FILAMIN AND GELSOLIN INFLUENCE CA2-SENSITIVITY OF SMOOTH-MUSCLE THIN-FILAMENTS()

Sheep aorta thin filaments were prepared by ultracentrifugation of an ATP-containing extract in the presence of different concentrations of ethanediol. Thin filaments prepared without ethanediol contained small quantities of tropomyosin (0.027 Tm:actin) and caldesmon (0.017 CD:ac tin) and activated the MgATPase of skeletal myosin independently of Ca 2+ Ultracentrifugation in the presence of 10-20% ethanediol resulted i n preparation of thin filaments with increased content of tropomyosin (0.17 Tm:actin) and caldesmon (0.04 CD:actin). These thin filaments po ssessed high Ca2+-sensitivity in activation of skeletal muscle myosin ATPase. Besides actin, tropomyosin and caldesmon, thin filaments conta ined gelsolin and filamin. Gelsolin content (0.007 gelsolin:actin) was independent of the presence of ethanediol. The filamin content decrea sed from 0.015 to 0.007 mol:mol actin when the ethanediol concentratio n was increased from 0 to 20%, and was negatively correlated with the Ca2+ sensitivity of thin filaments. In a reconstituted system, pure fi lamin or gelsolin affected caldesmon regulation of actomyosin ATPase. Gelsolin (0.01:actin) reduced the inhibition of actomyosin ATPase caus ed by caldesmon and increased the potency of Ca2+-calmodulin in revers ing this inhibition. Filamin (0.007:actin) also decreased the inhibito ry action of caldesmon on actin-activated myosin ATPase and also poten tiated the reversal of this inhibition by calmodulin. We conclude that minor components of smooth muscle thin filaments (gelsolin and filami n) significantly modify caldesmon mediated regulation of actomyosin AT Pase. We suggest a tropomyosin-mediated mechanism by which filamin or gelsolin could exert similar effects.