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

14-3-3 coordinates microtubules, Rac, and myosin II to control cell mechanics and cytokinesis

BACKGROUND

During cytokinesis, regulatory signals are presumed to emanate from the mitotic spindle. However, what these signals are and how they lead to the spatiotemporal changes in the cortex structure, mechanics, and regional contractility are not well understood in any system.

RESULTS

To investigate pathways that link the microtubule network to the cortical changes that promote cytokinesis, we used chemical genetics in Dictyostelium to identify genetic suppressors of nocodazole, a microtubule depolymerizer. We identified 14-3-3 and found that it is enriched in the cortex, helps maintain steady-state microtubule length, contributes to normal cortical tension, modulates actin wave formation, and controls the symmetry and kinetics of cleavage furrow contractility during cytokinesis. Furthermore, 14-3-3 acts downstream of a Rac small GTPase (RacE), associates with myosin II heavy chain, and is needed to promote myosin II bipolar thick filament remodeling.

CONCLUSIONS

14-3-3 connects microtubules, Rac, and myosin II to control several aspects of cortical dynamics, mechanics, and cytokinesis cell shape change. Furthermore, 14-3-3 interacts directly with myosin II heavy chain to promote bipolar thick filament remodeling and distribution. Overall, 14-3-3 appears to integrate several critical cytoskeletal elements that drive two important processes-cytokinesis cell shape change and cell mechanics.

Multiple myosin II heavy chain kinases: roles in filament assembly control and proper cytokinesis in Dictyostelium

Myosin II filament assembly in Dictyostelium discoideum is regulated via phosphorylation of residues located in the carboxyl-terminal portion of the myosin II heavy chain (MHC) tail. A series of novel protein kinases in this system are capable of phosphorylating these residues in vitro, driving filament disassembly. Previous studies have demonstrated that at least three of these kinases (MHCK A, MHCK B, and MHCK C) display differential localization patterns in living cells. We have created a collection of single, double, and triple gene knockout cell lines for this family of kinases. Analysis of these lines reveals that three MHC kinases appear to represent the majority of cellular activity capable of driving myosin II filament disassembly, and reveals that cytokinesis defects increase with the number of kinases disrupted. Using biochemical fractionation of cytoskeletons and in vivo measurements via fluorescence recovery after photobleaching (FRAP), we find that myosin II overassembly increases incrementally in the mutants, with the MHCK A(-)/B(-)/C(-) triple mutant showing severe myosin II overassembly. These studies suggest that the full complement of MHC kinases that significantly contribute to growth phase and cytokinesis myosin II disassembly in this organism has now been identified.

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

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.

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.

Differential requirement for the nonhelical tailpiece and the C terminus of the myosin rod in Caenorhabditis elegans muscle

Myosin heavy chain (MHC) is a large, multidomain protein important for both cellular structure and contraction. To examine the functional role of two C-terminal domains, the end of the coiled-coil rod and the nonhelical tailpiece, we have generated constructs in which residues within these domains are removed or mutated, and examined their behavior in Caenorhabditis elegans striated muscle. Genetic tests demonstrate that MHC lacking only tailpiece residues is competent to support the timely onset of embryonic contractions, and therefore viability, in animals lacking full-length MHC. Antibody staining experiments show that this truncated molecule localizes as wild type in early stages of development, but may be defective in processes important for thick filament organization later in embryogenesis. Ultrastructural analysis reveals thick filaments of normal morphology in disorganized arrangement, as well as occasional abnormal assemblages. In contrast, molecules in which the four terminal residues of the coiled coil are absent or mutated fail to rescue animals lacking endogenous MHC. Loss of these four residues is associated with delayed protein localization and delayed contractile function during early embryogenesis. Our results suggest that these two MHC domains, the rod and the tailpiece, are required for distinct steps during muscle development.

Novel myosin heavy chain kinase involved in disassembly of myosin II filaments and efficient cleavage in mitotic dictyostelium cells

We have cloned a full-length cDNA encoding a novel myosin II heavy chain kinase (mhckC) from Dictyostelium. Like other members of the myosin heavy chain kinase family, the mhckC gene product, MHCK C, has a kinase domain in its N-terminal half and six WD repeats in the C-terminal half. GFP-MHCK C fusion protein localized to the cortex of interphase cells, to the cleavage furrow of mitotic cells, and to the posterior of migrating cells. These distributions of GFP-MHCK C always corresponded with that of myosin II filaments and were not observed in myosin II-null cells, where GFP-MHCK C was diffusely distributed in the cytoplasm. Thus, localization of MHCK C seems to be myosin II-dependent. Cells lacking the mhckC gene exhibited excessive aggregation of myosin II filaments in the cleavage furrows and in the posteriors of the daughter cells once cleavage was complete. The cleavage process of these cells took longer than that of wild-type cells. Taken together, these findings suggest MHCK C drives the disassembly of myosin II filaments for efficient cytokinesis and recycling of myosin II that occurs during cytokinesis.

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.

Localization of fission yeast type II myosin, Myo2, to the cytokinetic actin ring is regulated by phosphorylation of a C-terminal coiled-coil domain and requires a functional septation initiation network

Myo2 truncations fused to green fluorescent protein (GFP) defined a C-terminal domain essential for the localization of Myo2 to the cytokinetic actin ring (CAR). The localization domain contained two predicted phosphorylation sites. Mutation of serine 1518 to alanine (S(1518)A) abolished Myo2 localization, whereas Myo2 with a glutamic acid at this position (S(1518)E) localized to the CAR. GFP-Myo2 formed rings in the septation initiation kinase (SIN) mutant cdc7-24 at 25 degrees C but not at 36 degrees C. GFP-Myo2S(1518)E rings persisted at 36 degrees C in cdc7-24 but not in another SIN kinase mutant, sid2-250. To further examine the relationship between Myo2 and the SIN pathway, the chromosomal copy of myo2(+) was fused to GFP (strain myo2-gc). Myo2 ring formation was abolished in the double mutants myo2-gc cdc7.24 and myo2-gc sid2-250 at the restrictive temperature. In contrast, activation of the SIN pathway in the double mutant myo2-gc cdc16-116 resulted in the formation of Myo2 rings which subsequently collapsed at 36 degrees C. We conclude that the SIN pathway that controls septation in fission yeast also regulates Myo2 ring formation and contraction. Cdc7 and Sid2 are involved in ring formation, in the case of Cdc7 by phosphorylation of a single serine residue in the Myo2 tail. Other kinases and/or phosphatases may control ring contraction.

On the mechanism of cleavage furrow ingression in Dictyostelium

The ability of Dictyostelium cells to divide without myosin II in a cell cycle-coupled manner has opened two questions about the mechanism of cleavage furrow ingression. First, are there other possible functions for myosin II in this process except for generating contraction of the furrow by a sliding filament mechanism? Second, what could be an alternative mechanical basis for the furrowing? Using aberrant changes of the cell shape and anomalous localization of the actin-binding protein cortexillin I during asymmetric cytokinesis in myosin II-deficient cells as clues, it is proposed that myosin II filaments act as a mechanical lens in cytokinesis. The mechanical lens serves to focus the forces that induce the furrowing to the center of the midzone, a cortical region where cortexillins are enriched in dividing cells. Additionally, continual disassembly of a filamentous actin meshwork at the midzone is a prerequisite for normal ingression of the cleavage furrow and a successful cytokinesis. If this process is interrupted, as it occurs in cells that lack cortexillins, an overassembly of filamentous actin at the midzone obstructs the normal cleavage. Disassembly of the crosslinked actin network can generate entropic contractile forces in the cortex, and may be considered as an alternative mechanism for driving ingression of the cleavage furrow. Instead of invoking different types of cytokinesis that operate under attached and unattached conditions in Dictyostelium, it is anticipated that these cells use a universal multifaceted mechanism to divide, which is only moderately sensitive to elimination of its constituent mechanical processes.

Protein kinase C phosphorylates nonmuscle myosin-II heavy chain from Drosophila but regulation of myosin function by this enzyme is not required for viability in flies

Conventional myosins (myosin-IIs) generate forces for cell shape change and cell motility. Myosin heavy chain phosphorylation regulates myosin function in simple eukaryotes and may also be important in metazoans. To investigate this regulation in a complex eukaryote, we purified the Drosophila myosin-II tail expressed in Escherichia coli and showed that it was phosphorylated in vitro by protein kinase C(PKC) at serines 1936 and 1944, which are located in the nonhelical globular tail piece. These sites are close to a conserved serine that is phosphorylated in vertebrate, nonmuscle myosin-IIs. If the two serines are mutagenized to alanine or aspartic acid, phosphorylation no longer occurs. Using a 341 amino acid tail fragment, we show that there is no difference in the salt-dependent assembly of wild-type phosphorylated and mutagenized polypeptides. Thus, the nonmuscle myosin heavy chain in Drosophila, which is encoded by the zipper gene, appears to be similar to rabbit nonmuscle myosin-IIA. In vivo, we generated transgenic flies that expressed the various myosin heavy chain variants in a zipper null or near-null genetic background. Like their wild-type counterparts, such variants are able to completely rescue the lethal phenotype due to severe zipper mutations. These results suggest that while the myosin-II heavy chain can be phosphorylated by PKC, regulation by this enzyme is not required for viability in Drosophila. Conservation during 530-1000 million years of evolution suggests that regulation by heavy chain phosphorylation may contribute to nonmuscle myosin-II function in some real, but minor, way.

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.