COOPERATIVE INTERACTIONS BETWEEN ADJACENT TROPONIN-TROPOMYOSIN COMPLEXES MAY BE TRANSMITTED THROUGH THE ACTIN FILAMENT

Recent analyses of the assembly of thin filaments containing altered f orms of troponin (or no troponin) suggested that the strongly cooperat ive nature of troponin-tropomyosin binding to actin might be primarily caused by indirect interactions involving the actin lattice, rather t han by direct contacts between neighboring troponin-tropomyosin molecu les. To test this hypothesis, thin filament assembly was examined usin g either cardiac tropomyosin digested with carboxypeptidase A (cbpTm) or a tropomyosin with defective function at both amino and carboxyl te rmini (unacetylated cbpTm). Compared to intact troponin-tropomyosin, b oth troponin-cbpTm and troponin-unacetylated cbpTm had much weaker bin ding to actin; however, cooperative interactions were only slightly re duced. These data support the implication that the primary source of t he cooperativity involves troponin-tropomyosin-promoted conformational changes within the actin polymer. Surprisingly, the effects of tropom yosin amino- and carboxyl-terminal structural defects on troponin-trop omyosin binding to actin were not additive. In the presence of troponi n, tropomyosin molecules with either defect had the same diminution in actin affinity as molecules with both defects. Finally, the Ca2+ sens itivity of troponin-tropomyosin binding to actin was increased by alte ration of either end of tropomyosin.