A structural model for phosphorylation control of Dictyostelium myosin II thick filament assembly

PTEN and the PTEN-like phosphatase CnrN have both distinct and overlapping roles in a Dictyostelium chemorepulsion pathway

Little is known about eukaryotic chemorepulsion. The enzymes phosphatase and tensin homolog (PTEN) and CnrN dephosphorylate phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. Dictyostelium discoideum cells require both PTEN and CnrN to induce chemorepulsion of cells away from the secreted chemorepellent protein AprA. How D. discoideum cells utilize two proteins with redundant phosphatase activities in response to AprA is unclear. Here, we show that D. discoideum cells require both PTEN and CnrN to locally inhibit Ras activation, decrease basal levels of PI(3,4,5)P3 and increase basal numbers of macropinosomes, and AprA prevents this increase. AprA requires both PTEN and CnrN to increase PI(4,5)P2 levels, decrease PI(3,4,5)P3 levels, inhibit proliferation, decrease myosin II phosphorylation and increase filopod sizes. PTEN, but not CnrN, decreases basal levels of PI(4,5)P2, and AprA requires PTEN, but not CnrN, to induce cell roundness. Together, our results suggest that CnrN and PTEN play unique roles in AprA-induced chemorepulsion.

One must reconstitute the functions of interest from purified proteins

I am often asked by students and younger colleagues and now by the editors of this issue to tell the history of the development of the in vitro motility assay and the dual-beam single-molecule laser trap assay for myosin-driven actin filament movement, used widely as key assays for understanding how both muscle and nonmuscle myosin molecular motors work. As for all discoveries, the history of the development of the myosin assays involves many people who are not authors of the final publications, but without whom the assays would not have been developed as they are. Also, early experiences shape how one develops ideas and experiments, and influence future discoveries in major ways. I am pleased here to trace my own path and acknowledge the many individuals involved and my early science experiences that led to the work I and my students, postdoctoral fellows, and sabbatical visitors did to develop these assays. Mentors are too often overlooked in historical descriptions of discoveries, and my story starts with those who mentored me.

PTEN and the PTEN-like phosphatase CnrN have both distinct and overlapping roles in a Dictyostelium chemorepulsion pathway

The directed movement of eukaryotic cells is crucial for processes such as embryogenesis and immune cell trafficking. The enzyme Phosphatase and tensin homolog (PTEN) dephosphorylates phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P 3 ] to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 ]. Dictyostelium discoideum cells require both PTEN and the PTEN-like phosphatase CnrN to locally inhibit Ras activation to induce biased movement of cells away from the secreted chemorepellent protein AprA. Both PTEN and CnrN decrease basal levels of PI(3,4,5)P 3 and increase basal numbers of macropinosomes, and AprA prevents this increase. AprA requires both PTEN and CnrN to increase PI(4,5)P 2 levels, decrease PI(3,4,5)P 3 levels, inhibit proliferation, decrease myosin II phosphorylation, and increase filopod sizes. AprA causes PTEN, similar to CnrN, to localize to the side of the cell towards AprA in an AprA gradient. However, PTEN and CnrN also have distinct roles in some signaling pathways. PTEN, but not CnrN, decreases basal levels of PI(4,5)P 2 , AprA requires PTEN, but not CnrN, to induce cell roundness, and CnrN and PTEN have different effects on the number of filopods and pseudopods, and the sizes of filopods. Together, our results suggest that CnrN and PTEN play unique roles in D. discoideum signaling pathways, and possibly dephosphorylate PI(3,4,5)P 3 in different membrane domains, to mediate chemorepulsion away from AprA.

Suggesting Dictyostelium as a Model for Disease-Related Protein Studies through Myosin II Polymerization Pathway

Dictyostelium myosin II displays remarkable dynamism within the cell, continually undergoing polymerization and depolymerization processes. Under low-ion conditions, it assumes a folded structure like muscle myosins and forms thick filaments through polymerization. In our study, we presented intermediate structures observed during the early stages of polymerization of purified myosin via negative staining electron microscopy, immediately crosslinked with glutaraldehyde at the onset of polymerization. We identified folded monomers, dimers, and tetramers in the process. Our findings suggest that Dictyostelium myosin II follows a polymerization pathway in vitro akin to muscle myosin, with folded monomers forming folded parallel and antiparallel dimers that subsequently associate to create folded tetramers. These folded tetramers eventually unfold and associate with other tetramers to produce long filaments. Furthermore, our research revealed that ATP influences filament size, reducing it regardless of the status of RLC phosphorylation while significantly increasing the critical polymerization concentrations from 0.2 to 9 nM. In addition, we demonstrate the morphology of fully matured Dictyostelium myosin II filaments.

Particle-based model of mechanosensory contractility kit assembly

Cell shape change processes, such as proliferation, polarization, migration, and cancer metastasis, rely on a dynamic network of macromolecules. The proper function of this network enables mechanosensation, the ability of cells to sense and respond to mechanical cues. Myosin II and cortexillin I, critical elements of the cellular mechanosensory machinery, preassemble in the cytoplasm of Dictyostelium cells into complexes that we have termed contractility kits (CKs). Two IQGAP proteins then differentially regulate the mechanoresponsiveness of the cortexillin I-myosin II elements within CKs. To investigate the mechanism of CK self-assembly and gain insight into possible molecular means for IQGAP regulation, we developed a coarse-grained excluded volume molecular model in which all protein polymers are represented by nm-sized spheres connected by spring-like links. The model is parameterized using experimentally measured parameters acquired through fluorescence cross-correlation spectroscopy and fluorescence correlation spectroscopy, which describe the interaction affinities and diffusion coefficients for individual molecular components, and which have also been validated via several orthogonal methods. Simulations of wild-type and null-mutant conditions implied that the temporal order of assembly of these kits is dominated by myosin II dimer formation and that IQGAP proteins mediate cluster growth. In addition, our simulations predicted the existence of "ambiguous" CKs that incorporate both classes of IQGAPs, and we confirmed this experimentally using fluorescence cross-correlation spectroscopy. The model serves to describe the formation of the CKs and how their assembly enables and regulates mechanosensation at the molecular level.

Higher order assembling of the mycobacterial polar growth factor DivIVA/Wag31

DivIVA or Wag31, which is an essential pole organizing protein in mycobacteria, can self-assemble at the negatively curved side of the membrane at the growing pole to form a higher order structural scaffold for maintaining cellular morphology and localizing various target proteins for cell-wall biogenesis. The structural organization of polar scaffold formed by polymerization of coiled-coil rich Wag31, which is implicated in the anti-tubercular activities of amino-pyrimidine sulfonamides, remains to be determined. A single-site phosphorylation in Wag31 regulates peptidoglycan biosynthesis in mycobacteria. We report biophysical characterizations of filaments formed by mycobacterial Wag31 using circular dichroism, atomic force microscopy and small angle solution X-ray scattering. Atomic force microscopic images of the wild-type, a phospho-mimetic (T73E) and a phospho-ablative (T73A) form of Wag31 show mostly linear filament formation with occasional curving, kinking and apparent branching. Solution X-ray scattering data indicates that the phospho-mimetic forms of the Wag31 polymers are on average more compact than their phospho-ablative counterparts, which is likely due to the extent of bending/branching. Observed structural features in this first view of Wag31 filaments suggest a basis for higher order Wag31 scaffold formation at the pole.

An endogenous chemorepellent directs cell movement by inhibiting pseudopods at one side of cells

Eukaryotic chemoattraction signal transduction pathways, such as those used by Dictyostelium discoideum to move toward cAMP, use a G protein-coupled receptor to activate multiple conserved pathways such as PI3 kinase/Akt/PKB to induce actin polymerization and pseudopod formation at the front of a cell, and PTEN to localize myosin II to the rear of a cell. Relatively little is known about chemorepulsion. We previously found that AprA is a chemorepellent protein secreted by Dictyostelium cells. Here we used 29 cell lines with disruptions of cAMP and/or AprA signal transduction pathway components, and delineated the AprA chemorepulsion pathway. We find that AprA uses a subset of chemoattraction signal transduction pathways including Ras, protein kinase A, target of rapamycin (TOR), phospholipase A, and ERK1, but does not require the PI3 kinase/Akt/PKB and guanylyl cyclase pathways to induce chemorepulsion. Possibly as a result of not using the PI3 kinase/Akt/PKB pathway and guanylyl cyclases, AprA does not induce actin polymerization or increase the pseudopod formation rate, but rather appears to inhibit pseudopod formation at the side of cells closest to the source of AprA.

Designed Protein Origami

Proteins are highly perfected natural molecular machines, owing their properties to the complex tertiary structures with precise spatial positioning of different functional groups that have been honed through millennia of evolutionary selection. The prospects of designing new molecular machines and structural scaffolds beyond the limits of natural proteins make design of new protein folds a very attractive prospect. However, de novo design of new protein folds based on optimization of multiple cooperative interactions is very demanding. As a new alternative approach to design new protein folds unseen in nature, folds can be designed as a mathematical graph, by the self-assembly of interacting polypeptide modules within the single chain. Orthogonal coiled-coil dimers seem like an ideal building module due to their shape, adjustable length, and above all their designability. Similar to the approach of DNA nanotechnology, where complex tertiary structures are designed from complementary nucleotide segments, a polypeptide chain composed of a precisely specified sequence of coiled-coil forming segments can be designed to self-assemble into polyhedral scaffolds. This modular approach encompasses long-range interactions that define complex tertiary structures. We envision that by expansion of the toolkit of building blocks and design strategies of the folding pathways protein origami technology will be able to construct diverse molecular machines.

Various Themes of Myosin Regulation

Members of the myosin superfamily are actin-based molecular motors that are indispensable for cellular homeostasis. The vast functional and structural diversity of myosins accounts for the variety and complexity of the underlying allosteric regulatory mechanisms that determine the activation or inhibition of myosin motor activity and enable precise timing and spatial aspects of myosin function at the cellular level. This review focuses on the molecular basis of posttranslational regulation of eukaryotic myosins from different classes across species by allosteric intrinsic and extrinsic effectors. First, we highlight the impact of heavy and light chain phosphorylation. Second, we outline intramolecular regulatory mechanisms such as autoinhibition and subsequent activation. Third, we discuss diverse extramolecular allosteric mechanisms ranging from actin-linked regulatory mechanisms to myosin:cargo interactions. At last, we briefly outline the allosteric regulation of myosins with synthetic compounds.

Kinetic Monte Carlo simulations of the assembly of filamentous biomacromolecules by dimer addition mechanism

In cells, several important biomacromolecules form oligomers through a dimer addition mechanism. Rate equations based on mean field approximations are usually employed to describe the assembly process. However, such equations often require multiple assumptions that mask some detailed changes of the biomolecular configurations during aggregations. Here, we present a Kinetic Monte Carlo simulation scheme to account for the diffusion and rotation of dimers on two-dimensional hexagonal lattices while naturally including the stochastic features. We investigate the effects of the interaction energy between dimers, the diffusion coefficient and the concentration of dimers on the aggregation by dimer addition mechanism. Our simulations identified unusual double-S shape evolutions of aggregation kinetics, which are probably associated with the formation of metastable clusters.

Association of polyalanine and polyglutamine coiled coils mediates expansion disease-related protein aggregation and dysfunction

The expansion of homopolymeric glutamine (polyQ) or alanine (polyA) repeats in certain proteins owing to genetic mutations induces protein aggregation and toxicity, causing at least 18 human diseases. PolyQ and polyA repeats can also associate in the same proteins, but the general extent of their association in proteomes is unknown. Furthermore, the structural mechanisms by which their expansion causes disease are not well understood, and these repeats are generally thought to misfold upon expansion into aggregation-prone β-sheet structures like amyloids. However, recent evidence indicates a critical role for coiled-coil (CC) structures in triggering aggregation and toxicity of polyQ-expanded proteins, raising the possibility that polyA repeats may as well form these structures, by themselves or in association with polyQ. We found through bioinformatics screenings that polyA, polyQ and polyQA repeats have a phylogenetically graded association in human and non-human proteomes and associate/overlap with CC domains. Circular dichroism and cross-linking experiments revealed that polyA repeats can form—alone or with polyQ and polyQA—CC structures that increase in stability with polyA length, forming higher-order multimers and polymers in vitro. Using structure-guided mutagenesis, we studied the relevance of polyA CCs to the in vivo aggregation and toxicity of RUNX2—a polyQ/polyA protein associated with cleidocranial dysplasia upon polyA expansion—and found that the stability of its polyQ/polyA CC controls its aggregation, localization and toxicity. These findings indicate that, like polyQ, polyA repeats form CC structures that can trigger protein aggregation and toxicity upon expansion in human genetic diseases.

Essential role of coiled coils for aggregation and activity of Q/N-rich prions and PolyQ proteins

The functional switch of glutamine/asparagine (Q/N)-rich prions and the neurotoxicity of polyQ-expanded proteins involve complex aggregation-prone structural transitions, commonly presumed to be forming β sheets. By analyzing sequences of interaction partners of these proteins, we discovered a recurrent presence of coiled-coil domains both in the partners and in segments that flank or overlap Q/N-rich and polyQ domains. Since coiled coils can mediate protein interactions and multimerization, we studied their possible involvement in Q/N-rich and polyQ aggregations. Using circular dichroism and chemical crosslinking, we found that Q/N-rich and polyQ peptides form α-helical coiled coils in vitro and assemble into multimers. Using structure-guided mutagenesis, we found that coiled-coil domains modulate in vivo properties of two Q/N-rich prions and polyQ-expanded huntingtin. Mutations that disrupt coiled coils impair aggregation and activity, whereas mutations that enhance coiled-coil propensity promote aggregation. These findings support a coiled-coil model for the functional switch of Q/N-rich prions and for the pathogenesis of polyQ-expansion diseases.

Zebrafish muscleblind-like genes: identification, structural features and expression

Muscleblind-like (MBNL) proteins are a family of RNA-binding proteins that participate in the regulation of tissue-specific alternative splicing. Misregulation of MBNL activity in humans leads to pathogenesis. Here, we report upon the identification and characterization of three muscleblind-like genes in zebrafish (zmbnl1, zmbnl2 and zmbnl3). Alternative splicing of the three zmbnl primary transcripts gives rise to at least four protein isoforms for zmbnl1, four for zmbnl2 and five for zmbnl3, respectively. All of the zmbnl proteins contain the characteristic CCCH zinc fingers required for RNA binding. In addition, several structural motifs, including a C-terminal Ser/Thr-rich region, are conserved among Mbnl orthologs in vertebrates, but not invertebrates. These genes are broadly expressed in most adult tissues. However, the relative expression levels of specific spliceforms vary across different tissues. During embryogenesis, zmbnl1 and zmbnl2 are both maternally and zygotically expressed. In contrast, zmbnl3 transcripts are not detected until the late pharyngula stage. Our results reveal the expression pattern of various mbnl spliceforms for the first time and suggest that they may play specific roles during fish development.

Filament-dependent and -independent localization modes of Drosophila non-muscle myosin II

Myosin II assembles into force-generating filaments that drive cytokinesis and the organization of the cell cortex. Regulation of myosin II activity can occur through modulation of filament assembly and by targeting to appropriate cellular sites. Here we show, using salt-dependent solubility and a novel fluorescence resonance energy transfer assay, that assembly of the Drosophila non-muscle myosin II heavy chain, zipper, is mediated by a 90-residue region (1849-1940) of the coiled-coil tail domain. This filament assembly domain, transiently expressed in Drosophila S2 cells, does not localize to the interphase cortex or the cytokinetic cleavage furrow, whereas a 500-residue region (1350-1865) that overlaps the NH(2) terminus of the assembly domain localizes to the interphase cortex but not the cytokinetic cleavage furrow. Targeting to these two sites appears to utilize distinct localization mechanisms as the assembly domain is required for cleavage furrow recruitment of a truncated coiled-coil tail region but not targeting to the interphase cortex. These results delineate the requirements for zipper filament assembly and indicate that the ability to form filaments is necessary for targeting to the cleavage furrow but not to the interphase cortex.

Calmodulin-binding proteins in the model organism Dictyostelium: a complete & critical review

Calmodulin is an essential protein in the model organism Dictyostelium discoideum. As in other organisms, this small, calcium-regulated protein mediates a diversity of cellular events including chemotaxis, spore germination, and fertilization. Calmodulin works in a calcium-dependent or -independent manner by binding to and regulating the activity of target proteins called calmodulin-binding proteins. Profiling suggests that Dictyostelium has 60 or more calmodulin-binding proteins with specific subcellular localizations. In spite of the central importance of calmodulin, the study of these target proteins is still in its infancy. Here we critically review the history and state of the art of research into all of the identified and presumptive calmodulin-binding proteins of Dictyostelium detailing what is known about each one with suggestions for future research. Two individual calmodulin-binding proteins, the classic enzyme calcineurin A (CNA; protein phosphatase 2B) and the nuclear protein nucleomorphin (NumA), which is a regulator of nuclear number, have been particularly well studied. Research on the role of calmodulin in the function and regulation of the various myosins of Dictyostelium, especially during motility and chemotaxis, suggests that this is an area in which future active study would be particularly valuable. A general, hypothetical model for the role of calmodulin in myosin regulation is proposed.

Desiccation of the resurrection plant Craterostigma plantagineum induces dynamic changes in protein phosphorylation

Reversible phosphorylation of proteins is an important mechanism by which organisms regulate their reactions to external stimuli. To investigate the involvement of phosphorylation during acquisition of desiccation tolerance, we have analysed dehydration-induced protein phosphorylation in the desiccation tolerant resurrection plant Craterostigma plantagineum. Several dehydration-induced proteins were shown to be transiently phosphorylated during a dehydration and rehydration (RH) cycle. Two abundantly expressed phosphoproteins are the dehydration- and abscisic acid (ABA)-responsive protein CDeT11-24 and the group 2 late embryogenesis abundant (LEA) protein CDeT6-19. Although both proteins accumulate in leaves and roots with similar kinetics in response to dehydration, their phosphorylation patterns differ. Several phosphorylation sites were identified on the CDeT11-24 protein using liquid chromatography-tandem mass spectrometry (LCMS/MS). The coincidence of phosphorylation sites with predicted coiled-coil regions leads to the hypothesis that CDeT11-24 phosphorylations influence the stability of coiled-coil interactions with itself and possibly other proteins.

The regulation of myosin II in Dictyostelium

Dictyostelium conventional myosin (myosin II) is an abundant protein that plays a role in various cellular processes such as cytokinesis, cell protrusion and development. This review will focus on the signal transduction pathways that regulate myosin II during cell movement. Myosin II appears to have two modes of action in Dictyostelium: local stabilization of the cytoskeleton by myosin filament association to the actin meshwork (structural mode) and force generation by contraction of actin filaments (motor mode). Some processes, such as cell movement under restrictive environment, require only the structural mode of myosin. However, cytokinesis in suspension and uropod retraction depend on motor activity as well. Myosin II can self-assemble into bipolar filaments. The formation of these filaments is negatively regulated by heavy chain phosphorylation through the action of a set of novel alpha kinases and is relatively well understood. However, only recently it has become clear that the formation of bipolar filaments and their translocation to the cortex are separate events. Translocation depends on filamentous actin, and is regulated by a cGMP pathway and possibly also by the cAMP phosphodiesterase RegA and the p21-activated kinase PAKa. Myosin motor activity is regulated by phosphorylation of the regulatory light chain through myosin light chain kinase A. Unlike conventional light chain kinases, this enzyme is not regulated by calcium but is activated by cGMP-induced phosphorylation via an upstream kinase and subsequent autophosphorylation.

Protein kinase Cgamma regulates myosin IIB phosphorylation, cellular localization, and filament assembly

Nonmuscle myosin II is an important component of the cytoskeleton, playing a major role in cell motility and chemotaxis. We have previously demonstrated that, on stimulation with epidermal growth factor (EGF), nonmuscle myosin heavy chain II-B (NMHC-IIB) undergoes a transient phosphorylation correlating with its cellular localization. We also showed that members of the PKC family are involved in this phosphorylation. Here we demonstrate that of the two conventional PKC isoforms expressed by prostate cancer cells, PKCbetaII and PKCgamma, PKCgamma directly phosphorylates NMHC-IIB. Overexpression of wild-type and kinase dead dominant negative PKCgamma result in both altered NMHC-IIB phosphorylation and subcellular localization. We have also mapped the phosphorylation sites of PKCgamma on NMHC-IIB. Conversion of the PKCgamma phosphorylation sites to alanine residues, reduces the EGF-dependent NMHC-IIB phosphorylation. Aspartate substitution of these sites reduces NMHC-IIB localization into cytoskeleton. These results indicate that PKCgamma regulates NMHC-IIB phosphorylation and cellular localization in response to EGF stimulation.

Critical regions for assembly of vertebrate nonmuscle myosin II

Myosin II molecules assemble and form filaments through their C-terminal rod region, and the dynamic filament assembly-disassembly process of nonmuscle myosin II molecules is important for cellular activities. To estimate the critical region for filament formation of vertebrate nonmuscle myosin II, we assessed the solubility of a series of truncated recombinant rod fragments of nonmuscle myosin IIB at various concentrations of NaCl. A C-terminal 248-residue rod fragment (Asp 1729-Glu 1976) was shown by its solubility behavior to retain native assembly features, and two regions within it were found to be necessary for assembly: 35 amino acid residues from Asp 1729 to Thr 1763 and 39 amino acid residues from Ala 1875 to Ala 1913, the latter containing a sequence similar to the assembly competence domain (ACD) of skeletal muscle myosin. Fragments lacking either of the two regions were soluble at any NaCl concentration. We referred to these two regions as nonmuscle myosin ACD1 (nACD1) and nACD2, respectively. In addition, we constructed an alpha-helical coiled-coil model of the rod fragment, and found that a remarkable negative charge cluster (termed N1) and a positive charge cluster (termed P2) were present within nACD1 and nACD2, respectively, besides another positive charge cluster (termed P1) in the amino-terminal vicinity of nACD2. From these results, we propose two major electrostatic interactions that are essential for filament formation of nonmuscle myosin II: the antiparallel interaction between P2 and N1 which is essential for the nucleation step and the parallel interaction between P1 and N1 which is important for the elongation step.

Dictyostelium myosin bipolar thick filament formation: importance of charge and specific domains of the myosin rod

Myosin-II thick filament formation in Dictyostelium is an excellent system for investigating the phenomenon of self-assembly, as the myosin molecule itself contains all the information required to form a structure of defined size. Phosphorylation of only three threonine residues can dramatically change the assembly state of myosin-II. We show here that the C-terminal 68 kDa of the myosin-II tail (termed AD-Cterm) assembles in a regulated manner similar to full-length myosin-II and forms bipolar thick filament (BTF) structures when a green fluorescent protein (GFP) “head” is added to the N terminus. The localization of this GFP-AD-Cterm to the cleavage furrow of dividing Dictyostelium cells depends on assembly state, similar to full-length myosin-II. This tail fragment therefore represents a good model system for the regulated formation and localization of BTFs. By reducing regulated BTF assembly to a more manageable model system, we were able to explore determinants of myosin-II self-assembly. Our data support a model in which a globular head limits the size of a BTF, and the large-scale charge character of the AD-Cterm region is important for BTF formation. Truncation analysis of AD-Cterm tail fragments shows that assembly is delicately balanced, resulting in assembled myosin-II molecules that are poised to disassemble due to the phosphorylation of only three threonines.

A portion of the myosin tail coupled to a green fluorescent protein "head" proves a valuable model for understanding myosin self-assembly

Myosin-II reorganization during mitosis is controlled temporally by its dephosphorylation and spatially by Mid1 in fission yeast

Cytokinesis in many eukaryotes requires an actomyosin contractile ring. Here, we show that in fission yeast the myosin-II heavy chain Myo2 initially accumulates at the division site via its COOH-terminal 134 amino acids independently of F-actin. The COOH-terminal region can access to the division site at early G2, whereas intact Myo2 does so at early mitosis. Ser1444 in the Myo2 COOH-terminal region is a phosphorylation site that is dephosphorylated during early mitosis. Myo2 S1444A prematurely accumulates at the future division site and promotes formation of an F-actin ring even during interphase. The accumulation of Myo2 requires the anillin homologue Mid1 that functions in proper ring placement. Myo2 interacts with Mid1 in cell lysates, and this interaction is inhibited by an S1444D mutation in Myo2. Our results suggest that dephosphorylation of Myo2 liberates the COOH-terminal region from an intramolecular inhibition. Subsequently, dephosphorylated Myo2 is anchored by Mid1 at the medial cortex and promotes the ring assembly in cooperation with F-actin.

The role of myosin heavy chain phosphorylation in Dictyostelium motility, chemotaxis and F-actin localization

To assess the role of myosin II heavy chain (MHC) phosphorylation in basic motility and natural chemotaxis, the Dictyostelium mhcA null mutant mhcA(-), mhcA(-) cells rescued with a myosin II gene that mimics the constitutively unphosphorylated state (3XALA) and mhcA(-) cells rescued with a myosin II gene that mimics the constitutively phosphorylated state (3XASP), were analyzed in buffer and in response to the individual spatial, temporal and concentration components of a cAMP wave using computer-assisted methods. Each mutant strain exhibited unique defects in cell motility and chemotaxis. Although mhcA(-) cells could crawl with some polarity and showed chemotaxis with highly reduced efficiency in a spatial gradient of cAMP, they were very slow, far less polar and more three-dimensional than control cells. They were also incapable of responding to temporal gradients of cAMP, of chemotaxis in a natural wave of cAMP or streaming late in aggregation. 3XASP cells were faster and chemotactically more efficient than mhcA(-) cells, but still incapable of responding to temporal gradients of cAMP, chemotaxis in natural waves of cAMP or streaming late in aggregation. 3XALA cells were fast, were able to respond to temporal gradients of cAMP, and responded to natural waves of cAMP. However, they exhibited a 50% reduction in chemotactic efficiency, could not stream late in aggregation and could not enter the streams of control cells in mixed cultures. F-actin staining further revealed that while the presence of unphosphorylated MHC was essential for the increase in F-actin in the cytoplasm in response to the increasing temporal gradient of cAMP in the front of a natural wave, the actual dephosphorylation event was essential for the associated increase in cortical F-actin. The results of these studies indicate that MHC phosphorylation-dephosphorylation, like myosin II regulatory light chain phosphorylation-dephosphorylation, represents a potential downstream target of the regulatory cascades emanating from the different phases of the wave.

Signaling pathways regulating Dictyostelium myosin II

Dictyostelium myosin II is a conventional, two-headed myosin that consists of two copies each of a myosin heavy chain (MHC), an essential light chain (ELC) and a regulatory light chain (RLC). The MHC is comprised of an amino-terminal motor domain, a neck region that binds the RLC and ELC and a carboxyl-terminal alpha-helical coiled-coil tail. Electrostatic interactions between the tail domains mediate the self-assembly of myosin II into bipolar filaments that are capable of interacting with actin filaments to generate a contractile force. In this review we discuss the regulation of Dictyostelium myosin II by a myosin light chain kinase (MLCK-A) that phosphorylates the RLC and increases motor activity and by MHC kinases (MHCKs) that phosphorylate the tail and prevent filament assembly. Dictyostelium may express as many as four MHCKs (MHCK A-D) consisting of an atypical alpha-kinase catalytic domain and a carboxyl-terminal WD repeat domain that targets myosin II filaments. A previously reported MHCK, termed MHC-PKC, now seems more likely to be a diacylglycerol kinase (DgkA). The relationship of the MHCKs to the larger family of alpha-kinases is discussed and key features of the structure of the alpha-kinase catalytic domain are reviewed. Potential upstream regulators of myosin II are described, including DgkA, cGMP, cAMP and PAKa, a target for Rac GTPases. Recent results point to a complex network of signaling pathways responsible for controling the activity and localization of myosin II in the cell.

Epidermal growth factor-mediated transient phosphorylation and membrane localization of myosin II-B are required for efficient chemotaxis

Epidermal growth factor (EGF) stimulation of prostate metastatic tumor cells results in transient phosphorylation and cellular localization of non-muscle myosin heavy chain II-B (NMHC II-B) with kinetics similar to those seen in chemotaxis. We demonstrate that expression of 18- and 72-kDa fragments derived from the NMHC II-B C terminus that contain EGF-dependent NMHC II-B phosphorylation sites serve as dominant-negative mutations for EGF-dependent NMHC II-B phosphorylation and localization. Both fragments inhibited the EGF-dependent phosphorylation by competing with NMHC II-B on the myosin heavy chain kinase. However, only expression of the 72-kDa fragment resulted in cells with abnormalities in cell shape, focal adhesions, and chemotaxis. We found that the 72-kDa (but not 18-kDa) fragment is capable of self-assembly. To our knowledge, these results provide the first strong evidence that EGF-dependent NMHC II-B phosphorylation is required for the cellular localization of NMHC II-B and that NMHC II-B is required for normal cell attachment and for chemotactic response.

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.

Tail chimeras of Dictyostelium myosin II support cytokinesis and other myosin II activities but not full development

Dictyostelium lacking myosin II cannot grow in suspension culture, develop beyond the mound stage or cap concanavalin A receptors and chemotaxis is impaired. Recently, we showed that the actin-activated MgATPase activity of myosin chimeras in which the tail domain of Dictyostelium myosin II heavy chain is replaced by the tail domain of either Acanthamoeba or chicken smooth muscle myosin II is unregulated and about 20 times higher than wild-type myosin. The Acanthamoeba chimera forms short bipolar filaments similar to, but shorter than, filaments of Dictyostelium myosin and the smooth muscle chimera forms much larger side-polar filaments. We now find that the Acanthamoeba chimera expressed in myosin null cells localizes to the periphery of vegetative amoeba similarly to wild-type myosin but the smooth muscle chimera is heavily concentrated in a single cortical patch. Despite their different tail sequences and filament structures and different localization of the smooth muscle chimera in interphase cells, both chimeras support growth in suspension culture and concanavalin A capping and colocalize with the ConA cap but the Acanthamoeba chimera subsequently disperses more slowly than wild-type myosin and the smooth muscle chimera apparently not at all. Both chimeras also partially rescue chemotaxis. However, neither supports full development. Thus, neither regulation of myosin activity, nor regulation of myosin polymerization nor bipolar filaments is required for many functions of Dictyostelium myosin II and there may be no specific sequence required for localization of myosin to the cleavage furrow.

Regulation of nonmuscle myosins by heavy chain phosphorylation

Functional activities of many nonmuscle myosin isoforms are (or are postulated to be) regulated by heavy chain phosphorylation. Depending on the myosin isoform, the serine or threonine residues located within the head (myosin I or myosin VI) or within the C-terminal tail domains (myosin II or myosin V) can be phosphorylated by more or less specific endogenous kinases. In some isoforms phosphorylation can occur both in the head and tail domains, as it has been found for myosin III. There are also isoforms that can be regulated both by the heavy and regulatory light chain phosphorylation, as for the example myosin II from slide mold Dictyostelium discoideum. The goal of this review was to describe recent findings on regulation of myosin I, myosin II, myosin III, myosin V and myosin VI isoforms by their heavy chain phosphorylation including the short charcteristics of the relevant kinases. The biological aspects of the phosphorylation are also discussed.

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.

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.

Coiled coils: a highly versatile protein folding motif

The alpha-helical coiled coil is one of the principal subunit oligomerization motifs in proteins. Its most characteristic feature is a heptad repeat pattern of primarily apolar residues that constitute the oligomer interface. Despite its simplicity, it is a highly versatile folding motif: coiled-coil-containing proteins exhibit a broad range of different functions related to the specific 'design' of their coiled-coil domains. The architecture of a particular coiled-coil domain determines its oligomerization state, rigidity and ability to function as a molecular recognition system. Much progress has been made towards understanding the factors that determine coiled-coil formation and stability. Here we discuss this highly versatile protein folding and oligomerization motif with regard to its structural architecture and how this is related to its biological functions.

Flightin is essential for thick filament assembly and sarcomere stability in Drosophila flight muscles

Flightin is a multiply phosphorylated, 20-kD myofibrillar protein found in Drosophila indirect flight muscles (IFM). Previous work suggests that flightin plays an essential, as yet undefined, role in normal sarcomere structure and contractile activity. Here we show that flightin is associated with thick filaments where it is likely to interact with the myosin rod. We have created a null mutation for flightin, fln(0), that results in loss of flight ability but has no effect on fecundity or viability. Electron microscopy comparing pupa and adult fln(0) IFM shows that sarcomeres, and thick and thin filaments in pupal IFM, are 25-30% longer than in wild type. fln(0) fibers are abnormally wavy, but sarcomere and myotendon structure in pupa are otherwise normal. Within the first 5 h of adult life and beginning of contractile activity, IFM fibers become disrupted as thick filaments and sarcomeres are variably shortened, and myofibrils are ruptured at the myotendon junction. Unusual empty pockets and granular material interrupt the filament lattice of adult fln(0) sarcomeres. Site-specific cleavage of myosin heavy chain occurs during this period. That myosin is cleaved in the absence of flightin is consistent with the immunolocalization of flightin on the thick filament and biochemical and genetic evidence suggesting it is associated with the myosin rod. Our results indicate that flightin is required for the establishment of normal thick filament length during late pupal development and thick filament stability in adult after initiation of contractile activity.

A computationally directed screen identifying interacting coiled coils from Saccharomyces cerevisiae

Computational methods can frequently identify protein-interaction motifs in otherwise uncharacterized open reading frames. However, the identification of candidate ligands for these motifs (e.g., so that partnering can be determined experimentally in a directed manner) is often beyond the scope of current computational capabilities. One exception is provided by the coiled-coil interaction motif, which consists of two or more alpha helices that wrap around each other: the ligands for coiled-coil sequences are generally other coiled-coil sequences, thereby greatly simplifying the motif/ligand recognition problem. Here, we describe a two-step approach to identifying protein-protein interactions mediated by two-stranded coiled coils that occur in Saccharomyces cerevisiae. Coiled coils from the yeast genome are first predicted computationally, by using the multicoil program, and associations between coiled coils are then determined experimentally by using the yeast two-hybrid assay. We report 213 unique interactions between 162 putative coiled-coil sequences. We evaluate the resulting interactions, focusing on associations identified between components of the spindle pole body (the yeast centrosome).

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.

Microtubules and mitotic cycle phase modulate spatiotemporal distributions of F-actin and myosin II in Drosophila syncytial blastoderm embryos

We studied cyclic reorganizations of filamentous actin, myosin II and microtubules in syncytial Drosophila blastoderms using drug treatments, time-lapse movies and laser scanning confocal microscopy of fixed stained embryos (including multiprobe three-dimensional reconstructions). Our observations imply interactions between microtubules and the actomyosin cytoskeleton. They provide evidence that filamentous actin and cytoplasmic myosin II are transported along microtubules towards microtubule plus ends, with actin and myosin exhibiting different affinities for the cell's cortex. Our studies further reveal that cell cycle phase modulates the amounts of both polymerized actin and myosin II associated with the cortex. We analogize pseudocleavage furrow formation in the Drosophila blastoderm with how the mitotic apparatus positions the cleavage furrow for standard cytokinesis, and relate our findings to polar relaxation/global contraction mechanisms for furrow formation.

Mutational analysis of phosphorylation sites in the Dictyostelium myosin II tail: disruption of myosin function by a single charge change

The dynamic assembly/disassembly of non-muscle myosin II filaments is critical for the regulation of enzymatic activities and localization. Phosphorylation of three threonines, 1823, 1833 and 2029, in the tail of Dictyostelium discoideum myosin II has been implicated in control of myosin filament assembly. By systematically replacing the three threonines to aspartates, mimicking a phosphorylated residue, we found that position 1823 is the most critical one for the regulation of myosin filament formation and in vivo function. Surprisingly, a single charge change is able to perturb filament formation and in vivo function of myosin II.