Altered Actin and Troponin Binding of Amino-terminal Variants of Chicken Striated Muscle Alpha-tropomyosin Expressed in Escherichia Coli

We have expressed two variants of chicken striated muscle alpha-tropomyosin in Escherichia coli: fusion tropomyosin containing 80 amino acids of a non-structural influenza virus protein (NS1) on the amino terminus and a non-fusion tropomyosin which is a variant because the amino-terminal methionine is not acetylated (unacetylated tropomyosin). From our analysis of purified proteins in vitro we suggest that the amino-terminal region, which is highly conserved in muscle tropomyosins, is crucial for all aspects of tropomyosin function. Both forms are altered in tropomyosin activity: neither shows head-to-tail polymerization, with or without troponin. Unacetylated tropomyosin binds weakly to actin, but in the presence of troponin it binds well and can regulate the actomyosin ATPase. Fusion tropomyosin binds well to actin, but binding of troponin is calcium-sensitive and it does not confer effective calcium sensitivity on the actomyosin ATPase. Our results indicate that the local charge at the amino terminus is critical for actin binding but that normal head-to-tail association is not required. The properties of fusion tropomyosin-troponin interaction are indicative of impaired troponin T binding to tropomyosin and provide evidence for its binding to the amino terminus of tropomyosin.