Effects of phosphorylation and nucleotides on the conformation of myosin II from Acanthamoeba castellanii

Non-muscle myosin 2 at a glance

Non-muscle myosin 2 (NM2) motors are the major contractile machines in most cell types. Unsurprisingly, these ubiquitously expressed actin-based motors power a plethora of subcellular, cellular and multicellular processes. In this Cell Science at a Glance article and the accompanying poster, we review the biochemical properties and mechanisms of regulation of this myosin. We highlight the central role of NM2 in multiple fundamental cellular processes, which include cell migration, cytokinesis, epithelial barrier function and tissue morphogenesis. In addition, we highlight recent studies using advanced imaging technologies that have revealed aspects of NM2 assembly hitherto inaccessible. This article will hopefully appeal to both cytoskeletal enthusiasts and investigators from outside the cytoskeleton field who have interests in one of the many basic cellular processes requiring actomyosin force production.

Intracellular localization and dynamics of myosin-II and myosin-IC in live Acanthamoeba by transient transfection of EGFP fusion proteins

We developed a reliable method for transient transfection of Acanthamoeba using Superfect (Qiagen) and a vector with the Acanthamoeba ubiquitin promoter and enhanced green fluorescent protein (EGFP) as the reporter gene. The transfection efficiency was 3% for profilin-I-EGFP and EGFP-myosin-II tail, and less than 0.5% for larger constructs such as full length myosin-II or myosin-IC. Profilin-I-EGFP was distributed throughout the cytoplasm as observed previously with rhodamine-labeled profilin, while EGFP alone accumulated in the nucleus. EGFP fused to full length myosin-II or to the C-terminal 256 residues of the myosin-II tail concentrated in fluorescent spots similar to thick filaments and minifilaments identified previously in fixed cells with fluorescent antibodies. Thick filaments were located in the dorsal cytoplasm and along the lateral margins of the back half of the cell. Thick filaments formed behind the leading edge and moved continuously towards the rear of the cell, where they disassembled. If phosphorylation of the myosin-II heavy chain was prevented by mutation of all three phosphorylated serines to alanine, thick filaments of unphosphorylated myosin-II accumulated around vesicles of various sizes. EGFP-myosin-IC was spread throughout the cytoplasm but concentrated transiently around contractile vacuoles and macropinocytosis cups providing that the construct included both the head and a tail with the SH3 domain.

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.

Phenylglyoxal reveals phosphorylation-dependent difference in the conformation of Acanthamoeba myosin II active site

Acanthamoeba myosin II is regulated in an unique way by phosphorylation of three serine residues located within nonhelical tailpiece of the rod domain. Phosphorylation inhibits functions associated with the NH2-terminal motor domain, i.e., actin-activated activity and ability to move actin filaments. Number of data indicate functional communication between these distant domains. In this work, effect of modification of arginine residues with phenylglyoxal on the Ca2+-ATPase activity and susceptibility to endoproteinase ArgC cleavage of monomeric phospho- and dephosphomyosin II has been investigated. Upon the phenylglyoxal treatment the activity of dephosphomyosin II was decreasing faster that the activity of phosphomyosin. The modification also affected the proteolytic fragmentation of phospho- and dephosphomyosin II: the cleavage of heavy chain was further inhibited for phosphomyosin and enhanced for dephosphomyosin with a concomitant exposure of an additional cleavage site within the head domain. No difference in the quantity of modified arginines was observed. These results indicate a difference between the conformation of active sites of phospho- and dephosphomyosin II.

Flexibility of Acanthamoeba myosin rod minifilaments

Previous electric birefringence experiments have shown that the actin-activated Mg2+-ATPase activity of Acanthamoeba myosin II correlates with the ability of minifilaments to cycle between flexible and stiff conformations. The cooperative transition between conformations was shown to depend on Mg2+ concentration, on ATP binding, and on the state of phosphorylation of three serines in the C-terminal end of the heavy chains. Since the junction between the heavy meromyosin (HMM) and light meromyosin (LMM) regions is expected to disrupt the alpha-helical coiled-coil structure of the rod, this region was anticipated to be the flexible site. We have now cloned and expressed the wild-type rod (residues 849-1509 of the full-length heavy chain) and rods mutated within the junction in order to test this. The sedimentation and electric birefringence properties of minifilaments formed by rods and by native myosin II are strikingly similar. In particular, the Mg2+-dependent flexible-to-stiff transitions of native myosin II and wild-type rod minifilaments are virtually superimposable. Mutations within the junction between the HMM and LMM regions of the rod modulate the ability of Mg2+ to stabilize the stiff conformation. Less Mg2+ is required to induce minifilament stiffening if proline-1244 is replaced with alanine. Deleting the entire junction region (25 amino acids) results in a even greater decrease in the Mg2+ concentration necessary for the transition. The HMM-LMM junction does indeed seem to act as a Mg2+-dependent flexible hinge.

Two-state thermal unfolding of a long dimeric coiled-coil: the Acanthamoeba myosin II rod

Acanthamoeba myosin II rod is a long alpha-helical coiled-coil with a flexible hinge containing a helix-breaking proline. The thermal stability of the complete rod domain of myosin II (residues 849-1509), a mutant in which the hinge proline was replaced by alanine (P398A), and a mutant with the whole hinge region deleted (delta(384-408)) was studied in 0.6 and 2.2 M KCl, pH 7.5. In analytical ultracentrifugation studies, the purified myosin II rods sedimented as monodisperse dimers with sedimentation coefficients s(20,w) = 3.8 S (wild-type, Mr = 149,000) and 3.6 S (P398A and delta(384-408)). Circular dichroism (CD) and differential scanning calorimetry (DSC) showed that the thermal unfolding of the myosin II rod is reversible and highly cooperative. The unfolding of the rod is coupled to a dissociation of the chains, as shown by HPLC gel filtration at high temperatures and by the concentration dependence of the transition temperature. The CD and DSC data are consistent with a two-state mechanism (Tm approximately 40 degrees C, deltaH approximately 400 kcal/mol) in which the dimeric rod unfolds with concomitant formation of two unfolded monomers. We found no evidence for independent unfolding of the two rod domains that are separated by the hinge region. The only difference observed in the unfolding of the mutant rods from that of the wild type was a approximately 2 degrees C increase in the thermal stability of the hinge-deletion mutant. Thus, the mechanism of unfolding the Acanthamoeba myosin II rod is different from those of skeletal muscle myosin rod and tropomyosin, for which non-two-state thermal transitions have been observed. The cooperative unfolding of the entire coiled-coil rod of Acanthamoeba myosin II may underlie the previously reported regulatory coupling between its N-terminal head and C-terminal tail.

Protein kinase A-catalyzed phosphorylation and its effect on conformation in phytochrome A

Phytochromes are ubiquitous red/far-red wavelength-sensitive photoreceptors in plants. Oat phytochrome A is a phosphoprotein. Phytochrome A (phyA) possesses two spatially different sites for phosphorylation with cAMP-dependent protein kinase (PKA) [McMichael & Lagarias (1990) Biochemistry 29, 3872-3878]. To assess the modulation of protein conformation by phosphorylation/dephosphorylation and its possible implication in phytochrome-mediated signal transduction, the conformations of phytochrome have been probed by PKA catalyzed phosphorylation. The phosphorylated species were purified and analyzed, along with untreated phytochrome, by limited proteolysis, circular dichroism (CD) and fluorescence quenching measurements. No significant changes in secondary structure of the phyA molecule after its phosphorylation were observed by CD. However, a subtle topographic and/or electrostatic effect of the phytochrome phosphorylation was detected by the time-resolved fluorescence quenching of Trp residues with Cs+ ions. N-Terminal phosphorylation at Ser17 was unique to the Pr form, but both Pr and Pfr phytochromes were phosphorylated at the hinge region to some extent. Phosphorylation at the hinge region resulted in noticeable changes in the proteolytic patterns, inhibiting cleavage near the phosphorylation site and favoring tryptic digestion of the Lys536-Asn537 peptide bond. Phosphorylation at the N-terminus did not cause observable changes in the helical structure of this region, but had an inhibitory effect on proteinase V8 accessibility at a site near the chromophore attachment. The functional relevance of protein phosphorylation of phyA is also discussed.

Nucleotides increase the internal flexibility of filaments of dephosphorylated Acanthamoeba myosin II

The actin-activated Mg(2+)-ATPase activity of Acanthamoeba myosin II minifilaments is dependent both on Mg2+ concentration and on the state of phosphorylation of three serine sites at the C-terminal end of the heavy chains. Previous electric birefringence experiments on minifilaments showed a large dependence of signal amplitude on the phosphorylation state and Mg2+ concentration, consistent with large changes in filament flexibility. These observations suggested that minifilament stiffness was important for function. We now report that the binding of nucleotides to dephosphorylated minifilaments at Mg2+ concentrations needed for optimal activity increases the flexibility by about 10-fold, as inferred from the birefringence signal amplitude increase. An increase in flexibility with nucleotide binding is not observed for dephosphorylated minifilaments at lower Mg2+ concentrations or for phosphorylated minifilaments at any Mg2+ concentrations examined. The relaxation times for minifilament rotations that are sensitive to the conformation myosin heads are also observed to depend on phosphorylation, Mg2+ concentration, and nucleotide binding. These latter experiments indicate that the actin-activated Mg2+ concentration, and nucleotide binding. These latter experiments indicate that the actin-activated Mg(2+)-ATPase activity of Acanthamoeba myosin II correlates with both changes in myosin head conformation and the ability of minifilaments to cycle between stiff and flexible conformations coupled to nucleotide binding and release.

Thermal unfolding of Acanthamoeba myosin II and skeletal muscle myosin

Studies on the thermal unfolding of monomeric Acanthamoeba myosin II and other myosins, in particular skeletal muscle myosin, using differential scanning calorimetry (DSC) are reviewed. The unfolding transitions for intact myosin or its head fragment are irreversible, whereas those of the rod part and its fragments are completely reversible. Acanthamoeba myosin II unfolds with a high degree of cooperativity from ca. 40-45 degrees C at pH 7.5 in 0.6 M KCl, producing a single, sharp endotherm in DSC. In contrast, thermal transitions of rabbit skeletal muscle myosin occur over a broader temperature range (ca. 40-60 degrees C) under the same conditions. The DSC studies on the unfolding of the myosin rod and its fragments allow identification of cooperative domains, each of which unfolds according to a two-state mechanism. Also, DSC data show the effect of the nucleotide-induced conformational changes in the myosin head on the protein stability.