Regulation of Dictyostelium myosin II by phosphorylation: what is essential and what is important?

Stretching actin filaments within cells enhances their affinity for the myosin II motor domain

To test the hypothesis that the myosin II motor domain (S1) preferentially binds to specific subsets of actin filaments in vivo, we expressed GFP-fused S1 with mutations that enhanced its affinity for actin in Dictyostelium cells. Consistent with the hypothesis, the GFP-S1 mutants were localized along specific portions of the cell cortex. Comparison with rhodamine-phalloidin staining in fixed cells demonstrated that the GFP-S1 probes preferentially bound to actin filaments in the rear cortex and cleavage furrows, where actin filaments are stretched by interaction with endogenous myosin II filaments. The GFP-S1 probes were similarly enriched in the cortex stretched passively by traction forces in the absence of myosin II or by external forces using a microcapillary. The preferential binding of GFP-S1 mutants to stretched actin filaments did not depend on cortexillin I or PTEN, two proteins previously implicated in the recruitment of myosin II filaments to stretched cortex. These results suggested that it is the stretching of the actin filaments itself that increases their affinity for the myosin II motor domain. In contrast, the GFP-fused myosin I motor domain did not localize to stretched actin filaments, which suggests different preferences of the motor domains for different structures of actin filaments play a role in distinct intracellular localizations of myosin I and II. We propose a scheme in which the stretching of actin filaments, the preferential binding of myosin II filaments to stretched actin filaments, and myosin II-dependent contraction form a positive feedback loop that contributes to the stabilization of cell polarity and to the responsiveness of the cells to external mechanical stimuli.

Role of B regulatory subunits of protein phosphatase type 2A in myosin II assembly control in Dictyostelium discoideum

In Dictyostelium discoideum, myosin II resides predominantly in a soluble pool as the result of phosphorylation of the myosin heavy chain (MHC), and dephosphorylation of the MHC is required for myosin II filament assembly, recruitment to the cytoskeleton, and force production. Protein phosphatase type 2A (PP2A) was identified in earlier studies in Dictyostelium as a key biochemical activity that can drive MHC dephosphorylation. We report here gene targeting and cell biological studies addressing the roles of candidate PP2A B regulatory subunits (phr2aBα and phr2aBβ) in myosin II assembly control in vivo. Dictyostelium phr2aBα- and phr2aBβ-null cells show delayed development, reduction in the assembly of myosin II in cytoskeletal ghost assays, and defects in cytokinesis when grown in suspension compared to parental cell lines. These results demonstrate that the PP2A B subunits phr2aBα and phr2aBβ contribute to myosin II assembly control in vivo, with phr2aBα having the predominant role facilitating MHC dephosphorylation to facilitate filament assembly.

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.

Type II myosin involved in cytokinesis in the fission yeast, Schizosaccharomyces pombe

We have cloned an unique gene encoding the heavy chain of a type II myosin in the fission yeast, Schizosaccharomyces pombe. The myo2+ gene encodes a protein of 1526 amino acids with a predicted molecular weight of 177 kDa and containing consensus binding motifs for both essential and regulatory light chains. The S. pombe myo2+ head domain is 45% identical to myosin IIs from Saccharomyces cerevisiae and Homo sapiens and 40% identical to Drosophila melanogaster Structurally, myo2+ most closely resembles budding yeast MYO1, the tails of both myosin IIs containing a number of proline residues that are predicted to substantially disrupt the ability of these myosins to form coiled coils. The myo2+ gene is located on chromosome III, 8.3 map units from ade6+. Deletion of approximately 70% of the coding sequence of myo2+ is lethal but myo2delta spores can acquire a suppressor mutation that allows them to form viable microcolonies consisting of filaments of branched cells with aberrant septa. Overexpression of myo2+ results in the inhibition of cytokinesis; cells become elongated and multinucleate and fail to assemble a functional cytokinetic actin ring and are either aseptate or form aberrant septa. These results suggest that a contractile actin-myosin based cytokinetic mechanism appeared early in the evolution of eukaryotic cells and further emphasise the utility of fission yeast as a model organism in which to study the molecular and cellular basis of cytokinesis.

Involvement of tail domains in regulation of Dictyostelium myosin II

The actin-dependent ATPase activity of Dictyostelium myosin II filaments is regulated by phosphorylation of the regulatory light chain. Four deletion mutant myosins which lack different parts of subfragment 2 (S2) showed phosphorylation-independent elevations in their activities. Phosphorylation-independent elevation in the activity was also achieved by a double point mutation to replace conserved Glu932 and Glu933 in S2 with Lys. These results suggested that inhibitory interactions involving the head and S2 are required for efficient regulation. Regulation of wild-type myosin was not affected by copolymerization with a S2 deletion mutant myosin in the same filaments. Furthermore, the activity linearly correlated with the fraction of phosphorylated molecules in wild-type filaments. These latter two results suggest that the inhibitory head-tail interactions are primarily intramolecular.

Molecular biological approaches to study myosin functions in cytokinesis of Dictyostelium

The cellular slime mold Dictyostelium discoideum is amenable to biochemical, cell biological, and molecular genetic analyses, and offers a unique opportunity for multifaceted approaches to dissect the mechanism of cytokinesis. One of the important questions that are currently under investigation using Dictyostelium is to understand how cleavage furrows or contractile rings are assembled in the equatorial region. Contractile rings consist of a number of components including parallel filaments of actin and myosin II. Phenotypic analyses and in vivo localization studies of cells expressing mutant myosin IIs have demonstrated that myosin II's transport to and localization at the equatorial region does not require regulation by phosphorylation of myosin II, specific amino acid sequences of myosin II, or the motor activity of myosin II. Rather, the transport appears to depend on a myosin II-independent flow of cortical cytoskeleton. What drives the flow of cortical cytoskeleton is still elusive. However, a growing number of mutants that affect assembly of contractile rings have been accumulated. Analyses of these mutations, identification of more cytokinesis-specific genes, and information deriving from other experimental systems, should allow us to understand the mechanism of contractile ring formation and other aspects of cytokinesis.

Biochemical characterization of a Dictyostelium myosin II heavy-chain phosphatase that promotes filament assembly

In Dictyostelium cells, myosin II is found as cytosolic nonassembled monomers and cytoskeletal bipolar filaments. It is thought that the phosphorylation state of three threonine residues in the tail of myosin II heavy chain regulates the molecular motor's assembly state and localization. Phosphorylation of the myosin heavy chain at threonine residues 1823, 1833 and 2029 is responsible for maintaining myosin in the nonassembled state, and subsequent dephosphorylation of these residues is a prerequisite for assembly into the cytoskeleton. We report here the characterization of myosin heavy-chain phosphatase activities in Dictyostelium utilizing myosin II phosphorylated by myosin heavy-chain kinase A as a substrate. One of the myosin heavy-chain phosphatase activities was identified as protein phosphatase 2A and the purified holoenzyme was composed of a 37-kDa catalytic subunit, a 65-kDa A subunit and a 55-kDa B subunit. The protein phosphatase 2A holoenzyme displays two orders of magnitude higher activity towards myosin phosphorylated on the heavy chains than it does towards myosin phosphorylated on the regulatory light chains, consistent with a role in the control of filament assembly. The purified myosin heavy-chain phosphatase activity promotes bipolar filament assembly in vitro via dephosphorylation of the myosin heavy chain. This system should provide a valuable model for studying the regulation and localization of protein phosphatase 2A in the context of cytoskeletal reorganization.

Phosphorylation of the myosin-II light chain does not regulate the timing of cytokinesis in fission yeast

Proper coordination of cytokinesis with chromosome separation during mitosis is crucial to ensure that each daughter cell inherits an equivalent set of chromosomes. It has been proposed that one mechanism by which this is achieved is through temporally regulated myosin regulatory light chain (RLC) phosphorylation (Satterwhite, L. L., and Pollard, T. D. (1992) Curr. Opin. Cell Biol. 4, 43-52). A variety of evidence is consistent with this model. A direct test of the importance of RLC phosphorylation in vivo has been done only in Dictyostelium and Drosophila; phosphorylation of the RLC is essential in Drosophila (Jordan, P., and Karess, R. (1997) J. Cell Biol. 139, 1805-1819) but not essential in Dictyostelium (Ostrow, B. D., Chen, P., and Chisholm, R. L. (1994) J. Cell Biol. 127, 1945-1955). The Schizosaccharomyces pombe myosin light chain Cdc4p is essential for cytokinesis, but it was unknown whether phosphorylation played a role in its regulation. Here we show that the S. pombe myosin light chain Cdc4p is phosphorylated in vivo on either serine 2 or 6 but not both. Mutation of either or both of these sites to alanine did not effect the ability of Cdc4p to bind the type II myosin Myo2p, and cells expressing only these mutated versions of Cdc4p grew and divided normally. Similarly, mutation of Ser-2, Ser-6, or both residues to aspartic acid did not affect growth or division of cells. Thus we conclude that phosphorylation of Cdc4p is not essential in vivo for the function of the protein.

Activation of myosin phosphatase targeting subunit by mitosis-specific phosphorylation

It has been demonstrated previously that during mitosis the sites of myosin phosphorylation are switched between the inhibitory sites, Ser 1/2, and the activation sites, Ser 19/Thr 18 (Yamakita, Y., S. Yamashiro, and F. Matsumura. 1994. J. Cell Biol. 124:129- 137; Satterwhite, L.L., M.J. Lohka, K.L. Wilson, T.Y. Scherson, L.J. Cisek, J.L. Corden, and T.D. Pollard. 1992. J. Cell Biol. 118:595-605), suggesting a regulatory role of myosin phosphorylation in cell division. To explore the function of myosin phosphatase in cell division, the possibility that myosin phosphatase activity may be altered during cell division was examined. We have found that the myosin phosphatase targeting subunit (MYPT) undergoes mitosis-specific phosphorylation and that the phosphorylation is reversed during cytokinesis. MYPT phosphorylated either in vivo or in vitro in the mitosis-specific way showed higher binding to myosin II (two- to threefold) compared to MYPT from cells in interphase. Furthermore, the activity of myosin phosphatase was increased more than twice and it is suggested this reflected the increased affinity of myosin binding. These results indicate the presence of a unique positive regulatory mechanism for myosin phosphatase in cell division. The activation of myosin phosphatase during mitosis would enhance dephosphorylation of the myosin regulatory light chain, thereby leading to the disassembly of stress fibers during prophase. The mitosis-specific effect of phosphorylation is lost on exit from mitosis, and the resultant increase in myosin phosphorylation may act as a signal to activate cytokinesis.

MLCK-A, an unconventional myosin light chain kinase from Dictyostelium, is activated by a cGMP-dependent pathway

Dictyostelium myosin II is activated by phosphorylation of its regulatory light chain by myosin light chain kinase A (MLCK-A), an unconventional MLCK that is not regulated by Ca2+/calmodulin. MLCK-A is activated by autophosphorylation of threonine-289 outside of the catalytic domain and by phosphorylation of threonine-166 in the activation loop by an unidentified kinase, but the signals controlling these phosphorylations are unknown. Treatment of cells with Con A results in quantitative phosphorylation of the regulatory light chain by MLCK-A, providing an opportunity to study MLCK-A's activation mechanism. MLCK-A does not alter its cellular location upon treatment of cells with Con A, nor does it localize to the myosin-rich caps that form after treatment. However, MLCK-A activity rapidly increases 2- to 13-fold when Dictyostelium cells are exposed to Con A. This activation can occur in the absence of MLCK-A autophosphorylation. cGMP is a promising candidate for an intracellular messenger mediating Con A-triggered MLCK-A activation, as addition of cGMP to fresh Dictyostelium lysates increases MLCK-A activity 3- to 12-fold. The specific activity of MLCK-A in cGMP-treated lysates is 210-fold higher than that of recombinant MLCK-A, which is fully autophosphorylated, but lacks threonine-166 phosphorylation. Purified MLCK-A is not directly activated by cGMP, indicating that additional cellular factors, perhaps a kinase that phosphorylates threonine-166, are involved.

Specific localization of serine 19 phosphorylated myosin II during cell locomotion and mitosis of cultured cells

Phosphorylation of the regulatory light chain of myosin II (RMLC) at Serine 19 by a specific enzyme, MLC kinase, is believed to control the contractility of actomyosin in smooth muscle and vertebrate nonmuscle cells. To examine how such phosphorylation is regulated in space and time within cells during coordinated cell movements, including cell locomotion and cell division, we generated a phosphorylation-specific antibody. Motile fibroblasts with a polarized cell shape exhibit a bimodal distribution of phosphorylated myosin along the direction of cell movement. The level of myosin phosphorylation is high in an anterior region near membrane ruffles, as well as in a posterior region containing the nucleus, suggesting that the contractility of both ends is involved in cell locomotion. Phosphorylated myosin is also concentrated in cortical microfilament bundles, indicating that cortical filaments are under tension. The enrichment of phosphorylated myosin in the moving edge is shared with an epithelial cell sheet; peripheral microfilament bundles at the leading edge contain a higher level of phosphorylated myosin. On the other hand, the phosphorylation level of circumferential microfilament bundles in cell-cell contacts is low. These observations suggest that peripheral microfilaments at the edge are involved in force production to drive the cell margin forward while microfilaments in cell-cell contacts play a structural role. During cell division, both fibroblastic and epithelial cells exhibit an increased level of myosin phosphorylation upon cytokinesis, which is consistent with our previous biochemical study (Yamakita, Y., S. Yamashiro, and F. Matsumura. 1994. J. Cell Biol. 124:129-137). In the case of the NRK epithelial cells, phosphorylated myosin first appears in the midzones of the separating chromosomes during late anaphase, but apparently before the formation of cleavage furrows, suggesting that phosphorylation of RMLC is an initial signal for cytokinesis.

Myosin light chain-activating phosphorylation sites are required for oogenesis in Drosophila

The Drosophila spaghetti squash (sqh) gene encodes the regulatory myosin light chain (RMLC) of nonmuscle myosin II. Biochemical analysis of vertebrate nonmuscle and smooth muscle myosin II has established that phosphorylation of certain amino acids of the RMLC greatly increases the actin-dependent myosin ATPase and motor activity of myosin in vitro. We have assessed the in vivo importance of these sites, which in Drosophila correspond to serine-21 and threonine-20, by creating a series of transgenes in which these specific amino acids were altered. The phenotypes of the transgenes were examined in an otherwise null mutant background during oocyte development in Drosophila females. Germ line cystoblasts entirely lacking a functional sqh gene show severe defects in proliferation and cytokinesis. The ring canals, cytoplasmic bridges linking the oocyte to the nurse cells in the egg chamber, are abnormal, suggesting a role of myosin II in their establishment or maintenance. In addition, numerous aggregates of myosin heavy chain accumulate in the sqh null cells. Mutant sqh transgene sqh-A20, A21 in which both serine-21 and threonine-20 have been replaced by alanines behaves in most respects identically to the null allele in this system, with the exception that no heavy chain aggregates are found. In contrast, expression of sqh-A21, in which only the primary phosphorylation target serine-21 site is altered, partially restores functionality to germ line myosin II, allowing cystoblast division and oocyte development, albeit with some cytokinesis failure, defects in the rapid cytoplasmic transport from nurse cells to cytoplasm characteristic of late stage oogenesis, and some damaged ring canals. Substituting a glutamate for the serine-21 (mutant sqh-E21) allows oogenesis to be completed with minimal defects, producing eggs that can develop normally to produce fertile adults. Flies expressing sqh-A20, in which only the secondary phosphorylation site is absent, appear to be entirely wild type. Taken together, this genetic evidence argues that phosphorylation at serine-21 is critical to RMLC function in activating myosin II in vivo, but that the function can be partially provided by phosphorylation at threonine-20.

Activation of Dictyostelium myosin light chain kinase A by phosphorylation of Thr166

Phosphorylation of the regulatory light chain is an important mechanism for the activation of myosin in non-muscle cells. Unlike most myosin light chain kinases (MLCKs), MLCK-A from Dictyostelium is not activated by Ca2+/calmodulin. Autophosphorylation increases activity, but only to a low level, suggesting that there is an additional activation mechanism. Here, we show that MLCK-A is autophosphorylated on Thr289, which is C-terminal to the catalytic domain. Phosphorylation of MLCK-A increases in response to concanavalin A (conA) treatment of cells, which was previously shown to activate MLCK-A. However, a mutant kinase with an alanine at position 289 (T289A) is also phosphorylated in vivo, indicating that there is an additional phosphorylated residue. Based on comparisons with other protein kinases, we tested whether phosphorylation of Thr166 drives activation of MLCK-A. Our data indicate that phosphorylation of Thr289 occurs in vivo, but is not associated with conA-induced activation, whereas phosphorylation of Thr166 by some as yet unidentified kinase is associated with activation. Replacement of Thrl66 with glutamate results in a 12-fold increase in activity as compared with the wild-type enzyme, supporting the idea that phosphorylation of Thr166 increases MLCK-A activity.

The sequence of the dictyostelium myo J heavy chain gene predicts a novel, dimeric, unconventional myosin with a heavy chain molecular mass of 258 kDa

The complete sequence of the Dictyostelium myo J heavy chain gene has been determined from overlapping genomic clones. The gene spans approximately 7400 base pairs, is split by two small introns, and encodes a 2241-residue, 258-kDa heavy chain polypeptide that that is composed of an N-terminal 944-residue myosin head domain, a central 863-residue domain that is predicted to form an alpha helical coiled-coil containing six hinges, and a C-terminal 434-residue globular domain. The head domain is notable in that it contains a approximately 30 residue insert near the nucleotide binding pocket, and five potential calmodulin/myosin light chain binding sites at the head/tail junction. The existence within the Myo J tail domain of both an extensive coiled-coil structure and a large globular domain suggests that this myosin is dimeric and incapable of self-assembly into filaments. While these properties, as well as the overall predicted structure of the Myo J protein, are reminiscent of class V myosins, the sequence of the 434-residue globular tail piece of Myo J shows no similarity to that of either yeast or vertebrate myosins V. Consistent with this, phylogenetic analyses based on myosin head sequence comparisons do not classify Myo J as a type V myosin. These and other sequence comparisons indicate that Myo J and two as-yet-unclassified unconventional myosins from Arabidopsis represent members of the newest class within the myosin superfamily (class XI). Northern blots analyses suggest that Myo J may function predominantly in vegetative Dictyostelium cells. Finally, Southern blot analyses suggest that Dictyostelium possesses another myosin that is very closely related to Myo J.