Effect of metal cations on the conformation of myosin subfragment-1- ADP-phosphate analog complexes: A near-UV circular dichroism study

The interaction of myosin with actin, coupled with hydrolysis of ATP, is the molecular basis of muscle contraction. The head segment of myosin, called S1, contains the distinct binding sites for ATP and actin and is responsible for the ATPase activity. The myosin-catalyzed ATP hydrolysis consists of several intermediate steps and each step is accompanied by conformational changes in the S1 segment. The rate-limiting step of the ATP hydrolysis is the dissociation of the S1·ADP·P(i) complex which is accelerated by actin. The substitution of P(i) with phosphate analogs (PA), such as vanadate, beryllium fluoride (BeF(x)) or aluminum fluoride (AlF₄^-), yields stable complexes which mimic the intermediates of the ATP hydrolysis. In this work, tertiary structure changes in S1 in the vicinity of aromatic residues was studied by comparing near-UV circular dichroism (CD) spectra from S1- nucleotide-phosphate analog complexes in the presence of Mg²⁺ and other cations. A significant difference between the MgATP and MgADP spectra indicated notable tertiary structural changes accompanying the M**ADP·P(i) → M*ADP transition. The spectra of the S1·MgADP·BeF(x) and S1·MgADP·AlF₄^- complexes resemble to those obtained upon addition of MgATPγS and MgATP to S1, and correspond to the M*·ATP and M**·ADP·P(i) intermediates, respectively. We have found recently that the presence of divalent metal cartons (Me²⁺) is essential for the formation of stable S1·MeADP·PA complexes. Moreover, the nature of the metal cations strongly influences the stability of these complexes [Peyser, Y. M., et al. (1996) Biochemistry 35, 4409-4416]. In the present work we studied the effect of Mg²⁺, Mn²⁺, Ca²⁺, Ni²⁺, Go²⁺, and Fe²⁺ on the near-UV CD spectrum of the ATP, ADP, ADP·BeF(x) and ADP·AlF₄^- containing S1 complexes. The CD spectra obtained with ADP, ATP ADP·BeF(x) and ADP·AlF₄^- were essentially identical in the presence of Co²⁺ and rather similar in the case of Ca²⁺, while they were partially different in other cases. An interesting correlation was found between actin activation and ATP versus ADP difference spectra in the presence of various metal ions. The distribution of the fractional concentration of the intermediates of ATP hydrolysis was estimated in the presence of each cation from the CD spectra with phosphate analogs. In the presence of Mg²⁺ the predominant intermediate is the M**·ADP·P(i) state, which is in accordance with the kinetic studies. On the other hand with non-native cations the predominant intermediate is the M*·ADP state and the release of ADP is the rate limiting step in the myosin-catalyzed ATP hydrolysis. According to the results, the near-UV CD spectrum originating from aromatic residues in S1 not only can distinguish identifiable states in the ATP hydrolysis cycle but can also pinpoint to changes in the tertiary structure caused by complex formation with nucleotide or nucleotide analog and various divalent metal cations. These findings, that are correlative with actin activation, and thus with the power stroke, suggest new strategies for perturbing S1 structure in the continuous efforts directed toward the elucidation of the mechanism of muscle contraction.