MOLECULAR POLARITY IN TROPOMYOSIN TROPONIN-T CO-CRYSTALS

New features of the structure and interactions of troponin T and tropo myosin have been revealed by electron microscopy of so-called double-d iamond co-crystals. These co-crystals were formed using rabbit alpha(2 ) tropomyosin complexed with troponin T from either skeletal or cardia c muscle, which have different lengths in the amino-terminal region, a s well as a bacterially expressed skeletal muscle troponin T fragment of 190 residues that lacks the amino-terminal region. Differences in t he images of the co-crystals have allowed us to establish the polariti es of both the troponin T subunit and tropomyosin in the projected lat tice. Moreover, in agreement with their sequences, the amino-terminal region of a bovine cardiac muscle troponin T isoform appears to be lon ger than that from the rabbit skeletal muscle troponin T isoform and t o span more of the amino terminus of tropomyosin at the head-to-tail f ilament joints. Images of crystals tilted relative to the electron bea m also reveal the supercoiling of the tropomyosin filaments in this la ttice. Based on these results, a three-dimensional model of the double -diamond lattice has been constructed.