Recruitment of a myosin heavy chain kinase to actin-rich protrusions in Dictyostelium.

Nonmuscle myosin II plays fundamental roles in cell body translocation during migration and is typically depleted or absent from actin-based cell protrusions such as lamellipodia, but the mechanisms preventing myosin II assembly in such structures have not been identified [1-3]. In Dictyostelium discoideum, myosin II filament assembly is controlled primarily through myosin heavy chain (MHC) phosphorylation. The phosphorylation of sites in the myosin tail domain by myosin heavy chain kinase A (MHCK A) drives the disassembly of myosin II filaments in vitro and in vivo [4]. To better understand the cellular regulation of MHCK A activity, and thus the regulation of myosin II filament assembly, we studied the in vivo localization of native and green fluorescent protein (GFP)-tagged MHCK A. MHCK A redistributes from the cytosol to the cell cortex in response to stimulation of Dictyostelium cells with chemoattractant in an F-actin-dependent manner. During chemotaxis, random migration, and phagocytic/endocytic events, MHCK A is recruited preferentially to actin-rich leading-edge extensions. Given the ability of MHCK A to disassemble myosin II filaments, this localization may represent a fundamental mechanism for disassembling myosin II filaments and preventing localized filament assembly at sites of actin-based protrusion.

Specific phosphorylation of threonine by the Dictyostelium myosin II heavy chain kinase family.

Dictyostelium myosin II heavy chain kinase A (MHCK A), MHCK B, and MHCK C contain a novel type of protein kinase catalytic domain that displays no sequence identity to the catalytic domain present in conventional serine, threonine, and/or tyrosine protein kinases. Several proteins, including myelin basic protein, myosin regulatory light chain, caldesmon, and casein were phosphorylated by the bacterially expressed MHCK A, MHCK B, and MHCK C catalytic domains. Phosphoamino acid analyses of the proteins showed that 91 to 99% of the phosphate was incorporated into threonine with the remainder into serine. Acceptor amino acid specificity was further examined using a synthetic peptide library (MAXXXX(S/T)XXXXAKKK; where X is any amino acid except cysteine, tryptophan, serine, and threonine and position 7 contains serine and threonine in a 1.7:1 ratio). Phosphorylation of the peptide library with the three MHCK catalytic domains resulted in 97 to 99% of the phosphate being incorporated into threonine, while phosphorylation with a conventional serine/threonine protein kinase, the p21-activated kinase, resulted in 80% of the phosphate being incorporated into serine. The acceptor amino acid specificity of MHCK A was tested directly by substituting serine for threonine in a synthetic peptide and a glutathione S-transferase fusion peptide substrate. The serine-containing substrates were phosphorylated at a 25-fold lower rate than the threonine-containing substrates. The results indicate that the MHCKs are specific for the phosphorylation of threonine.

WD repeat domains target dictyostelium myosin heavy chain kinases by binding directly to myosin filaments.

Myosin heavy chain kinase (MHCK) A phosphorylates mapped sites at the C-terminal tail of Dictyostelium myosin II heavy chain, driving disassembly of myosin filaments both in vitro and in vivo. MHCK A is organized into three functional domains that include an N-terminal coiled-coil region, a central kinase catalytic domain unrelated to conventional protein kinases, and a WD repeat domain at the C terminus. MHCK B is a homologue of MHCK A that possesses structurally related catalytic and WD repeat domains. In the current study, we explored the role of the WD repeat domains in defining the activities of both MHCK A and MHCK B using recombinant bacterially expressed truncations of these kinases either with or without their WD repeat domains. We demonstrate that substrate targeting is a conserved function of the WD repeat domains of both MHCK A and MHCK B and that this targeting is specific for Dictyostelium myosin II filaments. We also show that the mechanism of targeting involves direct binding of the WD repeat domains to the myosin substrate. To our knowledge, this is the first report of WD repeat domains physically targeting attached kinase domains to their substrates. The examples presented here may serve as a paradigm for enzyme targeting in other systems.

Green fluorescent protein and epitope tag fusion vectors for Dictyostelium discoideum.

We have constructed expression vectors for Dictyostelium discoideum which encode a green fluorescent protein (GFP) sequence upstream of a multicloning site for introduction of sequences of interest. Insertion of cDNAs into the multicloning site results in expression of fusion protein bearing an amino- or carboxyl-terminal GFP tag which can be used for fluorescent localization studies in Dictyostelium cells. A parallel construct fuses a FLAG epitope tag at the amino terminus of expressed protein. Each fusion cartridge was placed either in a G418-resistance vector allowing transactivated Ddp2-based extrachromosomal replication or in a vector allowing autonomous Ddp1-based replication. Distinct differences in expression stability were observed in the two vector types. When GFP-expressing cells were analyzed by fluorescence microscopy, significant cell-to-cell variability in expression level was observed when expression was based on the Ddp2 vector, while less cell-to-cell variation in expression level was observed when the Ddp1 backbone was used for expression.

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.

Molecular characterization and immunolocalization of Dictyostelium discoideum protein phosphatase 2A.

Protein phosphatase 2A (PP2A) was previously purified from Dictyostelium and biochemically characterized. The purified PP2A holoenzyme was composed of a 37 kDa catalytic 'C-subunit', a 65 kDa 'A-subunit' and a 55 kDa 'B-subunit'. We report here the characterization of the genes encoding the Dictyostelium PP2A subunits as well as the immunolocalization of the PP2A subunits in Dictyostelium. The cDNAs encoding the B- and C-subunits were isolated from a Dictyostelium library and the deduced amino acid sequences reveal strong conservation with the mammalian PP2A homologues. Southern blot analysis suggests that each of the PP2A subunit genes is present in a single copy. The PP2A subunits were localized mainly to the cytosol in Dictyostelium cells. However, immunofluorescence confocal microscopy demonstrates that the B-subunit of PP2A is highly enriched in centrosomes, suggesting a potential role for this PP2A regulatory subunit in the centrosomal function.

Dictyostelium myosin heavy chain kinase A subdomains. Coiled-coil and wd repeat roles in oligomerization and substrate targeting.

Myosin heavy chain kinase A (MHCK A) participates in the regulation of cytoskeletal myosin assembly in Dictyostelium, driving filament disassembly via phosphorylation of sites in the myosin tail. MHCK A contains an amino-terminal coiled-coil domain, a novel central catalytic domain, and a carboxyl-terminal domain containing a 7-fold WD repeat motif. We have overexpressed MHCK A truncation constructs to clarify the roles of each of these domains. Recombinant full-length MHCK A, MHCK A lacking the predicted coiled-coil domain, and MHCK A lacking the WD repeat domain were expressed at high levels in Dictyostelium cells lacking endogenous MHCK A. Biochemical analysis of the purified proteins demonstrates that the putative coiled-coil domain is responsible for the oligomerization of the MHCK A holoenzyme. Removal of the WD repeat domain had no effect on catalytic activity toward a synthetic peptide, but did result in a 95% loss of protein kinase activity when native myosin filaments were used as the substrate. Cellular analysis confirms that the same severe loss of activity against myosin occurs in vivo when the WD repeat domain is eliminated. These results suggest that the WD repeat domain of MHCK A serves to target this enzyme to its physiological substrate.

Identification of a protein kinase from Dictyostelium with homology to the novel catalytic domain of myosin heavy chain kinase A.

Myosin II assembly and localization into the cytoskeleton is regulated by heavy chain phosphorylation in Dictyostelium. The enzyme myosin heavy chain kinase A (MHCK A) has been shown previously to drive myosin filament disassembly in vitro and in vivo. MHCK A is noteworthy in that its catalytic domain is unrelated to the conventional families of eukaryotic protein kinases. We report here the cloning and initial biochemical characterization of another kinase from Dictyostelium that is related to MHCK A. When the segment of this protein that is similar to the MHCK A catalytic domain was expressed in bacteria, the resultant protein displayed efficient autophosphorylation, phosphorylated Dictyostelium myosin II, and also phosphorylated a peptide substrate corresponding to a portion of the myosin II tail. We have therefore named this gene myosin heavy chain kinase B. These results provide the first confirmation that sequences in other proteins that are related to the MHCK A catalytic domain can also encode protein kinase activity. It is likely that the related segments of homology present in rat eukaryotic elongation factor-2 kinase and a putative nematode eukaryotic elongation factor-2 kinase also encode the catalytic domains of those enzymes.

Mapping of the novel protein kinase catalytic domain of Dictyostelium myosin II heavy chain kinase A.

Myosin heavy chain kinase A (MHCK A) in Dictyostelium was identified as a biochemical activity that phosphorylates threonine residues in the myosin II tail domain and regulates myosin filament assembly. The catalytic domain of MHCK A has now been mapped through the functional characterization of a series of MHCK A truncation mutants expressed in Escherichia coli. A recombinant protein comprising the central nonrepetitive domain of MHCK A (residues 552-841) was isolated in a soluble form and shown to phosphorylate Dictyostelium myosin II, myelin basic protein, and a synthetic peptide substrate. The functionally mapped catalytic domain of MHCK A shows no detectable sequence similarity to known classes of eukaryotic protein kinases but shares substantial sequence similarity with a transcribed Caenorhabditis elegans gene and with the mammalian elongation factor-2 kinase (calcium/calmodulin-dependent protein kinase III). We suggest that MHCK A represents the prototype for a novel, widely occurring protein kinase family.

Cloning and characterization of a Dictyostelium myosin I heavy chain kinase activated by Cdc42 and Rac.

The motile activities of the small, single-headed class I myosins (myosin I) from the lower eukaryotes Acanthamoeba and Dictyostelium are activated by phosphorylation of a single serine or threonine residue in the head domain of the heavy chain. Recently, we purified a myosin I heavy chain kinase (MIHCK) from Dictyostelium based on its ability to activate the Dictyostelium myosin ID isozyme (Lee, S. -F., and Côté, G. P. (1995) J. Biol. Chem. 270, 11776-11782). The complete sequence of the Dictyostelium MIHCK has now been determined, revealing a protein of 98 kDa that is composed of an amino-terminal domain rich in proline, glutamine, and serine, a putative Cdc42/Rac binding motif, and a carboxyl-terminal kinase catalytic domain. MIHCK shares significant sequence identity with the Saccharomyces cerevisiae Ste20p kinase and the mammalian p21-activated kinase. Gel overlay assays and affinity chromatography experiments showed that MIHCK interacted with GTPgammaS (guanosine 5'-3-O-(thiotriphosphate))-labeled Cdc42 and Rac1 but not RhoA. In the presence of GTPgammaS-Rac1 MIHCK autophosphorylation increased from 1 to 9 mol of phosphate/mol, and the rate of Dictyostelium myosin ID phosphorylation was stimulated 10-fold. MIHCK may therefore provide a direct link between Cdc42/Rac signaling pathways and motile processes driven by myosin I molecules.

Myosin-based cortical tension in Dictyostelium resolved into heavy and light chain-regulated components.

Cortical tension in most nonmuscle cells is due largely to force production by conventional myosin (myosin II) assembled into the cytoskeleton. Cytoskeletal contraction in smooth muscle and nonmuscle cells is influenced by the degree of myosin filament assembly, and by activation of myosin motor function via regulatory light chain phosphorylation. Recombinant Dictyostelium discoideum cell lines have been generated bearing altered myosin heavy chains, resulting in either constitutive motor function or constitutive assembly into the cytoskeleton. Analysis of these cells allowed stiffening responses to agonists, measured on single cells, to be resolved into an regulatory light chain-mediated component reflecting activation of motor function, and a myosin heavy chain phosphorylation-regulated component reflecting assembly of filaments into the cytoskeleton. These two components can account for all of the cortical stiffening response seen during tested in vivo contractile events.

Dictyostelium myosin heavy chain kinase A regulates myosin localization during growth and development.

Phosphorylation of the Dictyostelium myosin II heavy chain (MHC) has a key role in regulating myosin localization in vivo and drives filament disassembly in vitro. Previous molecular analysis of the Dictyostelium myosin II heavy chain kinase (MHCK A) gene has demonstrated that the catalytic domain of this enzyme is extremely novel, showing no significant similarity to the known classes of protein kinases (Futey, L. M., Q. G. Medley, G. P. Côté, and T. T. Egelhoff. 1995. J. Biol. Chem. 270:523-529). To address the physiological roles of this enzyme, we have analyzed the cellular consequences of MHCK A gene disruption (mhck A- cells) and MHCK A overexpression (MHCK A++ cells). The mhck A- cells are viable and competent for tested myosin-based contractile events, but display partial defects in myosin localization. Both growth phase and developed mhck A- cells show substantially reduced MHC kinase activity in crude lysates, as well as significant overassembly of myosin into the Triton-resistant cytoskeletal fractions. MHCK A++ cells display elevated levels of MHC kinase activity in crude extracts, and show reduced assembly of myosin into Triton-resistant cytoskeletal fractions. MHCK A++ cells show reduced growth rates in suspension, becoming large and multinucleated, and arrest at the mound stage during development. These results demonstrate that MHCK A functions in vivo as a protein kinase with physiological roles in regulating myosin II localization and assembly in Dictyostelium cells during both growth and developmental stages.

Structural analysis of myosin heavy chain kinase A from Dictyostelium. Evidence for a highly divergent protein kinase domain, an amino-terminal coiled-coil domain, and a domain homologous to the beta-subunit of heterotrimeric G proteins.

We report here the cloning and characterization of the gene encoding the 130-kDa myosin heavy chain kinase (MHCK A) from the amoeba Dictyostelium. Previous studies have shown that purified MHCK A phosphorylates threonines in the carboxyl-terminal tail portion of the Dictyostelium myosin II heavy chain and that phosphorylation of these sites is critical in regulating the assembly and disassembly of myosin II filaments in vitro and in vivo. Biochemical analysis of MHCK A, together with analysis of the primary sequence, suggests that the amino-terminal approximately 500 amino acids form an alpha-helical coiled-coil domain and that residues from position approximately 860 to the carboxyl terminus (residue 1146) form a domain with significant similarity to the beta-subunit of heterotrimeric G proteins. No part of the MHCK A sequence displays significant similarity to the catalytic domain of conventional eukaryotic protein kinases. However, both native and recombinant MHCK A displayed autophosphorylation activity following renaturation from SDS gels, and MHCK A expressed in Escherichia coli phosphorylated purified Dictyostelium myosin, confirming that MHCK A is a bona fide protein kinase. Cross-linking studies demonstrated that native MHCK A is a multimer, consistent with the presence of an amino-terminal coiled-coil domain. Southern blot analysis indicates that MHCK A is encoded by a single gene that has no detectable introns.

Molecular genetic truncation analysis of filament assembly and phosphorylation domains of Dictyostelium myosin heavy chain.

Conventional myosin ('myosin II') is a major component of the cytoskeleton in a wide variety of eukaryotic cells, ranging from lower amoebae to mammalian fibroblasts and neutrophils. Gene targeting technologies available in the Dictyostelium discoideum system have provided the first genetic proof that this molecular motor protein is essential for normal cytokinesis, capping of cell surface receptors, normal chemotactic cell locomotion and morphogenetic shape changes during development. Although the roles of myosin in a variety of cell functions are becoming clear, the mechanisms that regulate myosin assembly into functional bipolar filaments within cells are poorly understood. Dictyostelium is currently the only system where mutant forms of myosin can be engineered in vitro, then expressed in their native context in cells that are devoid of the wild-type isoform. We have utilized this technology in combination with nested truncation and deletion analysis to map domains of the myosin tail necessary for in vivo and in vitro filament assembly, and for normal myosin heavy chain (MHC) phosphorylation. This analysis defines a region of 35 amino acids within the tail that is critical for filament formation both for purified myosin molecules and for myosin within the in vivo setting. Phosphorylation analysis of these mutants in intact cytoskeletons demonstrates that the carboxy-terminal tip of the myosin heavy chain is required for complete phosphorylation of the myosin tail.

Dictyostelium myosin heavy chain phosphorylation sites regulate myosin filament assembly and localization in vivo.

Three threonine residues in the tail region of Dictyostelium myosin II heavy chain have been implicated previously in control of myosin filament formation. Here we report the in vitro and in vivo consequences of converting these sites to alanine residues, which eliminates phosphorylation at these positions, or to aspartate residues, which mimics the negative charge state of the phosphorylated molecule. Alanine substitution allows in vitro assembly and in vivo contractile activity, although this myosin shows substantial over-assembly in vivo. Aspartate substitution eliminates filament assembly in vitro and renders the myosin unable to drive any tested contractile event in vivo. These results demonstrate that heavy chain phosphorylation plays a key modulatory role in controlling myosin function in vivo.

Molecular genetics of cell migration: Dictyostelium as a model system.

A central unresolved issue in modern cell biology concerns how eukaryotic cell migration is achieved. Although the underlying mechanics of cell locomotion appear similar in cells ranging from amoebae to leukocytes, the organisms that have been historically studied have not been amenable to the techniques of modern molecular genetics. The recent development of high-efficiency gene targeting technology for Dictyostelium discoideum, coupled with the classic cell migration behavior of this organism, offers an opportunity to resolve many of the controversial issues concerning cell locomotion.

Spatial and temporal control of nonmuscle myosin localization: identification of a domain that is necessary for myosin filament disassembly in vivo.

Myosin null mutants of Dictyostelium are defective for cytokinesis, multicellular development, and capping of surface proteins. We have used these cells as transformation recipients for an altered myosin heavy chain gene that encodes a protein bearing a carboxy-terminal 34-kD truncation. This truncation eliminates threonine phosphorylation sites previously shown to control filament assembly in vitro. Despite restoration of growth in suspension, development, and ability to cap cell surface proteins, these delta C34-truncated myosin transformants display severe cytoskeletal abnormalities, including excessive localization of the truncated myosin to the cortical cytoskeleton, impaired cell shaped dynamics, and a temporal defect in myosin dissociation from beneath capped surface proteins. These data demonstrate that the carboxy-terminal domain of myosin plays a critical role in regulating the disassembly of the protein from contractile structures in vivo.

Complementation of myosin null mutants in Dictyostelium discoideum by direct functional selection.

The eukaryotic slime mold Dictyostelium discoideum contains a single conventional myosin heavy chain gene (mhcA). Cell lines in which this gene was deleted via homologous recombination have been previously reported. These myosin null cells were shown to be defective for cytokinesis and for sporogenesis. We demonstrate here that the cloned mhcA gene can be reintroduced into these cells by the use of a direct functional selection. This selection was imposed by demanding that cells be capable of growth in suspension. The resulting transformants appear normal for cytokinesis, and also are fully competent for sporogenesis, confirming that reintroduction of the myosin gene is sufficient to restore these properties. These results demonstrate a method for rescuing mutants in Dictyostelium which may be generally applicable for genetically created mutations as well as for mutations which have been engineered.