Stability of segments of rabbit alpha-tropomyosin

Rabbit alpha-tropomyosin was cleaved into two pieces at the cysteine residue of each chain. The products were separated by chromatography and characterized by amino acid analysis, molecular weight determination in benign and denaturing solvents, optical rotation and circular dichroism. When the cleavage reaction was carried out under mild conditions which preserve the two-chain structure there was considerable loss of alpha-helix in each segment. Thermal stability studies, monitored by optical rotation and circular dichroism, showed that the transition temperature of the N-terminal fragment at pH 7.6 was approximately 17 degrees C higher than that of the C-terminal fragment. In acid solutions there is little difference in the thermal stability of the two segments. The least stable part of the molecule is concluded to be between residues 133 and 205 and this includes the troponin-binding site. The relative stabilities found for segments of rabbit alpha-tropomyosin differ from recent published conclusions and this may be a result of the different methods used to study the loss of the alpha-helical conformaton. The two tropomyosin fragments, unlike the parent tropomyosin, do not inhibit actomyosin adenosinetriphosphatase when mixed with troponin. The fragments did not show any of the aggregation properties of tropomyosin and did not combine with actin. The N-terminal fragment did not complex with troponin but there was some evidence for an interaction between the C-terminal fragment and troponin.