The essential role of tropomyosin in cooperative regulation of smooth muscle thin filament activity by caldesmon.

We compared the mechanisms by which caldesmon inhibits actin and actin-tropomyosin activation of myosin subfragment 1 (S1) MgATPase activity. Caldesmon always inhibited actin activation by displacing S1.ADP.Pi from actin and inhibition required at least 0.7 caldesmon molecules bond per actin for 90% inhibition. Caldesmon inhibited actin-tropomyosin without any displacement of S1.ADP.Pi; thus it inhibits a rate-limiting step. Inhibition is highly cooperative, requiring no more than one caldesmon bound per 10 actins for 90% inhibition of activation by actin and smooth muscle tropomyosin. The degree of cooperativity is defined by the tropomyosin since inhibition by skeletal tropomyosin requires up to one caldesmon bound per 4 actins for 90% inhibition under identical conditions. Both noncooperative inhibition of actin and cooperative, tropomyosin-dependent, inhibition are manifested by a fragment of caldesmon containing only the C-terminal 99 amino acids (658C), although this fragment does not itself bind to tropomyosin. The functional properties of 658C are very similar to striated muscle troponin I, consequently we propose a similar mechanism for tropomyosin-dependent regulation by caldesmon. Caldesmon binding switches actin-tropomyosin to the "off" or "weak" state and Ca2+/calmodulin binding to caldesmon blocks this switch and thus reactivates the actin filament.