Myosin I localizes to the midbody region during mammalian cytokinesis

Actin guides filamentous rhizoid growth and morphogenesis in the zoosporic fungus Chytriomyces hyalinus

The advantage of filamentous growth to the fungal lifestyle is so great that it arose multiple times. Most zoosporic fungi from phylum Chytridiomycota exhibit a monocentric thallus form consisting of anucleate filamentous rhizoids that anchor reproductive sporangia to substrata and absorb nutrients. Actin function during polarized growth and cytokinesis is well documented across eukaryotes, but its role in sculpting nonhyphal, nonyeast fungal cells is unknown. We sought to provide a basis for comparing actin organization among major fungal lineages and to investigate the effects of actin disruption on morphogenesis in a monocentric thallus. Using fluorescence microscopy, we observed fixed, rhodamine phalloidin-stained actin in chemically fixed Chytriomyces hyalinus, exemplifying monocentric thallus development within the diverse, zoosporic phylum Chytridiomycota. We also compared rhizoid lengths and rhizoid branching of thalli incubated with the actin inhibitor latrunculin B to determine the effects of actin disruption on morphology. Actin was concentrated at the tips of growing rhizoids. Actin cables typically formed cortical, parallel arrays in hyphae, but in mature sporangia they were concentrated in a funnel-shaped array in the central region. Thalli treated with latrunculin B had shorter rhizoids with fewer branches than controls. In both hyphae and monocentric thalli, actin localization coincides with active, polarized growth and cytokinesis. Specific actin localization patterns are largely shared between monocentric species but differ significantly from patterns observed in hyphae. Actin integrity is critical for sustaining filamentous growth in all fungi.

The ESCRT machinery: from the plasma membrane to endosomes and back again

The manipulation and reorganization of lipid bilayers are required for diverse cellular processes, ranging from organelle biogenesis to cytokinetic abscission, and often involves transient membrane disruption. A set of membrane-associated proteins collectively known as the endosomal sorting complex required for transport (ESCRT) machinery has been implicated in membrane scission steps, which transform a single, continuous bilayer into two distinct bilayers, while simultaneously segregating cargo throughout the process. Components of the ESCRT pathway, which include 5 distinct protein complexes and an array of accessory factors, each serve discrete functions. This review focuses on the molecular mechanisms by which the ESCRT proteins facilitate cargo sequestration and membrane remodeling and highlights their unique roles in cellular homeostasis.

Multiple Parallelisms in Animal Cytokinesis

The process of cytokinesis in animal cells is usually presented as a relatively simple picture: A cleavage plane is first positioned in the equatorial region by the astral microtubules of the anaphase mitotic apparatus, and a contractile ring made up of parallel filaments of actin and myosin II is formed and encircles the cortex at the division site. Active sliding between the two filament systems constricts the perimeter of the cortex, leading to separation of two daughter cells. However, recent studies in both animal cells and lower eukaryotic model organisms have demonstrated that cytokinesis is actually far more complex. It is now obvious that the three key processes of cytokinesis, cleavage plane determination, equatorial furrowing, and scission, are driven by different mechanisms in different types of cells. In some cases, moreover, multiple pathways appear to have redundant functions in a single cell type. In this review, we present a novel hypothesis that incorporates recent observations on the activities of mitotic microtubules and the biochemistry of Rho-type GTPase proteins and postulates that two different sets of microtubules are responsible for the two known mechanisms of cleavage plane determination and also for two distinct mechanisms of equatorial furrowing.

Dictyostelium cytokinesis: from molecules to mechanics

Cytokinesis is the mechanical process that allows the simplest unit of life, the cell, to divide, propagating itself. To divide, the cell converts chemical energy into mechanical energy to produce force. This process is thought to be active, due in large part to the mechanochemistry of the myosin-II ATPase. The cell's viscoelasticity defines the context and perhaps the magnitude of the forces that are required for cytokinesis. The viscoelasticity may also guide the force-generating apparatus, specifying the cell shape change that results. Genetic, biochemical, and mechanical measurements are providing a quantitative view of how real proteins control this essential life process.

Myosins and cell dynamics in cellular slime molds

Myosin is a mechanochemical transducer and serves as a motor for various motile activities such as cell migration, cytokinesis, maintenance of cell shape, phagocytosis, and morphogenesis. Nonmuscle myosin in vivo does not either stay static at specific subcellular regions or construct highly organized structures, such as sarcomere in skeletal muscle cells. The cellular slime mold Dictyostelium discoideum is an ideal "model organism" for the investigation of cell movement and cytokinesis. The advantages of this organism prompted researchers to carry out pioneering cell biological, biochemical, and molecular genetic studies on myosin II, which resulted in elucidation of many fundamental features of function and regulation of this most abundant molecular motor. Furthermore, recent molecular biological research has revealed that many unconventional myosins play various functions in vivo. In this article, how myosins are organized and regulated in a dynamic manner in Dictyostelium cells is reviewed and discussed.

Myosin localization during meiosis I of crane-fly spermatocytes gives indications about its role in division

We showed previously that in crane-fly spermatocytes myosin is required for tubulin flux [Silverman-Gavrila and Forer, 2000a: J Cell Sci 113:597-609], and for normal anaphase chromosome movement and contractile ring contraction [Silverman-Gavrila and Forer, 2001: Cell Motil Cytoskeleton 50:180-197]. Neither the identity nor the distribution of myosin(s) were known. In the present work, we used immunofluorescence and confocal microscopy to study myosin during meiosis-I of crane-fly spermatocytes compared to tubulin, actin, and skeletor, a spindle matrix protein, in order to further understand how myosin might function during cell division. Antibodies to myosin II regulatory light chain and myosin II heavy chain gave similar staining patterns, both dependent on stage: myosin is associated with nuclei, asters, centrosomes, chromosomes, spindle microtubules, midbody microtubules, and contractile rings. Myosin and actin colocalization along kinetochore fibers from prometaphase to anaphase are consistent with suggestions that acto-myosin forces in these stages propel kinetochore fibres poleward and trigger tubulin flux in kinetochore fibres, contributing in this way to poleward chromosome movement. Myosin and actin colocalization at the cell equator in cytokinesis, similar to studies in other cells [e.g., Fujiwara and Pollard, 1978: J Cell Biol 77:182-195], supports a role of actin-myosin interactions in contractile ring function. Myosin and skeletor colocalization in prometaphase spindles is consistent with a role of these proteins in spindle formation. After microtubules or actin were disrupted, myosin remained in spindles and contractile rings, suggesting that the presence of myosin in these structures does not require the continued presence of microtubules or actin. BDM (2,3 butanedione, 2 monoxime) treatment that inhibits chromosome movement and cytokinesis also altered myosin distributions in anaphase spindles and contractile rings, consistent with the physiological effects, suggesting also that myosin needs to be active in order to be properly distributed.

Cloning and expression of a novel nuclear matrix-associated protein that is regulated during the retinoic acid-induced neuronal differentiation

Retinoic acid (RA), a derivative of vitamin A, is essential for the normal patterning and neurogenesis during development. RA treatment induces growth arrest and terminal differentiation of a human embryonal carcinoma cell line (NT2) into postmitotic central nervous system neurons. Using RNA fingerprinting by arbitrarily primed polymerase chain reaction, we identified a novel serine/threonine-rich protein, RA-regulated nuclear matrix-associated protein (Ramp), that was down-regulated during the RA-induced differentiation of NT2 cells. Prominent mRNA expression of ramp could be detected in adult placenta and testis as well as in all human fetal tissues examined. The genomic clone of ramp has been mapped to the telomere of chromosome arm 1q, corresponding to band 1q32.1-32.2. Associated with the nuclear matrix of NT2 cells, Ramp translocates from the interphase nucleus to the metaphase cytoplasm during mitosis. During the late stage of cytokinesis, Ramp concentrates at the midzone of the dividing daughter cells. The transcript expression of ramp is closely correlated with the cell proliferation rate of NT2 cells. Moreover, overexpression of Ramp induces a transient increase in the proliferation rate of NT2 cells. Taken together, our data suggest that Ramp plays a role in the proliferation of the human embryonal carcinoma cells.

Contractile apparatus of the normal and abortive cytokinetic cells during mouse male meiosis

Mouse male meiotic cytokinesis was studied using immunofluorescent probes against various elements of cytokinetic apparatus and electron microscopy. In normal mice, some spermatocytes fail to undergo cytokinesis after meiotic I or II nuclear divisions, forming syncytial secondary spermatocytes and spermatids. Abnormal cytokinetic cells develop sparse and dispersed midzone spindles during the early stage. However, during late stages, single and compact midzone spindles are formed as in normal cells, but localize asymmetrically and attach to the cortex. Myosin and f-actin were observed in the midzone spindle and midbody regions of normally cleaving cells as well as in those cells that failed to develop a cytokinetic furrow, implying that cytokinetic failure is unlikely to be due to defect in myosin or actin assembly. Depolymerization of microtubules by nocodazole resulted in the loss of the midbody-associated f-actin and myosin. These observations suggest that actin-myosin localization in the midbody could be a microtubule-dependent process that may not play a direct role in cytokinetic furrowing. Anti-centrin antibody labels the putative centrioles while anti-(gamma)-tubulin antibody labels the minus-ends of the midzone spindles of late-stage normal and abnormal cytokinetic cells, suggesting that the centrosome and midzone spindle nucleation in abnormal cytokinetic cells is not different from those of normally cleaving cells. Possible use of mouse male meiotic cells as a model system to study cytokinesis has been discussed.

Dynacortin, a genetic link between equatorial contractility and global shape control discovered by library complementation of a Dictyostelium discoideum cytokinesis mutant

We have developed a system for performing interaction genetics in Dictyostelium discoideum that uses a cDNA library complementation/multicopy suppression strategy. Chemically mutagenized cells were screened for cytokinesis-deficient mutants and one mutant was subjected to library complementation. Isolates of four different genes were recovered as modifiers of this strain's cytokinesis defect. These include the cleavage furrow protein cortexillin I, a novel protein we named dynacortin, an ezrin-radixin-moesin-family protein, and coronin. The cortexillin I locus and transcript were found to be disrupted in the strain, identifying it as the affected gene. Dynacortin is localized partly to the cell cortex and becomes enriched in protrusive regions, a localization pattern that is similar to coronin and partly dependent on RacE. During cytokinesis, dynacortin is found in the cortex and is somewhat enriched at the poles. Furthermore, it appears to be reduced in the cleavage furrow. The genetic interactions and the cellular distributions of the proteins suggest a hypothesis for cytokinesis in which the contraction of the medial ring is a function of spatially restricted cortexillin I and myosin II and globally distributed dynacortin, coronin, and RacE.

Myosin Va associates with microtubule-rich domains in both interphase and dividing cells

Class V unconventional myosins are two-headed, nonfilamentous, actin-based mechanoenzymes that appear to be expressed ubiquitously. Mice possess at least two myosin V heavy chain genes (dilute and myr6) whose approximately 190 kDa protein products are referred to as myosin Va and Vb, respectively. Using antibodies that are specific for the Va isoform and immunofluorescence microscopy, we show here that myosin Va localizes to the microtubule organizing center (MTOC) in interphase cells, and to the mitotic asters, spindle, and midbody of dividing cells. These associations, which in the case of mitotic cells are characterized by the concentration of myosin Va in the immediate vicinity of the microtubules, were observed in a variety of cell types, including primary and immortal mouse melanocytes and fibroblasts, Hela cells, and Cos cells. Importantly, these associations were not observed in melanocytes and fibroblasts cultured from dilute null mice, indicating that the staining of these microtubule-rich domains was due to the presence of myosin Va, as opposed to another protein(s) containing a shared epitope(s) with myosin Va. When cells were extracted with detergent prior to fixation, myosin Va remained associated with each of these microtubule-rich domains, suggesting that these associations are not due to the possible presence of membranes at these sites. This fact, and our observation that these microtubule-rich domains contain little if any F-actin (based on phalloidin staining), suggest that myosin Va may bind to microtubules either directly or through a microtubule-associated protein. Finally, we found that dilute null fibroblasts in primary culture are twice as likely to be binucleate as wild type fibroblasts of the same genetic background (35% vs. 17%). Together, these results indicate that myosin Va associates with microtubule-rich domains in both interphase and dividing cells, and plays a role in the efficiency of cell division in culture.

Towards a molecular understanding of cytokinesis

In this review, we focus on recent discoveries regarding the molecular basis of cleavage furrow positioning and contractile ring assembly and contraction during cytokinesis. However, some of these mechanisms might have different degrees of importance in different organisms. This synthesis attempts to uncover common themes and to reveal potential relationships that might contribute to the biochemical and mechanical aspects of cytokinesis. Because the information about cytokinesis is still fairly rudimentary, our goal is not to present a definitive model but to present testable hypotheses that might lead to a better mechanistic understanding of the process.

Assembly of cytoskeletal proteins into cleavage furrows of tissue culture cells

We review results obtained after fluorescent actin and myosin II probes were microinjected into interphase and prophase PtK2 and LLC-PK tissue culture cells to follow the changing distribution of these cytoskeletal proteins in the live cells during division. The fluorescent probes first begin to assemble into the future furrow region during mid-anaphase before any sign of initial contractions. The total concentrations of F-actin and myosin in the cleavage furrow begin to decrease a few minutes after the onset of furrow contraction. The cell's shape and the position of its mitotic spindle affect the deposition of cytoskeletal proteins in the forming cleavage furrow. In cells with two spindles, contractile proteins were recruited not only to the cortex bordering the former metaphase plates but also to the cortex midway between each pair of adjacent non-daughter poles or centrosomes. The furrowing between adjacent poles seen in these cultured cells are similar to the furrows observed by Rappaport [(1961) J Exp Zool 148:81-89] when echinoderm eggs were manipulated into a torus shape so that the poles of two mitotic spindles were adjacent to one another. These observations on injected tissue culture cells suggest that vertebrate cells share common mechanisms for the establishment of the cleavage furrow with echinoderm cells.

Myosin II-independent cytokinesis in Dictyostelium: its mechanism and implications

Similar to higher animal cells, ameba cells of the cellular slime mold Dictyostelium discoideum form contractile rings containing filaments of myosin II during mitosis, and it is generally believed that contraction of these rings bisects the cells both on substrates and in suspension. In suspension, mutant cells lacking the single myosin II heavy chain gene cannot carry out cytokinesis, become large and multinucleate, and eventually lyze, supporting the idea that myosin II plays critical roles in cytokinesis. These mutant cells are however viable on substrates. Detailed analyses of these mutant cells on substrates revealed that, in addition to "classic" cytokinesis which depends on myosin II ("cytokinesis A"), Dictyostelium has two distinct, novel methods of cytokinesis, 1) attachment-assisted mitotic cleavage employed by myosin II null cells on substrates ("cytokinesis B"), and 2) cytofission, a cell cycle-independent division of adherent cells ("cytokinesis C"). Cytokinesis A, B, and C lose their function and demand fewer protein factors in this order. Cytokinesis B is of particular importance for future studies. Similar to cytokinesis A, cytokinesis B involves formation of a cleavage furrow in the equatorial region, and it may be a primitive but basic mechanism of efficiently bisecting a cell in a cell cycle-coupled manner. Analysis of large, multinucleate myosin II null cells suggested that interactions between astral microtubules and cortices positively induce polar protrusive activities in telophase. A model is proposed to explain how such polar activities drive cytokinesis B, and how cytokinesis B is coordinated with cytokinesis A in wild type cells.

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.

Staining of the midbody by an anti-digoxin-specific antibody

Using RNA in situ hybridization to reveal cytoplasmic localization patterns of mRNAs in cultured cells, we noted unexpected staining of a cytoplasmic component in telophase cells. Control experiments revealed that the anti-digoxin-specific antibody was responsible for this staining. Because the staining was observed only at a position where both daughter cells are still connected, we identified the stained component as the midbody. This was confirmed by double staining of cells with anti-digoxin and anti-alpha-tubulin antibodies. We concluded that anti-digoxin-specific antibody shows crossreactivity with a component present in the midbody.

Transport of myosin II to the equatorial region without its own motor activity in mitotic Dictyostelium cells

Fluorescently labeled myosin moved and accumulated circumferentially in the equatorial region of dividing Dictyostelium cells within a time course of 4 min, followed by contraction of the contractile ring. To investigate the mechanism of this transport process, we have expressed three mutant myosins that cannot hydrolyze ATP in myosin null cells. Immunofluorescence staining showed that these mutant myosins were also correctly transported to the equatorial region, although no contraction followed. The rates of transport, measured using green fluorescent protein-fused myosins, were indistinguishable between wild-type and mutant myosins. These observations demonstrate that myosin is passively transported toward the equatorial region and incorporated into the forming contractile ring without its own motor activity.

Myosin I

The class I myosins are single-headed, actin-binding, mechanochemical "motor" proteins with heavy chains in the molecular mass range of 110-130 kDa; they do not form filaments. Each myosin I heavy chain is associated with one to six light chains that bind to specific motifs known as IQ domains. In vertebrate myosin I isoforms, the light chain is calmodulin, which is thought to regulate motor activity. Proteins similar to calmodulin are associated with myosin I isoforms from lower eukaryotes. Some myosin I isoforms from lower eukaryotes are regulated by phosphorylation; however, the phosphorylation site is not present in vertebrate myosin I isoforms. Based on sequence analyses of the amino terminal "head" domains, myosin I can be subdivided into several subclasses. Analyses of the biochemical properties of the isolated molecules and localization studies support the proposal of roles for these molecules in intracellular trafficking and changes in membrane structure. Our present understanding of the properties of these molecules and their proposed roles is reviewed here.

Evidence for the presence of myosin I in the nucleus

We produced and affinity-purified polyclonal antibodies to adrenal myosin I. These antibodies recognize adrenal myosin I by Western blot analysis (116 kDa) and inhibit the actin-activated ATPase activity of purified adrenal myosin I. They also recognize a 120-kDa protein in extracts prepared from many different cell lines. Fluorescence microscopy demonstrated the presence of immunoreactive material in the perinuclear region, the leading edges, and the nuclei of 3T3 cells. Fluorescence microscopy also demonstrated nuclear staining in mouse oocytes at the germinal vesicle stage and in the pronuclei during fertilization. Confocal and immunoelectron microscopy confirmed the intranuclear localization. Electron microscopy also demonstrated staining of structures in nucleoli that are thought to be associated with rDNA transcription. Western blot analyses revealed the presence of the 120-kDa protein in extracts prepared from nuclei that are apparently free of cytosolic contamination. The same nuclear protein binds 125I-calmodulin and is photoaffinity labeled with [alpha-32P]ATP. The 120-kDa protein was partially purified from twice washed nuclei using ammonium sulfate fractionation and gel filtration chromatography. Column fractions containing 120-kDa protein as revealed by Western blot analysis also contain K+-EDTA ATPase activity. The 120-kDa protein was also shown to bind actin in the absence, but not the presence, of ATP. Since K+-EDTA ATPase activity, actin, and ATP binding are defining features of the members of the myosin superfamily of proteins, we propose that the 120-kDa protein is a previously undescribed myosin I isoform that is an intranuclear actin-based molecular motor.

Traction forces of cytokinesis measured with optically modified elastic substrata

Animal cells dividing in culture undergo a dramatic sequence of morphological changes, characterized by cytoskeletal disassembly as cells round up, redistribution of actin, myosins and other cytoplasmic and surface molecules into the cleavage furrow, and respreading, before daughter cells finally separate at the mid-body. Knowledge of forces governing these movements is critical to understanding their mechanisms, including whether formation of the cleavage furrow results from increased force generation at the equator or relaxation at the poles, and whether traction force subsequently mediates cytofission of the intercellular bridge. We have quantitatively mapped traction forces in dividing cells, by extending the silicone-rubber substratum method to detect forces of nanonewtons to micronewtons. We used a new silicone polymer to fabricate substrata whose compliance can be adjusted precisely by ultraviolet irradiation. We show that traction force appears locally at the furrow in the absence of relaxation at the poles during cleavage. Force also rises as connected daughter cells respread and attempt to separate, suggesting that tension contributes to the severing of the intercellular bridge when cytokinesis is completed.

Myosin II-independent processes in mitotic cells of Dictyostelium discoideum: redistribution of the nuclei, re-arrangement of the actin system and formation of the cleavage furrow

Mitosis was studied in multinucleated and mononucleated mutant cells of Dictyostelium discoideum that lack myosin II (Manstein et al. (1989) EMBO J. 8, 923–932). Multinucleated cells were produced by culture in suspension, mononucleated cells were enriched by growth on a solid surface (DeLozanne and Spudich (1987) Science 236, 1086–1091). The multinucleated cells disclosed interactions of mitotic complexes with the cell cortex that were not apparent in normal, mononucleated cells. During the anaphase stage, entire mitotic complexes consisting of spindle, microtubule asters, and separated sets of chromosomes were translocated to the periphery of the cells. These complexes were appended at a distance of about 3 microns from the cell surface, in a way that the spindle became orientated in parallel to the surface. Subsequently, lobes of the cell surface were formed around the asters of microtubules. These lobes were covered with tapered protrusions rich in coronin, an actin associated protein that typically accumulates in dynamic cell-surface projections (DeHostos et al. (1991) EMBO J. 10, 4097–4104). During their growth on a solid surface, mononucleated myosin II-null cells passed through the mitotic cleavage stages with a speed comparable to wild-type cells. Cytokinesis as linked to mitosis is distinguishable by several parameters from traction mediated cytofission, which results in the pinching off of pieces of a multinucleated cell in the interphase. The possibility is discussed that cells can cleave during mitosis without forming a contractile ring at the site of the cleavage furrow.

Myosin II transport, organization, and phosphorylation: evidence for cortical flow/solation-contraction coupling during cytokinesis and cell locomotion

The mechanism of cytokinesis has been difficult to define because of the short duration and the temporal-spatial dynamics involved in the formation, activation, force production, and disappearance of the cleavage furrow. We have investigated the structural and chemical dynamics of myosin II in living Swiss 3T3 cells from prometaphase through the separation and migration of daughter cells. The structural and chemical dynamics of myosin II have been defined using the semiautomated, multimode light microscope, together with a fluorescent analogue of myosin II and a fluorescent biosensor of myosin II regulatory light chain (RLC) phosphorylation at serine 19. The correlation of image data from live cells using different modes of light microscopy allowed interpretations not possible from single-mode investigations. Myosin II transported toward the equatorial plane from adjacent regions, forming three-dimensional fibers that spanned the volume of the equator during anaphase and telophase. A global phosphorylation of myosin II at serine 19 of the RLC was initiated at anaphase when cortical myosin II transport started. The phosphorylation of myosin II remained high near the equatorial plane through telophase and into cytokinesis, whereas the phosphorylation of myosin II at serine 19 of the RLC decreased at the poles. The timing and pattern of phosphorylation was the same as the shortening of myosin II-based fibers in the cleavage furrow. Myosin II-based fibers shortened and transported out of the cleavage furrow into the tails of the two daughter cells late in cytokinesis. The patterns of myosin II transport, phosphorylation, and shortening of fibers in the migrating daughter cells were similar to that previously defined for cells migrating in a wound in vitro. The temporal-spatial patterns and dynamics of myosin II transport, phosphorylation at serine 19 of the RLC, and the shortening and disappearance of myosin II-based fibers support the proposal that a combination of the cortical flow hypothesis and the solation-contraction coupling hypothesis explain key aspects of cytokinesis and polarized cell locomotion.

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.

Molecular motors, membrane movements and physiology: emerging roles for myosins

Myosins are a large family of structurally diverse mechanoenzymes which, upon interaction with actin filaments, convert energy from ATP hydrolysis into mechanical force. Consistent with the ubiquitous association of actin with membranes, many of these novel myosins are membrane-associated. In the past two years, evidence has emerged that suggests roles for actin-based molecular motors in a wide range of membrane phenomena such as cell locomotion, phagocytosis, secretion, organelle transport, signal transduction and mechanoregulation of membrane protein function.