A potentially exhaustive screening strategy reveals two novel divergent myosins in Dictyostelium

Myosin B of Plasmodium falciparum (PfMyoB): in silico prediction of its three-dimensional structure and its possible interaction with MTIP

The mobility and invasion strategy of Plasmodium falciparum is governed by a protein complex known as the glideosome, which contains an actin-myosin motor. It has been shown that myosin A of the parasite (PfMyoA) is the myosin of the glideosome, and the interaction of PfMyoA with myosin tail domain interacting protein (MTIP) determines its correct location and its ability to function in the complex. Because PfMyoA and myosin B of P. falciparum (PfMyoB) share high sequence identity, are both small proteins without a tail domain, belong to the class XIV myosins, and are expressed in late schizonts and merozoites, we suspect that these myosins may have similar or redundant functions. Therefore, this work examined the structural similarity between PfMyoA and PfMyoB and performed a molecular docking between PfMyoB and MTIP. Three-dimensional (3D) models obtained for PfMyoA and PfMyoB achieved high scores in the structural validation programs used, and their superimposition revealed high structural similarity, supporting the hypothesis of possible similar functions for these two proteins. The 3D interaction models obtained and energy values found suggested that interaction between PfMyoB and MTIP is possible. Given the apparent abundance of PfMyoA relative to PfMyoB in the parasite, we believe that the interaction between PfMyoB and MTIP would only be detectable in specific cellular environments because under normal circumstances, it would be masked by the interaction between PfMyoA and MTIP.

The SAH domain extends the functional length of the myosin lever

Stable, single alpha-helix (SAH) domains are widely distributed in the proteome, including in myosins, but their functions are unknown. To test whether SAH domains can act as levers, we replaced four of the six calmodulin-binding IQ motifs in the levers of mouse myosin 5a (Myo5) with the putative SAH domain of Dictyostelium myosin MyoM of similar length. The SAH domain was inserted between the IQ motifs and the coiled coil in a Myo5 HMM construct in which the levers were truncated from six to two IQ motifs (Myo5-2IQ). Electron microscopy of this chimera (Myo5-2IQ-SAH) showed the SAH domain was straight and 17 nm long as predicted, restoring the truncated lever to the length of wild-type (Myo5-6IQ). The powerstroke (of 21.5 nm) measured in the optical trap was slightly less than that for Myo5-6IQ but much greater than for Myo5-2IQ. Myo5-2IQ-SAH moved processively along actin at physiological ATP concentrations with similar stride and run lengths to Myo5-6IQ in in-vitro single molecule assays. In comparison, Myo5-2IQ is not processive under these conditions. Solution biochemical experiments indicated that the rear head did not mechanically gate the rate of ADP release from the lead head, unlike Myo5-6IQ. These data show that the SAH domain can form part of a functional lever in myosins, although its mechanical stiffness might be lower. More generally, we conclude that SAH domains can act as stiff structural extensions in aqueous solution and this structural role may be important in other proteins.

Mechanism, regulation, and functional properties of Dictyostelium myosin-1B

Myosin-1B is one of three long tailed class-1 myosins containing an ATP-insensitive actin-binding site in the tail region that are produced in Dictyostelium discoideum. Myosin-1B localizes to actin-rich structures at the leading edge of migrating cells where it has been implicated in the formation and retraction of membrane projections, the recycling of plasma membrane components, and intracellular particle transport. Here, we have used a combination of molecular engineering approaches to describe the kinetic and motile properties of the myosin-1B motor and its regulation by TEDS site phosphorylation. Our results show that myosin-1B is a low duty ratio motor and displays the fastest nucleotide binding kinetics of any of the Dictyostelium class-1 myosins studied so far. Different from Dictyostelium myosin-1D and myosin-1E, dephosphorylated myosin-1B is not inactivated but moves actin filaments efficiently, albeit at an up to 8-fold slower velocity in the in vitro motility assay. A further difference is that myosin-1B lacks the ability to switch between rapid movement and bearing tension upon physiological changes of free Mg2+ ions. In this respect, its motor properties appear to be more closely related to Dictyostelium myosin-2 and rabbit skeletal muscle myosin.

Thirteen is enough: the myosins of Dictyostelium discoideum and their light chains

BACKGROUND

Dictyostelium discoideum is one of the most famous model organisms for studying motile processes like cell movement, organelle transport, cytokinesis, and endocytosis. Members of the myosin superfamily, that move on actin filaments and power many of these tasks, are tripartite proteins consisting of a conserved catalytic domain followed by the neck region consisting of a different number of so-called IQ motifs for binding of light chains. The tails contain functional motifs that are responsible for the accomplishment of the different tasks in the cell. Unicellular organisms like yeasts contain three to five myosins while vertebrates express over 40 different myosin genes. Recently, the question has been raised how many myosins a simple multicellular organism like Dictyostelium would need to accomplish all the different motility-related tasks.

RESULTS

The analysis of the Dictyostelium genome revealed thirteen myosins of which three have not been described before. The phylogenetic analysis of the motor domains of the new myosins placed Myo1F to the class-I myosins and Myo5A to the class-V myosins. The third new myosin, an orphan myosin, has been named MyoG. It contains an N-terminal extension of over 400 residues, and a tail consisting of four IQ motifs and two MyTH4/FERM (myosin tail homology 4/band 4.1, ezrin, radixin, and moesin) tandem domains that are separated by a long region containing an SH3 (src homology 3) domain. In contrast to previous analyses, an extensive comparison with 126 class-VII, class-X, class-XV, and class-XXII myosins now showed that MyoI does not group into any of these classes and should not be used as a model for class-VII myosins.The search for calmodulin related proteins revealed two further potential myosin light chains. One is a close homolog of the two EF-hand motifs containing MlcB, and the other, CBP14, phylogenetically groups to the ELC/RLC/calmodulin (essential light chain/regulatory light chain) branch of the tree.

CONCLUSION

Dictyostelium contains thirteen myosins together with 6-8 MLCs (myosin light chain) to assist in a variety of actin-based processes in the cell. Although they are homologous to myosins of higher eukaryotes, the myosins of Dictyostelium should be considered with care as models for specific functions of vertebrate myosins.

Signal transduction pathways regulated by Rho GTPases in Dictyostelium

Rho GTPases are ubiquitously expressed across the eukaryotes where they act as molecular switches, cycling between an active GTP-bound state and an inactive GDP-bound state. Activation enables Rho GTPases to interact with a multitude of effectors that relay upstream signals to cytoskeletal and other components, eliciting rearrangements of the actin cytoskeleton and diverse other cellular responses. In Dictyostelium the Rho family comprises 15 members. Some of them (Rac1a/b/c, RacF1/F2, RacB) are members of the Rac subfamily, and one, RacA, belongs to the RhoBTB subfamily, however the Rho and Cdc42 subfamilies are not represented. Dictyostelium Rho GTPases regulate actin polymerization, cell morphology, endocytosis, cytokinesis, cell polarity and chemotaxis. Guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs) modulate the activation/inactivation cycle of the GTPases. In addition, guanine nucleotide-dissociation inhibitors (GDIs) regulate cycling of the GTPases between membranes and cytosol. Members of these three classes of regulatory molecules along with some effectors have been identified in Dictyostelium during the last years and their role in Rho signaling pathways has been investigated.

Identification and phylogenetic analysis of Drosophila melanogaster myosins

Myosins constitute a superfamily of motor proteins that convert energy from ATP hydrolysis into mechanical movement along the actin filaments. Phylogenetic analysis currently places myosins into 17 classes based on class-specific features of their conserved motor domain. Traditionally, the myosins have been divided into two classes depending on whether they form monomers or dimers. The conventional myosin of muscle and nonmuscle cells forms class II myosins. They are complex molecules of four light chains bound to two heavy chains that form bipolar filaments via interactions between their coiled-coil tails (type II). Class I myosins are smaller monomeric myosins referred to as unconventional myosins. Now, at least 15 other classes of unconventional myosins are known. How many myosins are needed to ensure the proper development and function of eukaryotic organisms? Thus far, three types of myosins were found in budding yeast, six in the nematode Caenorhabditis elegans, and at least 12 in human. Here, we report on the identification and classification of Drosophila melanogaster myosins. Analysis of the Drosophila genome sequence identified 13 myosin genes. Phylogenetic analysis based on the sequence comparison of the myosin motor domains, as well as the presence of the class-specific domains, suggests that Drosophila myosins can be divided into nine major classes. Myosins belonging to previously described classes I, II, III, V, VI, and VII are present. Molecular and phylogenetic analysis indicates that the fruitfly genome contains at least five new myosins. Three of them fall into previously described myosin classes I, VII, and XV. Another myosin is a homolog of the mouse and human PDZ-containing myosins, forming the recently defined class XVIII myosins. PDZ domains are named after the postsynaptic density, disc-large, ZO-1 proteins in which they were first described. The fifth myosin shows a unique domain composition and a low homology to any of the existing classes. We propose that this is classified when similar myosins are identified in other species.

Myosin diversity in Apicomplexa

A polymerase chain reaction (PCR) screen was used to examine the diversity of myosins in 7 Apicomplexan parasites: Toxoplasma gondii, Plasmodium falciparum, Neospora caninum, Eimeria tenella, Sarcocystis muris, Babesia bovis, and Cryptosporidium parvum. Using degenerate PCR primers compatible with the majority of known myosin classes, putative myosin sequences were obtained from all of these species. All of the sequences obtained showed greatest similarity to previously identified apicomplexan myosins, suggesting that the diversity of myosins in these parasites is limited. Myosin classes that are known to be widespread across the phylogenetic spectrum, e.g., the myosins I, II, and V, were not seen in the Apicomplexa. Thus, like the plants, the Apicomplexa may have evolved their own unique cohort of myosins that are responsible for the myosin-driven cellular functions observed in these parasites.

A millennial myosin census

The past decade has seen a remarkable explosion in our knowledge of the size and diversity of the myosin superfamily. Since these actin-based motors are candidates to provide the molecular basis for many cellular movements, it is essential that motility researchers be aware of the complete set of myosins in a given organism. The availability of cDNA and/or draft genomic sequences from humans, Drosophila melanogaster, Caenorhabditis elegans, Arabidopsis thaliana, Saccharomyces cerevisiae, Schizosaccharomyces pombe, and Dictyostelium discoideum has allowed us to tentatively define and compare the sets of myosin genes in these organisms. This analysis has also led to the identification of several putative myosin genes that may be of general interest. In humans, for example, we find a total of 40 known or predicted myosin genes including two new myosins-I, three new class II (conventional) myosins, a second member of the class III/ninaC myosins, a gene similar to the class XV deafness myosin, and a novel myosin sharing at most 33% identity with other members of the superfamily. These myosins are in addition to the recently discovered class XVI myosin with N-terminal ankyrin repeats and two human genes with similarity to the class XVIII PDZ-myosin from mouse. We briefly describe these newly recognized myosins and extend our previous phylogenetic analysis of the myosin superfamily to include a comparison of the complete or nearly complete inventories of myosin genes from several experimentally important organisms.

Regulation of Dictyostelium myosin I and II

Dictyostelium expresses 12 different myosins, including seven single-headed myosins I and one conventional two-headed myosin II. In this review we focus on the signaling pathways that regulate Dictyostelium myosin I and myosin II. Activation of myosin I is catalyzed by a Cdc42/Rac-stimulated myosin I heavy chain kinase that is a member of the p21-activated kinase (PAK) family. Evidence that myosin I is linked to the Arp2/3 complex suggests that pathways that regulate myosin I may also influence actin filament assembly. Myosin II activity is stimulated by a cGMP-activated myosin light chain kinase and inhibited by myosin heavy chain kinases (MHCKs) that block bipolar filament assembly. Known MHCKs include MHCK A and MHCK B, which have a novel type of kinase catalytic domain joined to a WD repeat domain, and MHC-protein kinase C (PKC), which contains both diacylglycerol kinase and PKC-related protein kinase catalytic domains. A Dictyostelium PAK (PAKa) acts indirectly to promote myosin II filament formation, suggesting that the MHCKs may be indirectly regulated by Rac GTPases.

Molecular motors and membrane traffic in Dictyostelium

Phagocytosis and membrane traffic in general are largely dependent on the cytoskeleton and their associated molecular motors. The myosin family of motors, especially the unconventional myosins, interact with the actin cortex to facilitate the internalization of external materials during the early steps of phagocytosis. Members of the kinesin and dynein motor families, which mediate transport along microtubules (MTs), facilitate the intracellular processing of the internalized materials and the movement of membrane. Recent studies indicate that some unconventional myosins are also involved in membrane transport, and that the MT- and actin-dependent transport systems might interact with each other. Studies in Dictyostelium have led to the discovery of many motors involved in critical steps of phagocytosis and membrane transport. With the ease of genetic and biochemical approaches, the established functional analysis to test phagocytosis and vesicle transport, and the effort of the Dictyostelium cDNA and Genome Projects, Dictyostelium will continue to be a superb model system to study phagocytosis in particular and cytoskeleton and motors in general.

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.

A myosin I is involved in membrane recycling from early endosomes

Geometry-based mechanisms have been proposed to account for the sorting of membranes and fluid phase in the endocytic pathway, yet little is known about the involvement of the actin-myosin cytoskeleton. Here, we demonstrate that Dictyostelium discoideum myosin IB functions in the recycling of plasma membrane components from endosomes back to the cell surface. Cells lacking MyoB (myoA(-)/B(-), and myoB(-) cells) and wild-type cells treated with the myosin inhibitor butanedione monoxime accumulated a plasma membrane marker and biotinylated surface proteins on intracellular endocytic vacuoles. An assay based on reversible biotinylation of plasma membrane proteins demonstrated that recycling of membrane components is severely impaired in myoA/B null cells. In addition, MyoB was specifically found on magnetically purified early pinosomes. Using a rapid-freezing cryoelectron microscopy method, we observed an increased number of small vesicles tethered to relatively early endocytic vacuoles in myoA(-)/B(-) cells, but not to later endosomes and lysosomes. This accumulation of vesicles suggests that the defects in membrane recycling result from a disordered morphology of the sorting compartment.

Novel Dictyostelium unconventional myosin, MyoM, has a putative RhoGEF domain

We have cloned a novel unconventional myosin gene myoM in Dictyostelium. Phylogenetic analysis of the motor domain indicated that MyoM does not belong to any known subclass of the myosin superfamily. Following the motor domain, two calmodulin-binding IQ motifs, a putative coiled-coil region, and a Pro, Ser and Thr-rich domain, lies a combination of dbl homology and pleckstrin homology domains. These are conserved in Rho GDP/GTP exchange factors (RhoGEFs). We have identified for the first time the RhoGEF domain in the myosin sequences. The growth and terminal developmental phenotype of Dictyostelium cells were not affected by the myoM(-) mutation. Green fluorescent protein-tagged MyoM, however, accumulated at crown-shaped projections and membranes of phase lucent vesicles in growing cells, suggesting its possible roles in macropinocytosis.

The role of unconventional myosins in Dictyostelium endocytosis

Dictyostelium discoideum is a simple eukaryote amenable to detailed molecular studies of the endocytic processes phagocytosis and macropinocytosis. Both the actin cytoskeleton and associated myosin motors are well-described and a range of mutants are now available that enable characterization of the role of the cytoskeleton in a range of cellular functions. Molecular genetic studies have uncovered roles for two different classes of Dictyostelium unconventional myosins in endocytosis. The class I myosins contribute to both macropinocytosis and phagocytosis by playing a general role in controlling actin-dependent manipulations of the actin-rich cortex. A class VII myosin has been shown to be important for phagocytosis. This brief review summarizes what is known about the role of these different myosins in both fluid and particle uptake in this system.

The tail domain of myosin M catalyses nucleotide exchange on Rac1 GTPases and can induce actin-driven surface protrusions

Members of the myosin superfamily play crucial roles in cellular processes including management of the cortical cytoskeleton, organelle transport and signal transduction. GTPases of the Rho family act as key control elements in the reorganization of the actin cytoskeleton in response to growth factors, and other functions such as membrane trafficking, transcriptional regulation, growth control and development. Here, we describe a novel unconventional myosin from Dictyostelium discoideum, MyoM. Primary sequence analysis revealed that it has the appearance of a natural chimera between a myosin motor domain and a guanine nucleotide exchange factor (GEF) domain for Rho GTPases. The functionality of both domains was established. Binding of the motor domain to F-actin was ATP-dependent and potentially regulated by phosphorylation. The GEF domain displayed selective activity on Rac1-related GTPases. Overexpression, rather than absence of MyoM, affected the cell morphology and viability. Particularly in response to hypo-osmotic stress, cells overexpressing the MyoM tail domain extended massive actin-driven protrusions. The GEF was enriched at the tip of growing protuberances, probably through its pleckstrin homology domain. MyoM is the first unconventional myosin containing an active Rac-GEF domain, suggesting a role at the interface of Rac-mediated signal transduction and remodeling of the actin cytoskeleton.

A dibasic motif in the tail of a class XIV apicomplexan myosin is an essential determinant of plasma membrane localization

Obligate intracellular parasites of the phylum Apicomplexa exhibit gliding motility, a unique form of substrate-dependent locomotion essential for host cell invasion and shown to involve the parasite actin cytoskeleton and myosin motor(s). Toxoplasma gondii has been shown to express three class XIV myosins, TgM-A, -B, and -C. We identified an additional such myosin, TgM-D, and completed the sequences of a related Plasmodium falciparum myosin, PfM-A. Despite divergent structural features, TgM-A purified from parasites bound actin in an ATP-dependent manner. Isoform-specific antibodies revealed that TgM-A and recombinant mycTgM-A were localized right beneath the plasma membrane, and subcellular fractionation indicated a tight membrane association. Recombinant TgM-D also had a peripheral although not as sharply defined localization. Truncation of their respective tail domains abolished peripheral localization and tight membrane association. Conversely, fusion of the tails to green fluorescent protein (GFP) was sufficient to confer plasma membrane localization and sedimentability. The peripheral localization of TgM-A and of the GFP-tail fusion did not depend on an intact F-actin cytoskeleton, and the GFP chimera did not localize to the plasma membrane of HeLa cells. Finally, we showed that the specific localization determinants were in the very C terminus of the TgM-A tail, and site-directed mutagenesis revealed two essential arginine residues. We discuss the evidence for a proteinaceous plasma membrane receptor and the implications for the invasion process.

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.