The localization of myosin I and myosin II in Acanthamoeba by fluorescence microscopy

A conserved pressure-driven mechanism for regulating cytosolic osmolarity

Controlling intracellular osmolarity is essential to all cellular life. Cells that live in hypo-osmotic environments, such as freshwater, must constantly battle water influx to avoid swelling until they burst. Many eukaryotic cells use contractile vacuoles to collect excess water from the cytosol and pump it out of the cell. Although contractile vacuoles are essential to many species, including important pathogens, the mechanisms that control their dynamics remain unclear. To identify the basic principles governing contractile vacuole function, we investigate here the molecular mechanisms of two species with distinct vacuolar morphologies from different eukaryotic lineages: the discoban Naegleria gruberi and the amoebozoan slime mold Dictyostelium discoideum. Using quantitative cell biology, we find that although these species respond differently to osmotic challenges, they both use vacuolar-type proton pumps for filling contractile vacuoles and actin for osmoregulation, but not to power water expulsion. We also use analytical modeling to show that cytoplasmic pressure is sufficient to drive water out of contractile vacuoles in these species, similar to findings from the alveolate Paramecium multimicronucleatum. These analyses show that cytoplasmic pressure is sufficient to drive contractile vacuole emptying for a wide range of cellular pressures and vacuolar geometries. Because vacuolar-type proton-pump-dependent contractile vacuole filling and pressure-dependent emptying have now been validated in three eukaryotic lineages that diverged well over a billion years ago, we propose that this represents an ancient eukaryotic mechanism of osmoregulation.

A conserved pressure-driven mechanism for regulating cytosolic osmolarity

Controlling intracellular osmolarity is essential to all cellular life. Cells that live in hypo-osmotic environments like freshwater must constantly battle water influx to avoid swelling until they burst. Many eukaryotic cells use contractile vacuoles to collect excess water from the cytosol and pump it out of the cell. Although contractile vacuoles are essential to many species, including important pathogens, the mechanisms that control their dynamics remain unclear. To identify basic principles governing contractile vacuole function, we here investigate the molecular mechanisms of two species with distinct vacuolar morphologies from different eukaryotic lineagesâ€"the discoban Naegleria gruberi , and the amoebozoan slime mold Dictyostelium discoideum . Using quantitative cell biology we find that, although these species respond differently to osmotic challenges, they both use actin for osmoregulation, as well as vacuolar-type proton pumps for filling contractile vacuoles. We also use analytical modeling to show that cytoplasmic pressure is sufficient to drive water out of contractile vacuoles in these species, similar to findings from the alveolate Paramecium multimicronucleatum . Because these three lineages diverged well over a billion years ago, we propose that this represents an ancient eukaryotic mechanism of osmoregulation.

Cellulose fibrils formation and organisation of cytoskeleton during encystment are essential for Acanthamoeba cyst wall architecture

Acanthamoebae success as human pathogens is largely due to the highly resistant cysts which represent a crucial problem in treatment of Acanthamoeba infections. Hence, the study of cyst wall composition and encystment play an important role in finding new therapeutic strategies. For the first time, we detected high activity of cytoskeletal elements - microtubular networks and filamentous actin, in late phases of encystment. Cellulose fibrils - the main components of endocyst were demonstrated in inter-cystic space, and finally in the ectocyst, hereby proving the presence of cellulose in both layers of the cyst wall. We detected clustering of intramembranous particles (IMPs) and their density alterations in cytoplasmic membrane during encystment. We propose a hypothesis that in the phase of endocyst formation, the IMP clusters represent cellulose microfibril terminal complexes involved in cellulose synthesis that after cyst wall completion are reduced. Cyst wall impermeability, due largely to a complex polysaccharide (glycans, mainly cellulose) has been shown to be responsible for Acanthamoeba biocide resistance and cellulose biosynthesis pathway is suggested to be a potential target in treatment of Acanthamoeba infections. Disruption of this pathway would affect the synthesis of cyst wall and reduce considerably the resistance to chemotherapeutic agents.

Superdiffusion dominates intracellular particle motion in the supercrowded cytoplasm of pathogenic Acanthamoeba castellanii

Acanthamoebae are free-living protists and human pathogens, whose cellular functions and pathogenicity strongly depend on the transport of intracellular vesicles and granules through the cytosol. Using high-speed live cell imaging in combination with single-particle tracking analysis, we show here that the motion of endogenous intracellular particles in the size range from a few hundred nanometers to several micrometers in Acanthamoeba castellanii is strongly superdiffusive and influenced by cell locomotion, cytoskeletal elements, and myosin II. We demonstrate that cell locomotion significantly contributes to intracellular particle motion, but is clearly not the only origin of superdiffusivity. By analyzing the contribution of microtubules, actin, and myosin II motors we show that myosin II is a major driving force of intracellular motion in A. castellanii. The cytoplasm of A. castellanii is supercrowded with intracellular vesicles and granules, such that significant intracellular motion can only be achieved by actively driven motion, while purely thermally driven diffusion is negligible.

Regulation of the filament structure and assembly of Acanthamoeba myosin II by phosphorylation of serines in the heavy-chain nonhelical tailpiece

Acanthamoeba myosin II (AMII) has two heavy chains ending in a 27-residue nonhelical tailpiece and two pairs of light chains. In a companion article, we show that five, and only five, serine residues can be phosphorylated both in vitro and in vivo: Ser639 in surface loop 2 of the motor domain and serines 1489, 1494, 1499, and 1504 in the nonhelical tailpiece of the heavy chains. In that paper, we show that phosphorylation of Ser639 down-regulates the actin-activated MgATPase activity of AMII and that phosphorylation of the serines in the nonhelical tailpiece has no effect on enzymatic activity. Here we show that bipolar tetrameric, hexameric, and octameric minifilaments of AMII with the nonhelical tailpiece serines either phosphorylated or mutated to glutamate have longer bare zones and more tightly clustered heads than minifilaments of unphosphorylated AMII, irrespective of the phosphorylation state of Ser639. Although antiparallel dimers of phosphorylated and unphosphorylated myosins are indistinguishable, phosphorylation inhibits dimerization and filament assembly. Therefore, the different structures of tetramers, hexamers, and octamers of phosphorylated and unphosphorylated AMII must be caused by differences in the longitudinal stagger of phosphorylated and unphosphorylated bipolar dimers and tetramers. Thus, although the actin-activated MgATPase activity of AMII is regulated by phosphorylation of Ser639 in loop 2 of the motor domain, the structure of AMII minifilaments is regulated by phosphorylation of one or more of four serines in the nonhelical tailpiece of the heavy chain.

Cytopede: a three-dimensional tool for modeling cell motility on a flat surface

When cultured on flat surfaces, fibroblasts and many other cells spread to form thin lamellar sheets. Motion then occurs by extension of the sheet at the leading edge and retraction at the trailing edge. Comprehensive quantitative models of these phenomena have so far been lacking and to address this need, we have designed a three-dimensional code called Cytopede specialized for the simulation of the mechanical and signaling behavior of plated cells. Under assumptions by which the cytosol and the cytoskeleton are treated from a continuum mechanical perspective, Cytopede uses the finite element method to solve mass and momentum equations for each phase, and thus determine the time evolution of cellular models. We present the physical concepts that underlie Cytopede together with the algorithms used for their implementation. We then validate the approach by a computation of the spread of a viscous sessile droplet. Finally, to exemplify how Cytopede enables the testing of ideas about cell mechanics, we simulate a simple fibroblast model. We show how Cytopede allows computation, not only of basic characteristics of shape and velocity, but also of maps of cell thickness, cytoskeletal density, cytoskeletal flow, and substratum tractions that are readily compared with experimental data.

Membrane associated nonmuscle myosin II functions as a motor for actin-based vesicle transport in clam oocyte extracts

Nonmuscle myosin II (Myo2) has been shown to associate with membranes of the trans-Golgi network and to be involved in Golgi to ER retrograde protein transport. Here, we provide evidence that Myo2 not only associates with membranes but functions to transport vesicles on actin filaments (AFs). We used extracts from unactivated clam oocytes for these studies. AFs assembled spontaneously in these extracts and myosin-dependent vesicle transport was observed upon activation. In addition, actin bundles formed and moved relative to each other at an average speed of 0.30 microm/s. Motion analysis revealed that vesicles moved on the spontaneously assembled AFs at speeds greater than 1 microm/s. The motor on these vesicles was identified as a member of the nonmuscle Myo2 family based on sequence determination by Edman chemistry. Vesicles in these extracts were purified by sucrose gradient centrifugation and movement was reconstituted in vitro using skeletal muscle actin coated coverslips. When peripheral membrane proteins of vesicles including Myo2 were removed by salt stripping or when extracts were treated with an antibody specific to clam oocyte nonmuscle Myo2, vesicle movement was inhibited. Blebbistatin, a Myo2 specific inhibitor, also blocked vesicle movement. Myo2 light chain kinase activity was found to be essential for vesicle movement and sliding of actin bundles. Together, our data provide direct evidence that nonmuscle Myo2 is involved in actin-dependent vesicle transport in clam oocytes.

The role of actin, actomyosin and microtubules in defining cell shape during the differentiation of Naegleria amebae into flagellates

Differentiation of Naegleria amebae into flagellates was used to examine the interaction between actin, actomyosin and microtubules in defining cell shape. Amebae, which lack microtubules except during mitosis, differentiate into flagellates with a fixed shape and a complex microtubule cytoskeleton in 120 min. Based on earlier models of ameboid motility it has been suggested that actomyosin is quiescent in flagellates. This hypothesis was tested by following changes in the cytoskeleton using three-dimensional reconstructions prepared by confocal microscopy of individual cells stained with antibodies against actin and tubulin as well as with phalloidin and DNase I. F-actin as defined by phalloidin staining was concentrated in expanding pseudopods. Most phalloidin staining was lost as cells rounded up before the onset of flagellum formation. Actin staining with a Naegleria-specific antibody that recognizes both F- and G-actin was confined to the cell cortex of both amebae and flagellates. DNase I demonstrated G-actin throughout all stages. Most of the actin in the cortex was not bound by phalloidin yet was resistant to detergent extraction suggesting that it was polymerized. The microtubule cytoskeleton of flagellates was intimately associated with this actin cortex. Treatment of flagellates with cytochalasin D produced a rapid loss of flagellate shape and the appearance of phalloidin staining while latrunculin A stabilized the flagellate shape. These results suggest that tension produced by an actomyosin network is required to maintain the flagellate shape. The rapid loss of the flagellate shape induced by drugs, which specifically block myosin light chain kinase, supports this hypothesis.

Characterization of myosin-II binding to Golgi stacks in vitro

In addition to important roles near the actin-rich cell cortex, ample evidence indicates that multiple myosins are also involved in membrane movements in the endomembrane system. Nonmuscle myosin-II has been shown to have roles in anterograde and retrograde trafficking at the Golgi. Myosin-II is present on Golgi stacks isolated from intestinal epithelial cells and has been localized to the Golgi in several polarized and unpolarized cell lines. An understanding of roles of myosin-II in Golgi physiology will be facilitated by understanding the molecular arrangement of myosin-II at the Golgi. Salt-washing removes endogenous myosin-II from isolated Golgi and purified brush border myosin-II can bind in vitro. Brush border myosin-II binds to a tightly bound Golgi peripheral membrane protein with a K(1/2) of 75 nM and binding is saturated at 0.7 pmol myosin/microg Golgi. Binding studies using papain cleavage fragments of brush border myosin-II show that the 120-kDa rod domain, but not the head domain, of myosin heavy chain can bind directly to Golgi stacks. The 120-kDa domain does not bind to Golgi membranes when phosphorylated in vitro with casein kinase-II. These results suggest that phosphorylation in the rod domain may regulate the binding and/or release of myosin-II from the Golgi. These data support a model in which myosin-II is tethered to the Golgi membrane by its tail and actin filaments by its head. Thus, translocation along actin filaments may extend Golgi membrane tubules and/or vesicles away from the Golgi complex.

Assembly of Acanthamoeba myosin-II minifilaments. Definition of C-terminal residues required to form coiled-coils, dimers, and octamers

Acanthamoeba myosin-II forms bipolar octamers by three successive steps of dimerization of the C-terminal, coiled-coil tail. In this study, we generated N-terminal and C-terminal truncation constructs and point mutants of the Acanthamoeba myosin-II tail to delineate the structural requirements for assembly of bipolar mini-filaments. By the use of light-scattering, CD spectroscopy, analytical ultracentrifugation, and tryptophan fluorescence experiments, we determined that: (1) the C-terminal 14 heptad repeats plus most of the tailpiece (residues 1381-1509) are required to form antiparallel dimers of coiled-coils; (2) amino acid residues within heptads 23-32 (residues 1254-1325) are required to form tetramers; (3) the C-terminal 32 heptad repeats suffice to assemble octameric minifilaments; (4) A1378 is outside of the interaction interface; (5) the mutation L1475W inhibits dimerization; and (6) F1443 is involved in the dimerization interface but is exposed to the solvent. We propose that the tailpiece (residues 1483-1509) interacts with two heptads (13 and 14, residues 1381-1393), which are important for dimerization and coiled-coil formation. These results support a model in which hydrophobic as well as electrostatic interactions control the register between myosin-II coiled-coils and guide sequential steps of dimerization that generate stable, octameric mini-filaments.

Dynamic localization of myosin-I to endocytic structures in Acanthamoeba

We used fluorescence microscopy of live Acanthamoeba to follow the time course of the concentration of myosin-I next to the plasma membrane at sites of macropinocytosis and phagocytosis. We marked myosin-I with a fluorescently labeled monoclonal antibody (Cy3-M1.7) introduced into the cytoplasm by syringe loading. M1.7 binds myosin-IA and -IC without affecting their activities, but does not bind myosin-IB. Cy3-M1.7 concentrates at two different macropinocytic structures: large circular membrane ruffles that fuse to create macropinosomes, and smaller endocytic structures that occur at the end of stalk-like pseudopodia. These dynamic structures enclose macropinosomes every 30-60 s. Cy3-M1.7 accumulates rapidly as these endocytic structures form and dissipate rapidly after they internalize. Double labeling fixed cells with Cy3-M1.7 and polyclonal antibodies specific for myosin-IA, -IB, or -IC revealed that all three myosin-I isoforms associate with macropinocytic structures, but individual structures vary in their myosin-I isoform composition. Myosin-I and actin also concentrate transiently at sites where amoebae ingest yeast or the pseudopodia of neighboring cells (heterophagy) by the process of phagocytosis. Within 3 min of yeast attachment to the amoeba, myosin-I concentrates around the phagocytic cup, yeast are internalized, and myosin-I de-localizes. Despite known differences in the regulation of macropinocytosis and phagocytosis, the morphology, protein composition, and dynamics of phagocytosis and macropinocytosis are similar, indicating that they share common structural properties and contractile mechanisms.

Intracellular localization and dynamics of myosin-II and myosin-IC in live Acanthamoeba by transient transfection of EGFP fusion proteins

We developed a reliable method for transient transfection of Acanthamoeba using Superfect (Qiagen) and a vector with the Acanthamoeba ubiquitin promoter and enhanced green fluorescent protein (EGFP) as the reporter gene. The transfection efficiency was 3% for profilin-I-EGFP and EGFP-myosin-II tail, and less than 0.5% for larger constructs such as full length myosin-II or myosin-IC. Profilin-I-EGFP was distributed throughout the cytoplasm as observed previously with rhodamine-labeled profilin, while EGFP alone accumulated in the nucleus. EGFP fused to full length myosin-II or to the C-terminal 256 residues of the myosin-II tail concentrated in fluorescent spots similar to thick filaments and minifilaments identified previously in fixed cells with fluorescent antibodies. Thick filaments were located in the dorsal cytoplasm and along the lateral margins of the back half of the cell. Thick filaments formed behind the leading edge and moved continuously towards the rear of the cell, where they disassembled. If phosphorylation of the myosin-II heavy chain was prevented by mutation of all three phosphorylated serines to alanine, thick filaments of unphosphorylated myosin-II accumulated around vesicles of various sizes. EGFP-myosin-IC was spread throughout the cytoplasm but concentrated transiently around contractile vacuoles and macropinocytosis cups providing that the construct included both the head and a tail with the SH3 domain.

Myosins in protists

This review focuses on selected papers that illustrate an historical perspective and the current knowledge of myosin structure and function in protists. The review contains a general description of myosin structure, a phylogenetic tree of the myosin classes, and descriptions of myosin isoforms identified in protists. Each myosin is discussed within the context of the taxonomic group of the organism in which the myosin has been identified. Domain structure, cellular location, function, and regulation are described for each myosin.

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.

Fission yeast myosin-II isoforms assemble into contractile rings at distinct times during mitosis

Myosin-II is required for cytokinesis in Schizosaccharomyces pombe [1-3], but unlike other unicellular organisms, S. pombe has two structurally distinct myosin-IIs, Myo2p and Myp2p, which are required under different conditions [4]. Disruption of myo2(+) is lethal, whereas disruption of myp2(+) leads to defects in cytokinesis when nutrients are limiting and to cold-sensitivity in 1 M KCl. In dividing cells, both myosin-IIs localize to a ring in the center of the cell, which is thought to contract, separating the cytoplasms of the daughter cells. Using deconvolution microscopy, we obtained three-dimensional reconstructions of fission yeast cells expressing green fluorescent protein-labeled (GFP)-myosin-II, providing for the first time detailed images of GFP-myosin-II rings. By time-lapse microscopy, we observed ring assembly and contraction in three dimensions using GFP-tubulin as a cell cycle marker. We determined that the Myo2p ring forms in metaphase/anaphase A whereas the Myp2p ring forms much later, at the end of anaphase B. Myo2p initiates ring formation while Myp2p acts later to increase the efficiency of cytokinesis.

LvsA, a protein related to the mouse beige protein, is required for cytokinesis in Dictyostelium

We isolated a Dictyostelium cytokinesis mutant with a defect in a novel locus called large volume sphere A (lvsA). lvsA mutants exhibit an unusual phenotype when attempting to undergo cytokinesis in suspension culture. Early in cytokinesis, they initiate furrow formation with concomitant myosin II localization at the cleavage furrow. However, the furrow is later disrupted by a bulge that forms in the middle of the cell. This bulge is bounded by furrows on both sides, which are often enriched in myosin II. The bulge can increase and decrease in size multiple times as the cell attempts to divide. Interestingly, this phenotype is similar to the cytokinesis failure of Dictyostelium clathrin heavy-chain mutants. Furthermore, both cell lines cap ConA receptors but form only a C-shaped loose cap. Unlike clathrin mutants, lvsA mutants are not defective in endocytosis or development. The LvsA protein shares several domains in common with the molecules beige and Chediak-Higashi syndrome proteins that are important for lysosomal membrane traffic. Thus, on the basis of the sequence analysis of the LvsA protein and the phenotype of the lvsA mutants, we postulate that LvsA plays an important role in a membrane-processing pathway that is essential for cytokinesis.

Profilin is predominantly associated with monomeric actin in Acanthamoeba

We used biochemical fractionation, immunoassays and microscopy of live and fixed Acanthamoeba to determine how much profilin is bound to its known ligands: actin, membrane PIP(2), Arp2/3 complex and polyproline sequences. Virtually all profilin is soluble after gentle homogenization of cells. During gel filtration of extracts on Sephadex G75, approximately 60% of profilin chromatographs with monomeric actin, 40% is free and none voids with Arp2/3 complex or other large particles. Selective monoclonal antibodies confirm that most of the profilin is bound to actin: 65% in extract immunoadsorption assays and 74–89% by fluorescent antibody staining. Other than monomeric actin, no major profilin ligands are detected in crude extracts. Profilin-II labeled with rhodamine on cysteine at position 58 retains its affinity for actin, PIP(2) and poly-L-proline. When syringe-loaded into live cells, it distributes throughout the cytoplasm, is excluded from membrane-bounded organelles, and concentrates in lamellapodia and sites of endocytosis but not directly on the plasma membrane. Some profilin fluorescence appears punctate, but since no particulate profilin is detected biochemically, these spots may be soluble profilin between organelles that exclude profilin. The distribution of profilin in fixed human A431 cells is similar to that in amoebas. Our results show that the major pool of polymerizable actin monomers is complexed with profilin and spread throughout the cytoplasm.

Vesicle transport: the role of actin filaments and myosin motors

The transport of vesicles and the retention of organelles at specific locations are fundamental processes in cells. Actin filaments and myosin motors have been shown to be required for both of these tasks. Most of the organelles in cells associate with actin filaments and some of the myosin motors required for movement on actin filaments have been identified. Myosin V has been shown to transport endoplasmic reticulum (ER) vesicles in neurons, pigment granules in melanocytes, and the vacuole in yeast. Myosin I has been shown to be involved in the transport of Golgi-derived vesicles in epithelial cells. Myosin VI has been shown to be associated with Golgi-derived vesicles, and cytoplasmic vesicles in living Drosophila embryos. Myosin II may be a vesicle motor but its role in vesicle transport has not been resolved. Secretory vesicles, endosomes and mitochondria appear to be transported on actin filaments but the myosin motors on these organelles have not been identified. Mitochondria in yeast may be transported by the dynamic assembly of an actin "tail." The model that has unified all of these findings is the concept that long-range movement of vesicles occurs on microtubules and short-range movement on actin filaments. The details of how the microtubule-dependent and the actin-dependent motors are coordinated are important questions in the field. There is now strong evidence that two molecular motors, kinesin and myosin V, interact with each other and perhaps function as a complex on vesicles. An understanding of the interrelationship of microtubules and actin filaments and the motors that move cargo on them will ultimately establish how vesicles and organelles are transported to their specific locations in cells.

Arp2/3 complex from Acanthamoeba binds profilin and cross-links actin filaments

The Arp2/3 complex was first purified from Acanthamoeba castellanii by profilin affinity chromatography. The mechanism of interaction with profilin was unknown but was hypothesized to be mediated by either Arp2 or Arp3. Here we show that the Arp2 subunit of the complex can be chemically cross-linked to the actin-binding site of profilin. By analytical ultracentrifugation, rhodamine-labeled profilin binds Arp2/3 complex with a Kd of 7 microM, an affinity intermediate between the low affinity of profilin for barbed ends of actin filaments and its high affinity for actin monomers. These data suggest the barbed end of Arp2 is exposed, but Arp2 and Arp3 are not packed together in the complex exactly like two actin monomers in a filament. Arp2/3 complex also cross-links actin filaments into small bundles and isotropic networks, which are mechanically stiffer than solutions of actin filaments alone. Arp2/3 complex is concentrated at the leading edge of motile Acanthamoeba, and its localization is distinct from that of alpha-actinin, another filament cross-linking protein. Based on localization and actin filament nucleation and cross-linking activities, we propose a role for Arp2/3 in determining the structure of the actin filament network at the leading edge of motile cells.

Three-dimensional structure of Brush Border Myosin-I at approximately 20 A resolution by electron microscopy and image analysis

Brush Border Myosin-I (BBMI) is a single-headed, unconventional myosin found in the microvilli of intestinal epithelial cells where it forms lateral bridges between the core bundle of actin filaments and the plasma membrane of the microvillus. A three-dimensional (3D) reconstruction of BBMI was made from images of negatively stained, two-dimensional (2D) crystals grown on lipid monolayers formed from mixtures of phosphatidylserine and phosphatidylcholine. The resolution of the 3D map extends to approximately 20 A and allows identification of all of the major structural domains of BBMI. The BBMI molecule is composed of three domains: a globular motor domain, a light-chain-binding domain and a lipid-binding domain. In our map, the putative motor domain is connected to an extended density, which we believe to be the light-chain-binding domain. This long, narrow region has three distinct bends, which may delineate the bound calmodulin light chains. Following the last calmodulin there is density which extends for a short distance across the lipid surface and is presumably the carboxy-terminal lipid-binding domain.

Myosin II function in non-muscle cells

Amongst the remarkable variety of motility that cells display, cytokinesis (cell division) is particularly striking. Dramatic changes in cell shape occur before, during and after cytokinesis. Myosin II is implicated in the 'rounding up' of cells prior to cytokinesis, and is essential in the formation of the contractile cleavage furrow during cytokinesis. Now it appears that myosin II plays a role in all stages of cytokinesis, as a recent report suggests that myosin II drives post-mitotic cell spreading. A similar type of motile mechanism operating in cell spreading may occur in other cell types in other situations.

Examination of the endosomal and lysosomal pathways in Dictyostelium discoideum myosin I mutants

The role of myosin Is in endosomal trafficking and the lysosomal system was investigated in a Dictyostelium discoideum myosin I double mutant myoB-/C-, that has been previously shown to exhibit defects in fluid-phase endocytosis during growth in suspension culture (Novak et al., 1995). Various properties of the endosomal pathway in the myoB-/C- double mutant as well as in the myoB- and myoC- single mutants, including intravesicular pH, and intracellular retention time and exocytosis of a fluid phase marker, were found to be indistinguishable from wild-type parental cells. The intimate connection between the contractile vacuole complex and the endocytic pathway in Dictyostelium, and the localization of a myosin I to the contractile vacuole in Acanthamoeba, led us to also examine the structure and function of this organelle in the three myosin I mutants. No alteration in contractile vacuole structure or function was observed in the myoB-, myoC- or myoB-/C- cell lines. The transport, processing, and localization of a lysosomal enzyme, alpha-mannosidase, were also unaltered in all three mutants. However, the myoB- and myoB-/C- cell lines, but not the myoC- cell line, were found to oversecrete the lysosomal enzymes alpha-mannosidase and acid phosphatase, during growth and starvation. None of the mutants oversecreted proteins following the constitutive secretory pathway. Two additional myosin I mutants, myoA- and myoA-/B-, were also found to oversecrete the lysosomally localized enzymes alpha-mannosidase and acid phosphatase. Taken together, these results suggest that these myosins do not play a role in the intracellular movement of vesicles, but that they may participate in controlling events that occur at the actin-rich cortical region of the cell. While no direct evidence has been found for the association of myosin Is with lysosomes, we predict that the integrity of the lysosomal system is tied to the fidelity of the actin cortex, and changes in cortical organization could influence lysosomal-related membrane events such as internalization or transit of vesicles to the cell surface.

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.

Identification and localization of three classes of myosins in pollen tubes of Lilium longiflorum and Nicotiana alata

The presence and localization of actin and myosin have been examined in pollen tubes of Lilium longiflorum and Nicotiana alata. Immunoblot analysis of pollen tube extracts with antibodies to actin, myosins IA and IB, myosin II, and myosin V reveals the presence of these contractile proteins. Immunofluorescence microscopy using various methods to preserve the pollen tubes; chemical fixation, rapid freeze fixation and freeze substitution (RF-FS) followed by rehydration or by embeddment in a methacrylate mixture, was performed to optimize preservation. Immunocytochemistry reaffirmed that actin is localized longitudinally in the active streaming lanes and near the cortical surface of the pollen tube. Myosin I was localized to the plasma membrane, larger organelles, the surface of the generative cell and the vegetative nucleus, whereas, myosin V was found in the vegetative cytoplasm in a punctate fashion representing smaller organelles. Myosin II subfragment 1 and light meromyosin were localized in a punctate fashion on the larger organelles throughout the vegetative cytoplasm. In addition, isolated generative cells and vegetative nuclei labeled only with the myosin I antibody. Competition studies indicated the specificity of the heterologous antibodies utilized in this study suggesting the presence of three classes of myosins in pollen. These results lead to the following hypothesis: Myosin I may move the generative cell and vegetative nucleus unidirectionally through the pollen tube to the tip, while myosin V moves the smaller organelles and myosins I and II move the larger organelles (bidirectionally) that are involved in growth.

Molecular genetic analysis of myoC, a Dictyostelium myosin I

The protozoan myosin Is are widely expressed actin-based motors, yet their in vivo roles remain poorly understood. Molecular genetic studies have been carried out to determine their in vivo function in the simple eukaryote Dictyostelium, an organism that contains a family of four myosin Is. Here we report the characterization of myoC, a gene that encodes a fifth member of this family. Analysis of the deduced amino acid sequence reveals that the myoC gene encodes a myosin that is homologous to the well-described Acanthamoeba myosin Is as well as to Dictyostelium myoB and -D. The expression pattern of the myoC mRNA is similar to that of myoB and myoD, with a peak of expression at times of maximal cell migration, around 6 hours development. Deletion of the myoB gene has been previously shown to result in mutant cells that are defective in pseudopod extension and phagocytosis. However, no obvious differences in cell growth, development, phagocytosis or motility were detected in cells in which the myoC gene had been disrupted by homologous recombination. F-actin localization and ultrastructural organization also appeared unperturbed in myoC- cells. This apparent ‘lack’ of phenotype in a myosin I single knockout cannot be simply explained by redundancy of function. Our results rather suggest that the present means of assessing myosin I function in vivo are insufficient to identify the unique roles of these actin-based motors.

Microtubule-associated movement of mitochondria and small particles in Acanthamoeba castellanii

Using video-enhanced differential interference microscopy and digital image processing, we have observed organelle motility in Acanthamoeba castellanii. In amoebae taken from cultures in rapid growth phase, mitochondria and small particles moved over distances of several microns and at an average velocity of approximately 2 microns/s. Mitochondrial motility was verified by intensified fluorescence microscopy of cells that were labeled in vivo with the DNA-binding dye DAPI or the mitochondria-specific dye MitoTracker. We further studied the role of microtubules (MTs) in the translocation of cell organelles. Double-labeling of fixed cells with mitochondrial markers (anti-F1 beta antibody, MitoTracker) and cytoskeletal markers (anti-tubulin antibody, rhodamine-phalloidin) demonstrate that the mitochondria colocalize with MTs in the subcortical cell area and are excluded from the F-actin-rich cell cortex. Colchicine treatment resulted in an almost complete depolymerization of MTs and an inhibition of organelle motility. Moreover, we have directly visualized MTs in vivo in flattened amoebae. Mitochondria and small particles moved along the MTs in a bidirectional mode at an average velocity of approximately 1 micron/s. We conclude that the observed movement of mitochondria and small particles in Acanthamoeba castellanii mainly occurs via microtubules and associated motor proteins.

F-actin distribution of Dictyostelium myosin I double mutants

The roles of the myosin I class of mechanoenzymes have been investigated by single and double gene knockout studies in the amoeba Dictyostelium discoideum. Cells lacking different myosin I pairs (myoA-/myoB-, myoB-/myoC-, and myoA-/myoC-) were examined with respect to their cytoskeletal organization. F-actin localization by rhodamine-phalloidin staining of cells indicates that the myoA-/myoB-, myoB-/myoC-, and myoA-/myoC- cells appear to redistribute their F-actin more slowly than wild type cells upon adhesion to a substrate. These studies suggest that Dictyostelium myoA, myoB, and myoC may have overlapping roles in maintaining the integrity or organization of the cortical membrane cytoskeleton.

Identification of coelomocyte unconventional myosin and its association with in vivo particle/vesicle motility

Sea urchin coelomocytes undergo an inducible structural transformation from petalloid to filopodial form during the ‘clotting’ response in sea urchins. Using a petalloid coelomocyte model, stimulated coelomocytes exhibited bidirectional particle/vesicle motility with a broad distribution of velocities, ranging from 0.02 to 0.12 microns s-1 in the outward bound direction. Coelomocytes treated with the microtubule-disrupting drug, nocodazole, continued to exhibit outward particle/vesicle movements along linear paths with an average velocity of 0.028 +/- 0.006 microns s-1. We partially purified a 110 kDa polypeptide possessing K+EDTA-, Ca2(+)-, Mg2(+)- and F-actin-activated Mg(2+)-ATPase activities characteristic of myosin-like motor proteins. The 110 kDa protein immuno-crossreacted with both affinity-purified, anti-brush border unconventional myosin-I polyclonal antibodies and anti-Acanthamoeba myosin head monoclonal antibodies. By indirect immunofluorescence, the 110 kDa unconventional myosin was localized to clusters of particles/vesicles within the perinuclear region of unstimulated coelomocytes, an area containing numerous mitochondria, acidic, lysosomal and Golgi organelles. Indirect immunofluorescence of partially transformed and filopodial coelomocytes detected a diminution of perinuclear staining with a concomitant appearance of stained linear arrays of particles/vesicles, enhanced staining of peripheral lamellae, and staining of the entire length of the filopodia. Subfractionation of unstimulated coelomocyte homogenates on linear sucrose gradients identified distinct peaks of ATPase activity associated with fractions containing conventional and 110 kDa unconventional myosin. Unconventional myosin-containing fractions were found to have numerous particles that stained with anti-brush border unconventional myosin-I antibodies and the lipophilic dye, DiOC6. Thus, coelomocytes demonstrate activatable movements of particles/vesicles in cells devoid of microtubules and possess an unconventional myosin, which may be the motor protein driving particle/vesicle translocation.

Movement of axoplasmic organelles on actin filaments assembled on acrosomal processes: evidence for a barbed-end-directed organelle motor

The directionality of the actin-dependent motors on squid axoplasmic organelles was determined using actin filaments assembled on the barbed ends of acrosomal processes. Acrosomal processes were isolated from Limulus polyphemus sperm and incubated in monomeric actin under conditions that promoted barbed end assembly only. Newly assembled actin was stabilized and stained with rhodamine-phalloidin and the presence of filaments at the barbed ends of the acrosomal processes was verified by fluorescence microscopy and negative contrast electron microscopy. Axoplasmic organelles that dissociated from extruded axoplasm were observed by video microscopy to move along the newly assembled actin filaments at an average velocity of 1.1 +/- 0.3 microns/second. All organelles moved in the direction away from the acrosomal fragment and towards the tip of the actin filaments. Therefore, the actin-dependent organelle motor on axoplasmic organelles is a barbed-end-directed motor like other myosins analyzed. These findings support the conclusions that axoplasmic organelles are driven by a myosin-like motor along actin filaments and that these filaments as well as microtubules function in fast axonal transport.

Membrane motility mediated by unconventional myosin

Largely on the basis of their physical properties and their localization to cell membranes, it has been proposed that the unconventional myosins are membrane motors. In the past year, a combination of immunological, biochemical and genetic approaches has begun to provide direct evidence that unconventional myosins have important roles in movements of the plasma membrane and cytoplasmic organelles.