INTERACTION OF ACTIN AND ADP WITH THE HEAD DOMAIN OF SMOOTH-MUSCLE MYOSIN - IMPLICATIONS FOR STRAIN-DEPENDENT ADP RELEASE IN SMOOTH-MUSCLE

Transient kinetic methods were used to study interactions between acti n, MgADP, and smooth muscle (chicken gizzard) myosin subfragment 1 (sm S1). The equilibrium dissociation constant (K-d) Of actin for smS1 was 3.5 nM, tighter than that of skeletal S1 (skS1). Actin binding to smS 1 was weakened 5-fold by saturation with ADP compared to 30-60-fold fo r skS1. The K-d Of ADP for smS1 was increased from 1.2 to 5 mu M by ac tin, whereas for skS1 values increased from 2 to 100 mu M. Thus, coupl ing between ADP and actin binding is weaker for smS1. Previous studies show that release of ADP from actin.smS1.ADP produces a tilt of the r egulatory domain [Whittaker, M., Wilson-Kubalek, E. M., Smith, J. E., Faust, L., Milligan, R. A., and Sweeney, H. L. (1995) Nature 378, 748- 751]. This result was confirmed by independent structural methods; til ting was absent for skS1, and the K-d for ADP was in agreement with th e values measured here [Gollub, J., Cremo, C. R., and Cooke, R. (1996) Nat. Struct. Biol. 3, 796-802; Poole, K. I. V., Lorenz, M., Ellison, P., Evans, G., Rosenbaum, G., Boesecke, P., Holmes, K. C., and Cremo, C. R. (1997) J. Muscle Res. Cell Motility 18, 264]. We discuss tilting upon ADP release with respect to our measurements, previous measureme nts with skS1, and nucleotide concentrations in smooth muscle. We prop ose that these data suggest a strain-dependent ADP release mechanism t hat may be accentuated in smooth muscles.