Functional analysis of a cardiac myosin rod in Dictyostelium discoideum

Biological, biochemical, and kinetic effects of mutations of the cardiomyopathy loop of Dictyostelium myosin II: importance of ALA400

The cardiomyopathy (CM)-loop of the heavy chain of class-II myosins begins with a highly conserved Arg residue (whose mutation in human beta-cardiac myosin II results in familial hypertrophic cardiomyopathy). The CM-loop of Dictyostelium myosin II (Arg397-Gln407) is essential for its biological functions and biochemical activities. We found that the CM-loop of smooth muscle myosin II substituted partially, and the CM-loop of beta-cardiac myosin II less well, for growth, capping of surface receptors and development, and the actin-activated MgATPase and in vitro motility activities of purified myosins. There was little correlation between the biochemical and biological activities of the two chimeras and 19 point mutants, but only the five mutants with k cat/K actin values equivalent to wild-type myosin supported essentially full biological function. The three point mutations of Arg397 equivalent to those that result in hypertrophic cardiomyopathy in humans had minimal biological effects and different biochemical effects. The A400V mutation rendered full-length wild-type myosin almost completely inactive, both in vitro and in vivo, and the reverse V400A mutation in the cardiac CM-loop chimera restored almost full activity, even though the sequence still differed from wild-type in 7 of 11 positions. Transient kinetic studies of acto-subfragment-1 (S1) showed that the chimeras and the Ala/Val, Val/Ala mutations do not affect the equilibrium or the association and dissociation rate constants for either ATP or ADP binding to acto-S1 or the rate of ATP-induced dissociation of acto-S1. We conclude that the Ala/Val, Val/Ala mutations affect the release of Pi from acto-S1.ADP.Pi. In addition, Val at position 400 substantially reduces the affinity of actin for S1 in the absence of nucleotide.

Dictyostelium and Acanthamoeba myosin II assembly domains go to the cleavage furrow of Dictyostelium myosin II-null cells

How myosin II localizes to the cleavage furrow of dividing cells is largely unknown. We show here that a 283-residue protein, assembly domain (AD)1, corresponding to the AD in the tail of Dictyostelium myosin II assembles into bundles of long tubules when expressed in myosin II-null cells and localizes to the cleavage furrow of dividing cells. AD1 mutants that do not polymerize in vitro do not go to the cleavage furrow in vivo. An assembly-competent polypeptide corresponding to the C-terminal 256 residues of Acanthamoeba myosin II also goes to the cleavage furrow of Dictyostelium myosin II-null cells. When overexpressed in wild-type cells, AD1 colocalizes with endogenous myosin II (possibly as a copolymer) in interphase, motile, and dividing cells and under caps of Con A receptors but has no effect on myosin II-dependent functions. These results suggest that neither a specific sequence, other than that required for polymerization, nor interaction with other proteins is required for localization of myosin II to the cleavage furrow.

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.

Inactivation of myosin heavy chain genes in the mouse: diverse and unexpected phenotypes

Myosin heavy chain (MyHC) is a critical component of the cellular contractile apparatus. The mammalian genome contains two nonmuscle, two smooth muscle, and eight striated muscle isoforms of MyHC. Within each class of genes, there is extremely high sequence homology among different MyHC isoforms, raising the question of whether these isoforms are functionally redundant or whether they perform unique roles in cell function. Recently, strains of mice null for four different MyHC isoforms have been generated. Mice null for the nonmuscle II-B isoform experience significant prenatal lethality and surviving animals have several cardiac abnormalities [Tullio et al. (1997) Proc Natl Acad Sci USA 94:12407-12412]. Mice homozygous null for alpha cardiac MyHC are embryonic lethal, while heterozygous mice are viable but also have numerous cardiac defects [Jones et al. (1996) J Clin Invest 98:1906-1917]. Mice null for IIb or IId adult skeletal MyHC are viable but have skeletal muscle abnormalities compared to wild type mice, despite compensation of a neighboring MyHC gene [Acakpo-Satchivi et al. (1997) J Cell Biol 139:1219-1229]. Both IIb and IId null mice show significant decreases in body mass. Mean muscle mass is also significantly decreased in both null strains but the extent and the pattern of affected muscles differs between the two strains. Both strains show evidence of skeletal muscle pathology but again the pattern and extent differ between the two strains. Finally, both adult skeletal strains demonstrate distinct impairments in contractile function when compared to wild type. Together these observations support the hypothesis that the different isoforms of MyHC are functionally unique and cannot substitute for one another.

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.

Role of myosin II tail sequences in its function and localization at the cleavage furrow in Dictyostelium

Cytoplasmic myosin II accumulates in the cleavage furrow and provides the force for cytokinesis in animal and amoeboid cells. One model proposes that a specific domain in the myosin II tail is responsible for its localization, possibly by interacting with a factor concentrated in the equatorial region. To test this possibility, we have expressed myosins carrying mutations in the tail domain in a strain of Dictyostelium cells from which the endogenous myosin heavy chain gene has been deleted. The mutations used in this study include four internal tail deletions: Mydelta824-941, Mydelta943-1464, Mydelta943-1194 and Mydelta1156-1464. Contrary to the prediction of the hypothesis, immunofluorescence staining demonstrated that all mutant myosins were able to move toward the furrow region. Chimeric myosins, which consisted of a Dictyostelium myosin head and chicken skeletal myosin tail, also efficiently localized to the cleavage furrow. All these deletion and chimeric mutant myosins, except for Mydelta943-1464, the largest deletion mutant, were able to support cytokinesis in suspension. Our data suggest that there is no single specific domain in the tail of Dictyostelium myosin II that is required for its functioning at and localization to the cleavage furrow.

Dictyostelium cell shape generation requires myosin II

Myosin II function has been implicated in non-muscle cell behaviors including movement, cytokinesis, and multicellular morphogenesis. Surprisingly, two-dimensional morphology and behavior of Dictyostelium amoebae lacking myosin II (mhcA-) is not dramatically altered when cells are examined using conventional imaging techniques. We have observed amoebae from the side (side-view microscopy or SVM) and find that wild-type but not mhcA- cells undergo extensive three-dimensional (3D) shape changes. These shape changes often occur above the plane of the substrate and are morphologically distinct from pseudopods. For example, unlike pseudopods, vertically extended cell regions are not enriched in F-actin and do not exclude organelles. In contrast, mhcA- cells generate F-actin-filled pseudopods and spread laterally but do not undergo vertical extension. When wild-type cells are removed from the substrate and shaken in suspension, they retain their irregular three-dimensional shape. Suspended mhcA- cells, however, rapidly become spherical. Thus, cells lacking myosin appear to be incapable of generating or maintaining 3D shape independent of a substrate. When a substrate is available, these cells can attach and spread on the surface in a myosin-independent manner. These previously undescribed defects of mhcA- cells reveal that 3D cell shape generation requires myosin II and is mechanistically distinct from pseudopod formation.

Recombinant glycoprotein production in the slime mould Dictyostelium discoideum

Dictyostelium discoideum is a well known amoeboid organism, with unicellular and multicellular life-cycle stages, that is used for studying cell and developmental biology. With advances in gene-disruption technology and transformation of this organism, many homologous proteins have been expressed either to complement defective proteins or to study basic cell biology. Now, D. discoideum is being used to express heterologous proteins that are difficult to study in other systems, and its unique cell biology is being exploited to facilitate a wide range of protein modifications. In the past year, substantial progress has been made in expressing correctly folded forms of malarial circumsporozoite antigen and rotavirus surface glycoprotein VP7. Exciting developments have also been made in expressing human muscarinic receptors.