Thermally induced unfolding of Acanthamoeba myosin II and skeletal muscle myosin: nucleotide effects

Screening for Ligands Using a Generic and High-Throughput Light-Scattering-Based Assay

Rapid identification of small molecules that interact with protein targets using a generic screening method greatly facilitates the development of therapeutic agents. The authors describe a novel method for performing homogeneous biophysical assays in a high-throughput format. The use of light scattering as a method to evaluate protein stability during thermal denaturation in a 384-well format yields a robust assay with a low frequency of false positives. This novel method leads to the identification of interacting small molecules without the addition of extraneous fluorescent probes. The analysis and interpretation of data is rapid, with sensitivity for protein stability comparable to differential scanning calorimetry. The authors propose potential uses in drug discovery, structural genomics, and functional genomics as a method to evaluate small-molecule interactions, identify natural cofactors that stabilize target proteins, and identify natural substrates and products for previously uncharacterized protein targets.

Regulation of nonmuscle myosins by heavy chain phosphorylation

Functional activities of many nonmuscle myosin isoforms are (or are postulated to be) regulated by heavy chain phosphorylation. Depending on the myosin isoform, the serine or threonine residues located within the head (myosin I or myosin VI) or within the C-terminal tail domains (myosin II or myosin V) can be phosphorylated by more or less specific endogenous kinases. In some isoforms phosphorylation can occur both in the head and tail domains, as it has been found for myosin III. There are also isoforms that can be regulated both by the heavy and regulatory light chain phosphorylation, as for the example myosin II from slide mold Dictyostelium discoideum. The goal of this review was to describe recent findings on regulation of myosin I, myosin II, myosin III, myosin V and myosin VI isoforms by their heavy chain phosphorylation including the short charcteristics of the relevant kinases. The biological aspects of the phosphorylation are also discussed.

Nucleotide binding induces global and local structural changes of myosin head in muscle fibres

Thermal stability and internal dynamics of myosin heads in fiber bundles from rabbit psoas muscle has been studied by electron paramagnetic resonance (EPR) spectroscopy and differential scanning calorimetry (DSC). Using ADP, ATP and orthovanadate (V(i)), three intermediate states of the ATP hydrolysis cycle were simulated in glycerinated muscle fibers. DSC transitions contained three overlapping endotherms in each state. Deconvolution showed that the transition temperature of 58.4 degrees C was almost independent of the intermediate state of myosin, while nucleotide binding shifted the melting temperatures of 54.0 and 62.3 degrees C, and changed the enthalpies. These changes suggest global rearrangements of the internal structure in myosin head. In the presence of ADP and ADP plus V(i), the conventional EPR spectra showed changes in the ordering of the probe molecules, suggesting local conformational and motional changes in the internal structure of myosin heads. Saturation transfer EPR measurements reported increased rotational mobility of spin labels in the presence of ATP plus orthovanadate corresponding to a weakly binding state of myosin to actin.

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.

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.