of these three domains contributes to the regulation of myosin I enzymatic activity, motor activity and subcellular localization.

Dictyostelium myosin IK is involved in the maintenance of cortical tension and affects motility and phagocytosis

Dictyostelium discoideum myosin Ik (MyoK) is a novel type of myosin distinguished by a remarkable architecture. MyoK is related to class I myosins but lacks a cargo-binding tail domain and carries an insertion in a surface loop suggested to modulate motor velocity. This insertion shows similarity to a secondary actin-binding site present in the tail of some class I myosins, and indeed a GST-loop construct binds actin. Probably as a consequence, binding of MyoK to actin was not only ATP- but also salt-dependent. Moreover, as both binding sites reside within its motor domain and carry potential sites of regulation, MyoK might represent a new form of actin crosslinker. MyoK was distributed in the cytoplasm with a significant enrichment in dynamic regions of the cortex. Absence of MyoK resulted in a drop of cortical tension whereas overexpression led to significantly increased tension. Absence and overexpression of MyoK dramatically affected the cortical actin cytoskeleton and resulted in reduced initial rates of phagocytosis. Cells lacking MyoK showed excessive ruffling, mostly in the form of large lamellipodia, accompanied by a thicker basal actin cortex. At early stages of development, aggregation of myoK null cells was slowed due to reduced motility. Altogether, the data indicate a distinctive role for MyoK in the maintenance and dynamics of the cell cortex.

Functions of unconventional myosins

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

Overlapping distribution of the 130- and 110-kDa myosin I isoforms on rat liver membranes

The biochemical and mechanochemical properties and localization of myosin I suggest the involvement of these small members of the myosin superfamily in some aspects of intracellular motility in higher cells. We have determined by quantitative immunoblotting with isoform-specific antibodies that the 130-kDa myosin I (myr 1 gene product) and 110-kDa myosin I (myr 2 gene product) account for 0.5 and 0.4%, respectively, of total rat liver protein. Immunoblot analyses reveal that the 130- and 110-kDa myosins I are found in several purified subcellular fractions from rat liver. The membrane-associated 130-kDa myosin I is found at the highest concentration in the plasma membrane (28 ng/microg plasma membrane protein) followed by the endoplasmic reticulum-like mitochondria-associated membrane fraction (MAM; 10 ng/microg MAM protein), whereas the 110-kDa myosin I is found at the highest concentration in Golgi (50 ng/¿g Golgi protein) followed by plasma membrane (20 ng/microg) and MAM (7 ng/microg). Our analyses indicate that myosin I is peripherally associated with Golgi and MAM and its presence in these fractions is not a consequence of myosin I bound to contaminating actin filaments. Although found in relatively low concentrations in microsomes, because of the abundance of microsomes, in liver most of the membrane-associated myosin I is associated with microsomes. Neither myosin I isoform is detected in purified mitochondria. This is the first quantitative analysis addressing the cellular distribution of these mammalian class I myosins.

A contractile activity that closes phagosomes in macrophages

Studies of Fc-mediated phagocytosis by mouse macrophages identified a contractile activity at the distal margins of forming phagosomes. Time-lapse video microscopic analysis of macrophages containing rhodamine-labeled actin and fluorescein dextran showed that actin was concentrated at the distal margins of closing phagosomes. Phagocytosis-related contractile activities were observed when one IgG-opsonized erythrocyte was engaged by two macrophages. Both cells extended pseudopodia until they met midway around the erythrocyte. It was then constricted and pulled into two phagosomes, which remained interconnected by a string of erythrocyte membrane. Butanedione monoxime, an uncompetitive inhibitor of class II and perhaps other myosins, and wortmannin and LY294002, inhibitors of phosphoinositide 3-kinase, prevented the constrictions without inhibiting the initial pseudopod extension. Immunofluorescence microscopy showed the presence of myosins IC, II, V and IXb in phagosomes. Of these, only myosin IC was concentrated around the strings connecting shared erythrocytes, suggesting that myosin IC mediates the purse-string-like contraction that closes phagosomes. The sequential processes of pseudopod extension and contraction can explain how macropinosomes and spacious phagosomes form without guidance from a particle surface.

Unconventional myosins at the crossroad of signal transduction and cytoskeleton remodeling

The cytoplasm of eukaryotic cells is a complex milieu and unraveling how its unique cytoarchitecture is achieved and maintained is a central theme in modern cell biology. The actin cytoskeleton is essential for the maintenance of cell shape and locomotion, and also provides tracks for active intracellular transport. Myosins, the actin-dependent motor proteins form a superfamily of at least 15 structural classes and have been identified in a wide variety of organisms, making the presence of actin and myosins a hallmark feature of eukaryotes. Direct connections of myosins to a variety of cellular tasks are now emerging, such as in cytokinesis, phagocytosis, endocytosis, polarized secretion and exocytosis, axonal transport. Recent studies reveal that myosins also play an essential role in many aspects of signal transduction and neurosensation.

Targeting of the myosin-I myr 3 to intercellular adherens type junctions induced by dominant active Cdc42 in HeLa cells

Myr 3, a member of the myosin-I family from rat, is shown in this study to be localized at adherens-type intercellular junctions in epithelial and nonepithelial tissues. Formation of intercellular junctions and the accompanying recruitment of myr 3 to these junctions involves signaling by the Rho subfamily of small GTP-binding proteins. This conclusion is based on studies with HtTA-1 HeLa cells that were induced by overexpression of constitutively active Cdc42Hs to form typical adherens-type intercellular junctions enriched in cadherins (N-cadherin), beta-catenin, filamentous actin and myr 3. Recruitement of myr 3 to Cdc42-induced adherens junctions in HeLa cells was dependent on a short region of the tail domain and a functional myosin motor domain, but was independent of its myosin-I tail homology and SH3 regions. Overexpression of constitutively active Rac1 induced a distinct type of adherens junction in HeLa cells that was characterized by elaborate intercellular interdigitations enriched in N-cadherin, beta-catenin and F-actin. Myr 3 was often present, but not specifically enriched in the intercellular junctions induced by constitutively active Rac1.

The Src homology domain 3 (SH3) of a yeast type I myosin, Myo5p, binds to verprolin and is required for targeting to sites of actin polarization

The budding yeast contains two type I myosins, Myo3p and Myo5p, with redundant functions. Deletion of both myosins results in growth defects, loss of actin polarity and polarized cell surface growth, and accumulation of intracellular membranes. Expression of myc-tagged Myo5p in myo3Delta myo5Delta cells fully restores wild-type characteristics. Myo5p is localized as punctate, cortical structures enriched at sites of polarized cell growth. We find that latrunculin-A-induced depolymerization of F-actin results in loss of Myo5p patches. Moreover, incubation of yeast cells at 37 degrees C results in transient depolarization of both Myo5p patches and the actin cytoskeleton. Mutant Myo5 proteins with deletions in nonmotor domains were expressed in myo3Delta myo5Delta cells and the resulting strains were analyzed for Myo5p function. Deletion of the tail homology 2 (TH2) domain, previously implicated in ATP-insensitive actin binding, has no detectable effect on Myo5p function. In contrast, myo3Delta myo5Delta cells expressing mutant Myo5 proteins with deletions of the src homology domain 3 (SH3) or both TH2 and SH3 domains display defects including Myo5p patch depolarization, actin disorganization, and phenotypes associated with actin dysfunction. These findings support a role for the SH3 domain in Myo5p localization and function in budding yeast. The proline-rich protein verprolin (Vrp1p) binds to the SH3 domain of Myo3p or Myo5p in two-hybrid tests, coimmunoprecipitates with Myo5p, and colocalizes with Myo5p. Immunolocalization of the myc-tagged SH3 domain of Myo5p reveals diffuse cytoplasmic staining. Thus, the SH3 domain of Myo5p contributes to but is not sufficient for localization of Myo5p either to patches or to sites of polarized cell growth. Consistent with this, Myo5p patches assemble but do not localize to sites of polarized cell surface growth in a VRP1 deletion mutant. Our studies support a multistep model for Myo5p targeting in yeast. The first step, assembly of Myo5p patches, is dependent upon F-actin, and the second step, polarization of actin patches, requiresVrp1p and the SH3 domain of Myo5p.

The ATPase activity of Myr3, a rat myosin I, is allosterically inhibited by its own tail domain and by Ca2+ binding to its light chain calmodulin

We purified Myr3 (third unconventional myosin from rat), a mammalian "amoeboid" subclass myosin I, from rat liver. The heavy chain of purified Myr3 is associated with a single calmodulin light chain. Myr3 exhibits K/EDTA-ATPase and Mg-ATPase activity. The Mg-ATPase activity is stimulated by increasing F-actin concentrations in a complex triphasic manner similar to the Mg-ATPase activity of myosin I molecules from protozoa. Although purified Myr3 was observed to cross-link actin filaments, it bound in an ATP regulated manner to F-actin, and no evidence for a nucleotide-independent high affinity actin binding site that could explain the triphasic activation pattern was obtained. Micromolar concentrations of free Ca2+ reversibly inhibit the Mg-ATPase activity of Myr3 by binding to its light chain calmodulin, which remains bound to the Myr3 heavy chain irrespective of the free Ca2+ concentration. Polyclonal antibodies and Fab fragments directed against the tail domain were found to stimulate the Mg-ATPase activity. A similar stimulation of the Myr3 Mg-ATPase activity is observed upon proteolytic removal of the very C-terminal SH3 domain. These results demonstrate that Myr3 is subject to negative regulation by free calcium and its own tail domain and possibly positive regulation by a tail-domain binding partner.

Distinct functions of calmodulin are required for the uptake step of receptor-mediated endocytosis in yeast: the type I myosin Myo5p is one of the calmodulin targets

The uptake step of receptor-mediated endocytosis in yeast is dependent on the calcium binding protein calmodulin (Cmd1p). In order to understand the role that Cmd1p plays, a search was carried out for possible targets among the genes required for the internalization process. Co-immunoprecipitation, two-hybrid and overlay assays demonstrated that Cmd1p interacts with Myo5p, a type I unconventional myosin. Analysis of the endocytic phenotype and the Cmd1p-Myo5p interaction in thermosensitive cmd1 mutants indicated that the Cmd1p-Myo5p interaction is required for endocytosis in vivo. However, the Cmd1p-Myo5p interaction requirement was partially overcome by deleting the calmodulin binding sites (IQ motifs) from Myo5p, suggesting that these motifs inhibit Myo5p function. Additionally, genetic and biochemical evidence obtained with a collection of cmd1 mutant alleles strongly suggests that Cmd1p plays an additional role in the internalization step of receptor-mediated endocytosis in yeast.

Myogenic cells express multiple myosin isoforms

In vivo and in vitro, proliferating motile myoblasts form aligned groups of cells, with a characteristic bipolar morphology, subsequently become post-mitotic, begin to express skeletal myosin and fuse. We were interested in whether members of the myosin superfamily were involved in myogenesis. We found that the myoblasts expressed multiple myosin isoforms, from at least five different classes of the myosin superfamily (classes I, II, V, VII and IX), using RT-PCR and degenerate primers to conserved regions of myosin. All of these myosin isoforms were expressed most highly in myoblasts and their expression decreased as they differentiated into mature myotubes, by RNAse protection assays, and Western analysis. However, only myosin I alpha, non-muscle myosin IIA and IIB together with actin relocalize in response to the differentiative state of the cell. In single cells, myosin I alpha was found at the leading edge, in rear microspikes and had a punctate cytoplasmic staining, and non-muscle myosin was associated with actin bundles as previously described for fibroblasts. In aligned groups of cells, all these proteins were found at the plasma membrane. Co-staining for skeletal myosin II, and myosin I alpha showed that myosin I alpha also appeared to be expressed at higher levels in post-mitotic myoblasts that had begun to express skeletal myosin prior to fusion. In early myotubes, actin and non-muscle myosin IIA and IIB remained localized at the membrane. All of the other myosin isoforms we looked at, myosin V, myosin IX and a second isoform of myosin I (mouse homologue to myr2) showed a punctate cytoplasmic staining which did not change as the myoblasts differentiated. In conclusion, although we found that myoblasts express many different isoforms of the myosin superfamily, only myosin I alpha, non-muscle myosin IIA and IIB appear to play any direct role in myogenesis.

Cloning of the genes encoding two murine and human cochlear unconventional type I myosins

Several lines of evidence indicate a crucial role for unconventional myosins in the function of the sensory hair cells of the inner ear. We report here the characterization of the cDNAs encoding two unconventional type I myosins from a mouse cochlear cDNA library. The first cDNA encodes a putative protein named Myo1c, which is likely to be the murine orthologue of the bullfrog myosin I beta and which may be involved in the gating of the mechanotransduction channel of the sensory hair cells. This myosin belongs to the group of short-tailed myosins I, with its tail ending shortly after a polybasic, TH-1-like domain. The second cDNA encodes a novel type I myosin Myo1f which displays three regions: a head domain with the conserved ATP- and actin-binding sites, a neck domain with a single IQ motif, and a tail domain with the tripartite structure initially described in protozoan myosins I. The tail of Myo1f includes (1) a TH-1 region rich in basic residues, which may interact with anionic membrane phospholipids; (2) a TH-2 proline-rich region, expected to contain an ATP-insensitive actin-binding site; and (3) a SH-3 domain found in a variety of cytoskeletal and signaling proteins. Northern blot analysis indicated that the genes encoding Myo1c and Myo1f display a widespread tissue expression in the adult mouse. Myo1c and Myo1f were mapped by in situ hybridization to the chromosomal regions 11D-11E and 17B-17C, respectively. The human orthologuous genes MYO1C and MYO1F were also characterized, and mapped to the human chromosomal regions 17p18 and 19p13.2-19p13.3, respectively.

Evidence for physical and functional interactions among two Saccharomyces cerevisiae SH3 domain proteins, an adenylyl cyclase-associated protein and the actin cytoskeleton

In a variety of organisms, a number of proteins associated with the cortical actin cytoskeleton contain SH3 domains, suggesting that these domains may provide the physical basis for functional interactions among structural and regulatory proteins in the actin cytoskeleton. We present evidence that SH3 domains mediate at least two independent functions of the Saccharomyces cerevisiae actin-binding protein Abp1p in vivo. Abp1p contains a single SH3 domain that has recently been shown to bind in vitro to the adenylyl cyclase-associated protein Srv2p. Immunofluorescence analysis of Srv2p subcellular localization in strains carrying mutations in either ABP1 or SRV2 reveals that the Abp1p SH3 domain mediates the normal association of Srv2p with the cortical actin cytoskeleton. We also show that a site in Abp1p itself is specifically bound by the SH3 domain of the actin-associated protein Rvs167p. Genetic analysis provides evidence that Abp1p and Rvs167p have functions that are closely interrelated. Abp1 null mutations, like rvs167 mutations, result in defects in sporulation and reduced viability under certain suboptimal growth conditions. In addition, mutations in ABP1 and RVS167 yield similar profiles of genetic "synthetic lethal" interactions when combined with mutations in genes encoding other cytoskeletal components. Mutations which specifically disrupt the SH3 domain-mediated interaction between Abp1p and Srv2p, however, show none of the shared phenotypes of abp1 and rvs167 mutations. We conclude that the Abp1p SH3 domain mediates the association of Srv2p with the cortical actin cytoskeleton, and that Abp1p performs a distinct function that is likely to involve binding by the Rvs167p SH3 domain. Overall, work presented here illustrates how SH3 domains can integrate the activities of multiple actin cytoskeleton proteins in response to varying environmental conditions.

Chicken myosin IB mRNA is highly expressed in lymphoid tissues

Little is known about the functions of members of the myosin I family in vertebrates. Chicken myosin IB is a member of the amoeba-type subclass of myosin I molecules and tissue localisation studies may provide possible clues to the functions of these myosin I molecules. The expression of the mRNA of this unconventional myosin IB was analysed by in situ hybridization and compared with that of the well characterised brush border myosin I on frozen sections of tissues from the adult domestic chicken. High levels of myosin IB mRNA were found in the intestine and spleen, but were not found in other tissues examined such as brain, heart, lung, liver and kidney. In the intestine, myosin IB mRNA was much more abundant in the lamina propria than in the enterocytes, whereas brush border myosin I mRNA was restricted to the enterocytes. In the spleen, myosin IB mRNA expression was abundant in regions of white pulp, namely germinal centres, periellipsoid lymphocyte sheaths and periarteriolar lymphocyte sheaths. Lymphocytes are the major cell type in both the lamina propria and the white pulp of the spleen, which suggests that chicken myosin IB is highly expressed in lymphocytes. Lymphocyte recirculation depends on their migration through the endothelial layer and it is possible that myosin IB may have a role to play in this type of cell motility.

Role of type I myosins in receptor-mediated endocytosis in yeast

Type I myosins are thought to drive actin-dependent membrane motility, but the direct demonstration in vivo of their involvement in specific cellular processes has been difficult. Deletion of the genes MYO3 and MYO5, which encode the yeast type I myosins, almost abolished growth. A double-deleted mutant complemented with a MYO5 temperature-sensitive allele (myo5-1) showed a strong defect in the internalization step of receptor-mediated endocytosis, whereas the secretory pathway remained apparently unaffected. Thus, myosin I activity is required for a budding event in endocytosis but not for several other aspects of membrane traffic.

Dictyostelium mutants lacking multiple classic myosin I isoforms reveal combinations of shared and distinct functions

Dictyostelium cells that lack the myoB isoform were previously shown to exhibit reduced efficiencies of phagocytosis and chemotactic aggregation ("streaming") and to crawl at about half the speed of wild-type cells. Of the four other Dictyostelium myosin I isoforms identified to date, myoC and myoD are the most similar to myoB in terms of tail domain sequence. Furthermore, we show here that myoC, like myoB and myoD, is concentrated in actin-rich cortical regions like the leading edge of migrating cells. To look for evidence of functional overlap between these isoforms, we analyzed myoB, myoC, and myoD single mutants, myoB/myoD double mutants, and myoB/myoC/myoD triple mutants, which were created using a combination of gene targeting techniques and constitutive expression of antisense RNA. With regard to the speed of locomoting, aggregation-stage cells, of the three single mutants, only the myoB mutant was significantly slower. Moreover, double and triple mutants were only slightly slower than the myoB single mutant. Consistent with this, the protein level of myoB alone rises dramatically during early development, suggesting that a special demand is placed on this one isoform when cells become highly motile. We also found, however, that the absolute amount of myoB protein in aggregation-stage cells is much higher than that for myoC and myoD, suggesting that what appears to be a case of nonoverlapping function could be the result of large differences in the amounts of functionally overlapping isoforms. Streaming assays also suggest that myoC plays a significant role in some aspect of motility other than cell speed. With regard to phagocytosis, both myoB and myoC single mutants exhibited significant reductions in initial rate, suggesting that these two isoforms perform nonredundant roles in supporting the phagocytic process. In triple mutants these defects were not additive, however. Finally, because double and triple mutants exhibited significant and progressive decreases in doubling times, we also measured the kinetics of fluid phase endocytic flux (uptake, transit time, efflux). Not only do all three isoforms contribute to this process, but their contributions are synergistic. While these results, when taken together, refute the simple notion that these three "classic" myosin I isoforms perform exclusively identical functions, they do reveal that all three share in supporting at least one cellular process (endocytosis), and they identify several other processes (motility, streaming, and phagocytosis) that are supported to a significant extent by either individual isoforms or various combinations of them.

Myosins on the move to signal transduction

The continuous discovery of diverse myosin molecules reveals that the motor domain of myosin is used for many different types of functions. Recent data imply that myosins play active roles in signal transduction pathways, in which they modulate the activities of other proteins and properly position signalling activities at the cytoskeleton-membrane interface.

A Dictyostelium myosin I plays a crucial role in regulating the frequency of pseudopods formed on the substratum

Analysis of the motile behavior of a strain of Dictyostelium lacking a myosin I, myoA, revealed that this mutant strain formed pseudopods and turned twice as frequently as wild type cells [Titus et al., 1993: Mol. Biol. Cell 4:233-246]. The basis for this aberrant behavior has been explored using three-dimensional reconstructions of translocating cells. Wild type cells form approximately 40% of pseudopods on the substratum and 60% off the substratum. The majority of pseudopods formed on the substratum initiate sharp turns while the majority of pseudopods formed off the substratum are retracted. Although myoA- cells form pseudopods at roughly twice the frequency of wild type cells, the increase in frequency is specific for only those pseudopods formed on the substratum. This increase is the basis for the aberrant increase in turning in myoA- cells. The selective increase in the frequency of pseudopods formed on the substratum correlates with a number of additional abnormalities in myoA- pseuodpod formation. First, myoA- cells can simultaneously extend more than one pseudopod, whereas wild type cells extend only one pseudopod at a time. Second, although wild type and myoA- pseudopods achieve the same final volumes, myoA- pseudopods grow at half the rate of wild type pseudopods and, therefore, take longer to achieve final volume. Third, while a wild type pseudopod grows in a continuous fashion, a myoA- pseudopod grows in a discontinuous fashion. Together, these results demonstrate that myoA plays a fundamental role in controlling the frequency of only those pseudopods formed on the substratum, and that maintenance of the normal frequency of pseudopod formation appears to be necessary for the normal velocity of cellular translocation, the normal frequency of turning, the normal rate of average pseudopod growth, and the high efficiency of chemotaxis. These results in turn indicate that pseudopod formation is precisely coordinated in space and time, and actin-associated proteins like myoA play key roles in coordination.

Localization of the rat myosin I molecules myr 1 and myr 2 and in vivo targeting of their tail domains

Myr 1 is a widely distributed mammalian myosin I molecule related to brush border myosin 1. A second widely distributed myosin I molecule similar to myr 1 and brush border myosin I, called myr 2, has now been identified. Specific antibodies and expression of epitope-tagged molecules were used to determine the subcellular localization of myr 1 and myr 2 in NRK cells. Myr 1 was detected at the plasma membrane and was particularly enriched in cell protrusions like lamellipodia, membrane ruffles and filopodia. In dividing cells myr 1 localized to the cleavage furrow. Myr 2 was localized in a discrete punctate pattern in resting cells and in cells undergoing cytokinesis. In subcellular fractionation experiments myr 1 and myr 2 were both partly soluble and partly associated with smooth membranes of medium density. The tail domains of myosin I molecules have been proposed to interact with a receptor and thereby determine the subcellular localization. To test this hypothesis we expressed the tail domains of myr 1 and myr 2 that lack the F-actin-binding myosin head domain in NRK cells. These tail domains also partly copurified with smooth membranes of medium density and immunolocalized similar to the respective endogenous myosin I; however, they exhibited a lower affinity for membranes and an increased diffuse cytosolic localization. These results suggest that the tail domains of myr 1 and myr 2 are sufficient for subcellular targeting but that their head domains also contribute significantly to maintaining a proper subcellular localization.

Identification of Acan125 as a myosin-I-binding protein present with myosin-I on cellular organelles of Acanthamoeba

We have discovered the first protein to bind to a non-filamentous myosin, aside from actin. This protein, Acan125, is a 125-kDa protein from Acanthamoeba that associates with the SH3 domain of Acanthamoeba myosin-IC and not the SH3 domain of human fodrin. Antibodies raised against Acan125 recognize a single protein of 125 kDa from a whole cell lysate of Acanthamoeba; antibodies to myosin-I (M1.7 and M1.8) do not recognize Acan125 on the same blot. Double labeling of Acanthamoeba show Acan125 and myosin-I to be present on the same intracellular organelle, most likely amoebastomes. Immunoprecipitation with either anti-myosin-I or anti-Acan125 antibodies coprecipitates both Acan125 and myosin-I from a lysate of Acanthamoeba, demonstrating that Acan125 interacts with native myosin-I.

Regulation of calmodulin-binding myosins

Myosins constitute a diverse superfamily of actin-based mechanoenzymes that are involved in many essential cellular motilities. In addition to conventional muscle myosin II, ten other classes of unconventional myosins are known. Many unconventional myosins bind multiple calmodulin light chains and Ca2+, which can dramatically alter their mechanochemical and enzymatic activity. Calmodulin-binding myosins can also be regulated by phospholipid binding, phosphorylation of the heavy chain and actin-binding proteins. The molecular details linking unconventional-myosin regulation and function are just beginning to emerge.