Differential scanning calorimetric study of myosin subfragment 1 with tryptic cleavage at the N-terminal region of the heavy chain

Biophysical Derangements in Genetic Cardiomyopathies

This article focuses on three "bins" that comprise sets of biophysical derangements elicited by cardiomyopathy-associated mutations in the myofilament. Current therapies focus on symptom palliation and do not address the disease at its core. We and others have proposed that a more nuanced classification could lead to direct interventions based on early dysregulation changing the trajectory of disease progression in the preclinical cohort. Continued research is necessary to address the complexity of cardiomyopathic progression and develop efficacious therapeutics.

Intermolecular Interactions of Myosin Subfragment 1 Induced by the N-Terminal Extension of Essential Light Chain 1

We applied dynamic light scattering (DLS) to compare aggregation properties of two isoforms of myosin subfragment 1 (S1) containing different "essential" (or "alkali") light chains, A1 or A2, which differ by the presence of an N-terminal extension in A1. Upon mild heating (up to 40°C), which was not accompanied by thermal denaturation of the protein, we observed a significant growth in the hydrodynamic radius of the particles for S1(A1), from ~18 to ~600-700 nm, whereas the radius of S1(A2) remained unchanged and equal to ~18 nm. Similar difference between S1(A1) and S1(A2) was observed in the presence of ADP. In contrast, no differences were observed by DLS between these two S1 isoforms in their complexes S1-ADP-BeF_x and S1-ADP-AlF₄^- which mimic the S1 ATPase intermediate states S1*-ATP and S1**-ADP-P_i. We propose that during the ATPase cycle the A1 N-terminal extension can interact with the motor domain of the same S1 molecule, and this can explain why S1(A1) and S1(A2) in S1-ADP-BeF_x and S1-ADP-AlF₄^- complexes do not differ in their aggregation properties. In the absence of nucleotides (or in the presence of ADP), the A1 N-terminal extension can interact with actin, thus forming an additional actin-binding site on the myosin head. However, in the absence of actin, this extension seems to be unable to undergo intramolecular interaction, but it probably can interact with the motor domain of another S1 molecule. These intermolecular interactions of the A1 N-terminus can explain unusual aggregation properties of S1(A1).

Does Interaction between the Motor and Regulatory Domains of the Myosin Head Occur during ATPase Cycle? Evidence from Thermal Unfolding Studies on Myosin Subfragment 1

Myosin head (myosin subfragment 1, S1) consists of two major structural domains, the motor (or catalytic) domain and the regulatory domain. Functioning of the myosin head as a molecular motor is believed to involve a rotation of the regulatory domain (lever arm) relative to the motor domain during the ATPase cycle. According to predictions, this rotation can be accompanied by an interaction between the motor domain and the C-terminus of the essential light chain (ELC) associated with the regulatory domain. To check this assumption, we applied differential scanning calorimetry (DSC) combined with temperature dependences of fluorescence to study changes in thermal unfolding and the domain structure of S1, which occur upon formation of the ternary complexes S1-ADP-AlF4- and S1-ADP-BeFx that mimic S1 ATPase intermediate states S1**-ADP-Pi and S1*-ATP, respectively. To identify the thermal transitions on the DSC profiles (i.e. to assign them to the structural domains of S1), we compared the DSC data with temperature-induced changes in fluorescence of either tryptophan residues, located only in the motor domain, or recombinant ELC mutants (light chain 1 isoform), which were first fluorescently labeled at different positions in their C-terminal half and then introduced into the S1 regulatory domain. We show that formation of the ternary complexes S1-ADP-AlF4- and S1-ADP-BeFx significantly stabilizes not only the motor domain, but also the regulatory domain of the S1 molecule implying interdomain interaction via ELC. This is consistent with the previously proposed concepts and also adds some new interesting details to the molecular mechanism of the myosin ATPase cycle.

Thermal denaturation and aggregation of myosin subfragment 1 isoforms with different essential light chains

We compared thermally induced denaturation and aggregation of two isoforms of the isolated myosin head (myosin subfragment 1, S1) containing different “essential” (or “alkali”) light chains, A1 or A2. We applied differential scanning calorimetry (DSC) to investigate the domain structure of these two S1 isoforms. For this purpose, a special calorimetric approach was developed to analyze the DSC profiles of irreversibly denaturing multidomain proteins. Using this approach, we revealed two calorimetric domains in the S1 molecule, the more thermostable domain denaturing in two steps. Comparing the DSC data with temperature dependences of intrinsic fluorescence parameters and S1 ATPase inactivation, we have identified these two calorimetric domains as motor domain and regulatory domain of the myosin head, the motor domain being more thermostable. Some difference between the two S1 isoforms was only revealed by DSC in thermal denaturation of the regulatory domain. We also applied dynamic light scattering (DLS) to analyze the aggregation of S1 isoforms induced by their thermal denaturation. We have found no appreciable difference between these S1 isoforms in their aggregation properties under ionic strength conditions close to those in the muscle fiber (in the presence of 100 mM KCl). Under these conditions kinetics of this process was independent of protein concentration, and the aggregation rate was limited by irreversible denaturation of the S1 motor domain.

Nucleotide-induced and actin-induced structural changes in SH1-SH2-modified myosin subfragment 1

We compared the structural properties of myosin subfragment 1 (S1) modified at both reactive SH-groups, SH1 (Cys707) and SH2 (Cys697), with the properties of unmodified S1 and SH1-modified S1. It is shown using differential scanning calorimetry (DSC) that SH1 modification has no noticeable influence on the changes in S1 thermal unfolding induced by the formation of S1 ternary complexes with ADP and P(i) analogs (V(i), AlF(4)(-), and BeF(x)). These changes, however, normally expressed in a significant increase of S1 thermal stability, are almost fully prevented by modification of both SH1 and SH2. In contrast, SH2 modification had no effect on the changes induced by the formation of the ternary complexes S1-ADP-V(i), S1-ADP-AlF(4)(-), and S1-ADP-BeF(x) in EPR spectra of S1 spin-labeled at SH1 group. Interaction of S1 with F-actin substantially increased the thermal stability of S1; a similar effect was observed by DSC with both SH1- and SH1-SH2-modified S1. Overall, our results demonstrate that modification of both reactive SH-groups on S1 has no influence on the actin-induced changes of S1 and on the local nucleotide-induced conformational changes in the SH1 group region, but strongly prevents the global nucleotide-induced structural changes in the entire S1 molecule. The results suggest that modification of SH1 and SH2 impairs the spread of nucleotide-induced conformational changes from the ATPase site throughout the structure of the entire S1 molecule, thus disturbing a coupling between the motor and regulatory domains in the myosin head.

Physicochemical properties of native adzuki bean (Vigna angularis) 7S globulin and the molecular cloning of its cDNA isoforms

7S globulin (vicilin), the major seed storage protein in adzuki bean [Vigna angularis], was purified by ammonium sulfate fractionation, gel filtration column chromatography, and anion-exchange column chromatography that resulted in two fractions. On SDS-PAGE, both fractions gave two major and some minor bands, but there was a difference in the minor band compositions between the two fractions. Thermal stability, solubility, surface hydrophobicity, and emulsifying ability of these three samples were analyzed. Although there was no difference in solubility and emulsifying ability among the samples, thermal stability and surface hydrophobicity were different. These differences might be due to the differences in subunit compositions. cDNAs were cloned by reverse transcription-polymerase chain reaction (RT-PCR) using primers designed on the basis of the determined N-terminal sequences of the major bands. We obtained three isoforms of cDNAs, which had highest homology with the mung bean 8Salpha globulin (7S globulin), and then soybean beta-conglycinin (7S globulin) beta subunit among legume plants. Adzuki bean 7S globulin isoforms contain more methionine and tryptophan than mung bean 8Salpha globulin and soybean beta-conglycinin beta subunit. In addition, high mannose types of glycans were attached to two or one N-glycosylation sites of adzuki bean 7S globulins.

Dissociative mechanism of F-actin thermal denaturation

We have applied differential scanning calorimetry to investigate thermal unfolding of F-actin. It has been shown that the thermal stability of F-actin strongly depends on ADP concentration. The transition temperature, T(m), increases with increasing ADP concentration up to 1 mM. The T(m) value also depends on the concentration of F-actin: it increases by almost 3 degrees C as the F-actin concentration is increased from 0.5 to 2.0 mg/ml. Similar dependence of the T(m) value on protein concentration was demonstrated for F-actin stabilized by phalloidin, whereas it was much less pronounced in the presence of AlF4(-). However, T(m) was independent of protein concentration in the case of monomeric G-actin. The results suggest that at least two reversible stages precede irreversible thermal denaturation of F-actin; one of them is dissociation of ADP from actin subunits, and another is dissociation of subunits from the ends of actin filaments. The model explains why unfolding of F-actin depends on both ADP and protein concentration.

Functional characterization of the N-terminal region of myosin-2

All class 2 myosins contain an N-terminal extension of approximately 80 residues that includes an Src homology 3 (SH3)-like subdomain. To explore the functional importance of this region, which is also present in most other myosin classes, we generated truncated constructs of Dictyostelium discoideum myosin-2. Truncation at position 80 resulted in the complete loss of myosin-2 function in vivo. Actin affinity was more than 80-fold, and the rate of ADP release approximately 40-fold decreased in this mutant. In contrast, a myosin construct that lacks only the SH3-like subdomain, corresponding to residues 33-79, displayed much smaller functional defects. In complementation experiments with myosin-2 null cells, this construct rescued myosin-2-dependent processes such as cytokinesis, fruiting body formation, and sporogenesis. An 8-fold reduction in motile activity and changes of similar extent in the affinity for ADP and filamentous actin indicate the importance of the SH3-like subdomain for correct communication between the functional regions within the myosin motor domain and suggest that local perturbations in this region can play a role in modulating myosin-2 motor activity.

Inhibition of skeletal muscle S1-myosin ATPase by peroxynitrite

Exposure of myosin subfragment 1 (S1) to 3-morpholinosydnonimine (SIN-1) produced a time-dependent inhibition of the F-actin-stimulated S1 Mg(2+)-ATPase activity, reaching 50% inhibition with 46.7 +/- 8.3 microM SIN-1 for 8.7 microM S1, that is, at a SIN-1/S1 molar ratio of approximately 5.5. The inhibition was due to the peroxynitrite produced by SIN-1 decomposition because (1) decomposed SIN-1 was found to have no effect on S1 ATPase activity, (2) addition of SIN-1 in the presence of superoxide dismutase and catalase fully prevented inhibition by SIN-1, and (3) micromolar pulses of chemically synthesized peroxynitrite produced inhibition of F-actin-stimulated S1 Mg(2+)-ATPase activity. In parallel, SIN-1 produced the inhibition of the nonphysiological Ca(2+)-dependent and K(+)/EDTA-dependent S1 ATPase activity of S1 and, therefore, suggested that the inhibition of F-actin-stimulated S1 Mg(2+)-ATPase activity is produced by the oxidation of highly reactive cysteines of S1 (Cys(707) and Cys(697)), located close to the catalytic center. This point was further confirmed by the titration of S1 cysteines with 5,5'-dithiobis(2-nitrobenzoic acid) and by the parallel decrease of Cys(707) labeling by 5-(iodoacetamido)fluorescein, and it was reinforced by the fact that other common protein modifications produced by peroxynitrite, for example, protein carbonyl and nitrotyrosine formation, were barely detected at the concentrations of SIN-1 that produced more than 50% inhibition of the F-actin-stimulated S1 Mg(2+)-ATPase activity. Differential scanning calorimetry of S1 (untreated and treated with different SIN-1 concentrations) pointed out that SIN-1, at concentrations that generate micromolar peroxynitrite fluxes, impaired the ability of ADP.V(1) to induce the intermediate catalytic transition state and also produced the partial unfolding of S1 that leads to an enhanced susceptibility of S1 to trypsin digestion, which can be fully protected by 2 mM GSH.

Actomyosin systems of biological motility

Evolution of notions on the molecular mechanism of muscle contraction and other events based on the actin-myosin interaction, from the middle of XX century to the present time, is briefly reviewed, including recent views on the functioning of the myosin head as a "molecular motor". The results of structural and functional studies on the myosin head performed by the author and his colleagues using differential scanning calorimetry are also reviewed.

Two opposite effects of cofilin on the thermal unfolding of F-actin: a differential scanning calorimetric study

Differential scanning calorimetry was used to examine the effects of cofilin on the thermal unfolding of actin. Stoichiometric binding increases the thermal stability of both G- and F-actin but at sub-saturating concentrations cofilin destabilizes F-actin. At actin:cofilin molar ratios of 1.5-6 the peaks corresponding to stabilized (66-67 degrees C) and destabilized (56-57 degrees C) F-actin are observed simultaneously in the same thermogram. Destabilizing effects of sub-saturating cofilin are highly cooperative and are observed at actin:cofilin molar ratios as low as 100:1. These effects are abolished by the addition of phalloidin or aluminum fluoride. Conversely, at saturating concentrations, cofilin prevents the stabilizing effects of phalloidin and aluminum fluoride on the F-actin thermal unfolding. These results suggest that cofilin stabilizes those actin subunits to which it directly binds, but destabilizes F-actin with a high cooperativity in neighboring cofilin-free regions.