Genetically engineered truncated myosin in Dictyostelium: the carboxyl-terminal regulatory domain is not required for the developmental cycle

Lysosome and proteasome pathways are distributed in laticifers of Euphorbia helioscopia L

At present, the lysosome pathway (LP) and proteasome pathway (PP) are known as major clearance systems in eukaryotic cells. The laticifer, a secretory tissue, degrades some cytoplasm during development. In this study, we investigated the distribution of LP and PP in non-articulated laticifers of Euphorbia helioscopia L. Electron microscopy revealed that, plastids, mitochondria and some cyotsol were degraded in the late development laticifers, where there were numerous vesicles originated from dicytosomes. Accordingly, some key proteins in LP and PP were detected in E. helioscopia latex using isobaric tags for relative and absolute quantitation (iTRAQ) proteomics. Further immunohistochemistry analysis revealed that the clathrin heavy chain (CHC) belonging to LP and the ubiquitin-mediated proteasome degradation increases gradually as the laticifer develops. Immuno-electron microscopy revealed that the cysteine protease, CHC and AP-2 complex subunit beta-1 belonging to LP were mainly distributed in vesicles deriving from dicytosomes, which we called lysosome-like vesicles. Ubiquitin was widely distributed in the cytosol, and proteasome activity was significantly reduced when various concentrations of the inhibitor MG132 were added to the latex total protein. We hypothesize that LP and PP are distributed in E. helioscopia laticifers; and it was speculated that LP and PP might be involved in the degradation of organelles and some cytoplasmic matrix in E. helioscopia laticifers.

A global, myosin light chain kinase-dependent increase in myosin II contractility accompanies the metaphase-anaphase transition in sea urchin eggs

Myosin II is the force-generating motor for cytokinesis, and although it is accepted that myosin contractility is greatest at the cell equator, the temporal and spatial cues that direct equatorial contractility are not known. Dividing sea urchin eggs were placed under compression to study myosin II-based contractile dynamics, and cells manipulated in this manner underwent an abrupt, global increase in cortical contractility concomitant with the metaphase-anaphase transition, followed by a brief relaxation and the onset of furrowing. Prefurrow cortical contractility both preceded and was independent of astral microtubule elongation, suggesting that the initial activation of myosin II preceded cleavage plane specification. The initial rise in contractility required myosin light chain kinase but not Rho-kinase, but both signaling pathways were required for successful cytokinesis. Last, mobilization of intracellular calcium during metaphase induced a contractile response, suggesting that calcium transients may be partially responsible for the timing of this initial contractile event. Together, these findings suggest that myosin II-based contractility is initiated at the metaphase-anaphase transition by Ca2+-dependent myosin light chain kinase (MLCK) activity and is maintained through cytokinesis by both MLCK- and Rho-dependent signaling. Moreover, the signals that initiate myosin II contractility respond to specific cell cycle transitions independently of the microtubule-dependent cleavage stimulus.

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.

Insertion or deletion of a single residue in the strut sequence of Dictyostelium myosin II abolishes strong binding to actin

The strut loop, one of the three loops that connects the upper and lower 50K subdomains of myosin, plays a role as a "strut" to keep the relative disposition of the two subdomains. A single residue was either inserted into or deleted from this loop. The insertion or deletion mutation abolished the in vivo motor functions of myosin, as revealed by the fact that the mutant myosins did not complement the phenotypic defects of the myosin-null cells. In vitro studies of purified full-length myosins and their subfragment-1s (S1s) revealed that the insertion mutants virtually lost the strong binding to actin although their motor functions in the absence of actin remained almost normal, showing that only the hydrophobic actin-myosin association was selectively affected by the insertion mutations. Unlike the insertion mutants, the deletion mutant showed defects both in the strong-binding state and the rate-limiting step of ATPase cycle. These results indicate the functional importance of the strut loop in establishing the strong-binding state of myosin and thereby achieving successful power strokes.

Deletion of the myopathy loop of Dictyostelium myosin II and its impact on motor functions

One of the putative actin-binding sites of Dictyostelium myosin II is the beta-strand-turn-beta-strand structure (Ile(398)-Leu-Ala-Gly-Arg-Asp(403)-Leu-Val(405)), the "myopathy loop, " which is located at the distal end of the upper 50-kDa subdomain and next to the conserved arginine (Arg(397)), whose mutation in human cardiac myosin results in familial hypertrophic cardiomyopathy. The myopathy loop contains the TEDS residue (Asp(403)), which is a target of the heavy-chain kinase in myosin I. Moreover, the loop contains a cluster of hydrophobic residues (Ile(398), Leu(399), Leu(404), and Val(405)), whose side chains are fully exposed to the solvent. In our study, the myopathy loop was deleted from Dictyostelium myosin II to investigate its functional roles. The mutation abolished hydrophobic interactions of actin and myosin in the strong binding state during the ATPase cycle. Association of the mutant myosin and actin was maintained only through ionic interactions under these conditions. Without strong hydrophobic interactions, the mutant myosin still exhibited motor functions, although at low levels. It is likely that the observed defects resulted mainly from a loss of the cluster of hydrophobic residues, since replacement of Asp(403) or Arg(402) with alanine generated a mutant with less severe or no defects compared with those of the deletion mutant.

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

Myosin II thick filament assembly in Dictyostelium is regulated by phosphorylation at three threonines in the tail region of the molecule. Converting these three threonines to aspartates (3 x Asp myosin II), which mimics the phosphorylated state, inhibits filament assembly in vitro, and 3 x Asp myosin II fails to rescue myosin II-null phenotypes. Here we report a suppressor screen of Dictyostelium myosin II-null cells containing 3 x Asp myosin II, which reveals a 21-kD region in the tail that is critical for the phosphorylation control. These data, combined with new structural evidence from electron microscopy and sequence analyses, provide evidence that thick filament assembly control involves the folding of myosin II into a bent monomer, which is unable to incorporate into thick filaments. The data are consistent with a structural model for the bent monomer in which two specific regions of the tail interact to form an antiparallel tetrameric coiled-coil structure.

Mutational analysis of the switch II loop of Dictyostelium myosin II

A loop comprising residues 454-459 of Dictyostelium myosin II is structurally and functionally equivalent to the switch II loop of the G-protein family. The consensus sequence of the "switch II loop" of the myosin family is DIXGFE. In order to determine the functions of each of the conserved residues, alanine scanning mutagenesis was carried out on the Dictyostelium myosin II heavy chain gene. Examination of in vivo and in vitro motor functions of the mutant myosins revealed that the I455A and S456A mutants retained those functions, whereas the D454A, G457A, F458A and E459A mutants lost them. Biochemical analysis of the latter myosins showed that the G457A and E459A mutants lost the basal ATPase activity by blocking of the isomerization and hydrolysis steps of the ATPase cycle, respectively. The F458A mutant, however, lost the actin-activated ATPase activity without loss of the basal ATPase activity. These results are discussed in terms of the crystal structure of the Dictyostelium myosin motor domain.

Conditional loss-of-myosin-II-function mutants reveal a position in the tail that is critical for filament nucleation

Myosin-II must be assembled into filaments to perform its cellular functions. Two conditional loss-of-myosin-II-function mutants were recovered from a previous genetic screen with defects that were mapped to the coiled-coil tail region of Dictyostelium myosin-II. Strikingly, both tail mutations affected the same arginine residue at position 1880. A single amino acid substitution, R1880P, disrupted both the dimerization and tetramerization steps of filament nucleation. Even a single charge reversal at this position, R1880D, was sufficient to inhibit filament assembly, while other single charge reversals in the region of antiparallel contract suppressed these filament assembly mutants. The considerable impact of small electrostatic forces on nucleation suggests that these steps are delicately balanced and easily reversible.

Single-headed myosin II acts as a dominant negative mutation in Dictyostelium

Conventional myosin II is an essential protein for cytokinesis, capping of cell surface receptors, and development of Dictyostelium cells. Myosin II also plays an important role in the polarization and movement of cells. All conventional myosins are double-headed molecules but the significance of this structure is not understood since single-headed myosin II can produce movement and force in vitro. We found that expression of the tail portion of myosin II in Dictyostelium led to the formation of single-headed myosin II in vivo. The resultant cells contain an approximately equal ratio of double- and single-headed myosin II molecules. Surprisingly, these cells were completely blocked in cytokinesis and capping of concanavalin A receptors although development into fruiting bodies was not impaired. We found that this phenotype is not due to defects in myosin light chain phosphorylation. These results show that single-headed myosin II cannot function properly in vivo and that it acts as a dominant negative mutation for myosin II function. These results suggest the possibility that cooperativity of myosin II heads is critical for force production in vivo.

Expression of light meromyosin in Dictyostelium blocks normal myosin II function

The ability of myosin II to form filaments is essential for its function in vivo. This property of self association is localized in the light meromyosin (LMM) region of the myosin II molecules. To explore this property in more detail within the context of living cells, we expressed the LMM portion of the Dictyostelium myosin II heavy chain gene in wild-type Dictyostelium cells. We found that the LMM protein was expressed at high levels and that it folded properly into alpha-helical coiled-coiled molecules. The expressed LMM formed large cytoplasmic inclusions composed of entangled short filaments surrounded by networks of long tubular structures. Importantly, these abnormal structures sequestered the cell's native myosin II, completely removing it from its normal cytoplasmic distribution. As a result the cells expressing LMM displayed a myosin-null phenotype: they failed to undergo cytokinesis and became multinucleate, failed to form caps after treatment with Con A, and failed to complete their normal developmental cycle. Thus, expression of the LMM fragment in Dictyostelium completely abrogates myosin II function in vivo. The dominant-negative character of this phenotype holds promise as a general method to disrupt myosin II function in many cell types without the necessity of gene targeting.

Functional analysis of a cardiac myosin rod in Dictyostelium discoideum

Manipulation of the single conventional myosin heavy chain (mhc) gene in Dictyostelium discoideum (Dd) has delineated an essential role for the filament-forming, or light meromyosin (LMM) domain of the myosin molecule in cytokinesis, development, and in the capping of cell surface receptors (see Spudich: Cell Regulation 1:1-11, 1989; Egelhoff et al.: Journal of Cell Biology, 112:677-688, 1991a). In order to assess the functional relationship between sarcomeric and cytoplasmic myosins, a chimeric gene encoding the Dd myosin head and subfragment 2 fused to rat beta cardiac LMM was transfected into both wild-type and Dd mhc null cells. Chimeric myosin was organized into dense cortical patches in the cytoplasm of both wild-type and Dd mhc null cells. Although null cells expressing chimeric mhc at approximately 10% of Dd mhc levels were unable to grow in shaking suspension or to complete development, chimeric myosin was able to rescue capping of cell surface receptors, to associate with filamentous actin, and to localize to the correct subcellular position during aggregation. Deletion of 29 amino acids in the rod corresponding to a previously defined filament assembly competent region eliminated the cortical patches and the posterior localization during chemotaxis. Taken together, these observations suggest that sarcomeric and cytoplasmic myosin rods are functionally interchangeable in several aspects of nonmuscle motility.

The telophase disc: its possible role in mammalian cell cleavage

The molecular signals that determine the position and timing of the furrow that forms during mammalian cell cytokinesis are presently unknown. It is apparent, however, that these signals are generated by the mitotic spindle after the onset of anaphase. Recently we have described a structure that bisects the cell during telophase at the position of the cytokinetic furrow. This structure, the telophase disc, appears to be templated by the mitotic spindle during anaphase, and precedes the formation of the cytokinetic furrow. The relationship of the telophase disc to the myosin and actin based furrowing mechanism is discussed here. We propose that the telophase disc may determine the position and timing of cleavage by recruitment and alignment of myosin.

Clathrin heavy chain is required for pinocytosis, the presence of large vacuoles, and development in Dictyostelium

To investigate the intracellular role of the clathrin heavy chain in living cells, we have used "antisense" RNA to engineer mutant Dictyostelium discoideum cells that are severely deficient in clathrin heavy chain expression. Immunoblots stained with an anti-clathrin heavy chain antiserum revealed that mutant cells contained undetectable amounts of clathrin heavy chain protein. Similarly, Northern blots showed an absence of clathrin heavy chain mRNA. Clathrin heavy chain-deficient Dictyostelium cells were viable, but exhibited growth rates twofold slower than parental cells. Whereas many morphological features of the mutant cells were normal, mutant cells lacked coated pits and coated vesicles. Clathrin-deficient cells were also missing large translucent vacuoles that serve as endosomes and contractile vacuoles. In the absence of clathrin heavy chain, mutant cells displayed three distinct functional defects: (a) impairment in endocytosis of fluid phase markers, but competence in another endocytic pathway, the phagocytosis of solid particles; (b) defects in osmoregulation; and (c) inability to complete the starvation-induced development cycle.

Characterization of the clathrin heavy chain from Dictyostelium discoideum

We report the cloning and analysis of a clathrin heavy-chain cDNA from the eukaryotic microorganism, Dictyostelium discoideum. A single gene, designated chcA, for the clathrin heavy chain encoded a protein of 1,694 amino acids with a molecular mass of 193,618 daltons. Comparison of the amino acid sequence with the rat and with the yeast sequence showed that the highly conserved protein was more similar to the mammalian clathrin heavy chain (57% identity) than to the yeast heavy chain (45% identity). The mRNA for the clathrin heavy chain was regulated during development. mRNA levels were highest during vegetative growth and declined as the cells progressed through the 24-hr developmental cycle. The concentration of clathrin heavy-chain protein was the same in cells grown in liquid media (high rates of pinocytosis) as in cells grown with bacteria (low rates of pinocytosis), which suggests that regulation of pinocytosis in these cells is not achieved by altering the concentration of clathrin.

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.