Dynamic interaction between actin and myosin subfragment 1 in the presence of ADP.
The equilibrium and dynamics of the interaction between actin, myosin subfragment 1 (S1), and ADP have been investigated by using actin which has been covalently labeled at Cys-374 with a pyrene group. The results are consistent with actin binding to S1.ADP (M.D) in a two-step reaction, A + M.D K1 equilibrium A-M.D K2 equilibrium A.M.D, in which the pyrene fluorescence only monitors the second step. In this model, K1 = 2.3 X 10(4) M-1 (k+1 = 4.6 X 10(4) M-1 s-1) and K2 = 10 (k+2 less than or equal to 4 s-1); i.e., both steps are relatively slow compared to the maximum turnover of the ATPase reaction. ADP dissociates from both M.D and A-M.D at 2 s-1 and from A.M.D at greater than or equal to 500 s-1; therefore, actin only accelerates the release of product from the A.M.D state. This model is consistent with the actomyosin ATPase model proposed by Geeves et al. [(1984) J. Muscle Res. Cell Motil. 5, 351]. The results suggest that A-M.D cannot break down at a rate greater than 4 s-1 by dissociation of ADP, by dissociation of actin, or by isomerizing to A.M.D. It is therefore unlikely to be significantly occupied in a rapidly contracting muscle, but it may have a role in a muscle contracting against a load where the ATPase rate is markedly inhibited. Under these conditions, this complex may have a role in maintaining tension with a low ATP turnover rate.
Geeves
M A
MA
Department of Biochemistry, School of Medical Sciences, University of Bristol, U.K.
D000199:Actins / Q000378:metabolism*; D000244:Adenosine Diphosphate / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D066298:In Vitro Techniques; D007700:Kinetics; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism; D015879:Myosin Subfragments; D009218:Myosins / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism*; D011817:Rabbits
D000199:Actins; D015879:Myosin Subfragments; D010446:Peptide Fragments; D000244:Adenosine Diphosphate; D000251:Adenosine Triphosphatases; D009218:Myosins