STRUCTURAL COUPLING OF TROPONIN-C AND ACTOMYOSIN IN MUSCLE-FIBERS

EPR of spin labeled TnC at Cys98 was used to explore the possible stru ctural coupling between TnC in the thin filament and myosin trapped in the intermediate states of ATPase cycle. Weakly attached myosin heads (trapped by low ionic strength, low temperature and ATP) did not indu ce structural changes in TnC as compared to relaxed muscle, as spin la beled TnC displayed the same narrow orientational distribution [Li, H. -C., and Fajer, P. G. (1994) Biochemistry 33, 14324]. Ca2+-binding alo ne resulted in disordering of the labeled domain of TnC. Additional co nformational changes of TnC occurred upon the attachment of strongly b ound, prepower stroke myosin heads (trapped by AlF4-), These changes w ere not present in ghost fibers which myosin had been removed, excludi ng direct effects of AlF4- on the orientation of TnC in muscle fibers. The postpower stroke heads (rigor ADP/Ca2+ and rigor/Ca2+) induced fu rther changes in the orientational distribution of labeled domain of T nC irrespective of the degree of cooperativity in thin filaments. We t hus conclude that troponin C in thin filaments detects structural chan ges in myosin during force generation, implying that there is a struct ural coupling between actomyosin and TnC.