ROLE OF ATP IN THE BINDING OF CALDESMON TO SMOOTH-MUSCLE MYOSIN

We have reported earlier that ATP causes both an increase in the affin ity of caldesmon for smooth muscle myosin and a change in stoichiometr y from 2 caldesmon molecules per myosin to 1:1 (Hemric and Chalovich, 1990). We now show that this ATP effect does not occur with skeletal m uscle myosin, indicating that ATP has a specific effect on the structu re of filamentous smooth muscle myosin. This ATP effect does not appea r to be due to stabilization of a 10S type of filamentous smooth muscl e myosin like that reported earlier (Ikebe and Hartshorne, 1984) since neither phosphorylaton nor extensive modification of myosin with MalN Et (both which stabilize the 6S state of monomeric myosin) eliminates the effect of ATP. Caldesmon does bind more tightly to a form of smooc h muscle myosin which is resistant to papain digestion. These results suggest that the ATP effect is due to stabilization of a local conform ation of smooth muscle myosin which is independent of the larger 10S/6 S conformational change (Suzuki et al., 1988). In the presence of ATP, the two heads of smooth muscle myosin and the S-2 region form a singl e, higher affinity binding region for caldesmon.