Changes in the thermal unfolding of p-phenylenedimaleimide-modified myosin subfragment 1 induced by its 'weak' binding to F-actin

A novel DSC approach for evaluating protectant drugs efficacy against dementia

Differential scanning calorimetry was applied to evaluate the efficacy of preventive treatments with biologically active compounds of plant origin against neurodegenerative disorder in mice. As we reported recently, large differences exist between the heat capacity profiles of water-soluble brain proteome fractions from healthy animals and from animals with scopolamine-induced dementia: the profiles for healthy animals displayed well expressed exothermic event peaking at 40-45°C, by few degrees above body temperature, but still preceding in temperature the proteome endothermic denaturational transitions; the low-temperature exotherm was completely abolished by the scopolamine treatment. Here we explored this signature difference in the heat capacity profiles to assess the efficacy of preventive treatments with protectant drugs anticipated to slow down or block progression of dementia (myrtenal, ellagic acid, lipoic acid and their combinations, including also ascorbic acid). We found that these neuroprotectants counteract the scopolamine effect and partially or completely preserve the 'healthy' thermogram, and specifically the low-temperature exotherm. These results well correlate with the changes in the cognitive functions of the animals assessed using the Step Through Test for learning and memory. The exothermic event is deemed to be associated with a reversible process of fibrillization and/or aggregation of specific water-soluble brain protein fractions preceding their denaturation. Most importantly, the results demonstrate that the effect of scopolamine and its prevention by protectant substances are clearly displayed in the heat capacity profiles of the brain proteome, thus identifying DSC as a powerful method in drug testing and discovery.

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.

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.

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.

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

The thermal unfolding of myosin subfragment 1 (S1) cleaved by trypsin was studied by differential scanning calorimetry. In the absence of nucleotides, trypsin splits the S1 heavy chain into three fragments (25, 50, and 20 kDa). This cleavage has no appreciable influence on the thermal unfolding of S1 examined in the presence of ADP, in the ternary complexes of S1 with ADP and phosphate analogs, such as orthovanadate (Vi) or beryllium fluoride (BeFx), and in the presence of F-actin. In the presence of ATP and in the complexes S1.ADP.Vi or S1.ADP.BeFx, trypsin produces two additional cleavages in the S1 heavy chain: a faster cleavage in the N-terminal region between Arg23 and Ile24, and a slower cleavage at the 50 kDa fragment. It has been shown that the N-terminal cleavage strongly decreases the thermal stability of S1 by shifting the maximum of its thermal transition by about 7 degrees C to a lower temperature, from 50 degrees C to 42.4 degrees C, whereas the cleavage at both these sites causes dramatic destabilization of the S1 molecule leading to total loss of its thermal transition. Our results show that S1 with ATP-induced N-terminal cleavage is able, like uncleaved S1, to undergo global structural changes in forming the stable ternary complexes with ADP and Pi analogs (Vi, BeFx). These changes are reflected in a pronounced increase of S1 thermal stability. However, S1 cleaved by trypsin in the N-terminal region is unable, unlike S1, to undergo structural changes induced by interaction with F-actin that are expressed in a 4-5 degrees C shift of the S1 thermal transition to higher temperature. Thus, the cleavage between Arg23 and Ile24 does not significantly affect nucleotide-induced structural changes in the S1, but it prevents structural changes that occur when S1 is bound to F-actin. The results suggest that the N-terminal region of the S1 heavy chain plays an important role in structural stabilization of the entire motor domain of the myosin head, and a long-distance communication pathway may exist between this region and the actin-binding sites.