EFFECT OF CALDESMON ON THE ASSEMBLY OF SMOOTH-MUSCLE MYOSIN

Smooth muscle myosin filaments are much less stable than the skeletal muscle counterpart. Smooth myosin requires higher concentration of Mg2 + than skeletal myosin to form thick filaments and addition of ATP dis assembles the dephosphorylated smooth muscle myosin filaments into mon omers but not phosphorylated ones. We found that the addition of calde smon to dephosphorylated myosin induced the formation of the filaments under the conditions where myosin by itself is soluble or disassemble d. Although the induced filaments mere short at 1 mM Mg2+, they became medium sized and seemed like side polar filaments with prominent 14 n m periodicity at higher Mg2+ conditions (8 mM). In the presence of F-a ctin, myosin filaments induced by caldesmon were associated along acti n filaments to form large structures. The association of actin and myo sin filaments was observed only in the presence of caldesmon, suggesti ng that caldesmon cross-linked actin and myosin filaments. This cross- linking was disrupted by the addition of calmodulin. Caldesmon-induced filament formation of dephosphorylated myosin in the presence of Mg2-ATP may explain the existence of myosin filaments in relaxed smooth m uscle fibers. A similar effect of telokin on myosin filament assembly was also examined and is discussed.