The Role of Sulfhydryl Groups in the Polymerization and Adenosine Triphosphate Binding of G-Actin

Binding of phosphorylase a and b to skeletal muscle thin filament proteins

Phosphorylase plays an important role in energy generation during muscle contraction. We have demonstrated that purified rabbit skeletal muscle phosphorylase a and phosphorylase b bind to rabbit muscle F-actin, F-actin-tropomyosin, F-actin-tropomyosin-troponin, and myofibrils. Neither phosphorylase a nor phosphorylase b binds to myosin. Phosphorylase a and b bind to F-actin with S0.5 values of 1.5 X 10(-6) and 2.1 X 10(-6) M, respectively. At saturation, 0.035 mol of phosphorylase a and b is bound for every seven G-actin monomers in the F-actin polymer. Using the F-actin-tropomyosin-troponin complex as opposed to F-actin as a binding target, there are five- and threefold increases in the maximal binding capacity for phosphorylase a and phosphorylase b, respectively, without a significant change in the S0.5 value for either form of the enzyme. A similar stoichiometry and affinity of phosphorylase binding are observed when myofibrils are used as the binding target. Ca2+ ions and AMP increase the maximal binding capacity for phosphorylase a to myofibrils while ATP decreases the Bmax. Our study suggests that in skeletal muscle, phosphorylase a and phosphorylase b may interact with the thin filament, and that this binding to thin filament proteins may be controlled by changes in sarcoplasmic concentration of Ca2+ and ligands of phosphorylase during muscle contraction.

Thin filament proteins and thin filament-linked regulation of vertebrate muscle contraction

Recent developments in the field of myofibrillar proteins will be reviewed. Consideration will be given to the proteins that participate in the contractile process itself as well as to those involved in Ca-dependent regulation of striated (skeletal and cardiac) and smooth muscle. The relation of protein structure to function will be emphasized and the relation of various physiologically and histochemically defined fiber types to the proteins found in them will be discussed.

Kinetic and thermodynamic properties of the ternary complex between F-actin, myosin subfragment 1 and adenosine 5'-[beta, gamma-imido]triphosphate

Equilibrium constants for the formation of a ternary complex between actin, myosin subfragment 1 (S1) and the non-hydrolyzable ATP analog adenosine 5'-[beta, gamma-imido]triphosphate (Ado PP[NH]P) were determined from light-scattering titrations under a variety of conditions. The affinities of S1 (binding constant K1) and acto . S1 (K4) for AdoPP[NH]P have relatively low dependencies on temperature (delta H degrees approximately equal to - 15 - 30 kJ mol-1) and ionic strength, in contrast to the affinities of S1 (K2) and S1 . AdoPP[NH]P (K3) for actin which are influenced quite strongly by temperature (delta H degrees approximately equal to 50 - 65 kJ mol-1) and ionic strength, K2 decreasing by a factor of 10 - 15 between I = 0.05 M and I = 0.2 M and K3 decreasing by a factor of 5.K1, and by detailed balance K2 as well, were found to be about 10-times higher than hitherto reported values (K1 = 3.4 X 10(7) M-1, K2 = 6 X 10(8) M-1, at 24 degrees C,I = 0.09 M, pH 8.0). The binding of ADP to S1 is about 10-fold weaker than that of AdoPP[NH]P, being however much more exothermic (delta H degrees = - 70 kJ mol-1 at I = 0.1 M) and having a negative standard entropy change (delta S = - 125 J mol-1 K-1), in contrast to AdoPP[NH]P binding for which the calculated delta S had positive values. The observed rate constant of dissociation of acto . S1 by AdoPP[NH]P showed an almost hyperbolic dependence on the nucleotide concentration, reaching a maximum of 15 s-1 at I = 0.055 M and 5 s-1 at I = 0.275 M, pH 8.0, 23 degrees C; at 5 degrees C this value was somewhat higher. The rate constant of dissociation of AdoPP[NH]P from its complex with acto . S1 was estimated to exceed 400 s-1 at 23 degrees C, and to be of the order of 150 s-1 at 4 degrees C. The observed rate constant for the association of the S1 . nucleotide complex and actin was proportional to actin concentrations up to 60 microM, thus defining an apparent second-order rate constant of 2 X 10(4) M-1 s-1 at I = 0.125 M and 23 degrees C. A reaction scheme is proposed in which isomerizations of the acto . S1 and acto . S1 . nucleotide complexes can occur.

Stereochemical aspects of the interaction of myosin and actomyosin with nucleotides

The five possible analogues of ATP and the three possible analogues of ADP which contain single non-bridging sulphur atoms instead of oxygen in the polyphosphate structure have been used as probes of the interaction of nucleotides with myosin and actomyosin. Evidence is presented for the requirement of an alpha, beta, gamma-tridentate complex of magnesium and ATP as the substrate for myosin. Of the four possible tridentate MgATP diastereomers, the delta exo isomer (nomenclature of Cornelius & Cleland, 1978) appears to be the actual substrate.

The interaction of bovine pancreatic deoxyribonuclease I and skeletal muscle actin

The rate of exchange of actin-bound nucleotide is decreased by a factor of about 20 when actin is complexed with DNAase I without affecting the binding constant of calcium for actin. Binding constants of DNAase I to monomeric and filamentous actin were determined to be 5 X 10(8) M-1 and 1.2 X 10(4) M-1 respectively. The depolymerisation of F-actin by DNAase I appears to be due to a shift in the G-F equilibrium of actin by DNAase I. Inhibition of the DNA-degrading activity of DNAase I by G-actin is of the partially competitive type.