FUNCTIONAL ALPHA-TROPOMYOSIN PRODUCED IN ESCHERICHIA-COLI - A DIPEPTIDE EXTENSION CAN SUBSTITUTE THE AMINO-TERMINAL ACETYL GROUP

Unlike the muscle protein, alpha-tropomyosin expressed in Escherichia coli does not bind actin, does not exhibit head-to-tail polymerization , and does not inhibit actomyosin ATPase activity in the absence of tr oponin. The only chemical difference between recombinant and muscle tr opomyosins is that the first methionine is not acetylated in the recom binant protein (Hitchcock-DeGregori, S. E., and Heald, R. W. (1987) J. Biol. Chem. 262, 9730-9735). We expressed three fusion tropomyosins i n E. coli with 2, 3, and 17 amino acids fused to its amino terminus. A h three fusions restored actin binding, head-to-tail polymerization, a nd the capacity to inhibit the actomyosin ATPase to these unacetylated tropomyosins. Unlike larger fusions, the small fusions of 2 and 3 ami no acids do not interfere with regulatory function. Therefore the pres ence of a fused dipeptide at the amino terminus of unacetylated tropom yosin is sufficient to replace the function of the N-acetyl group pres ent in muscle tropomyosin. A structural interpretation for the functio n of the acetyl group, based on our results and the coiled coil struct ure of tropomyosin, is presented.