Molecular Weight and Subunit Structure of Tropomyosin B

Abstract Sedimentation equilibrium studies on rabbit tropomyosin have indicated heterogeneity under conditions in which it has previously been considered to exist as a single molecular species. At neutral pH, solutions of tropomyosin consist of a monomer in equilibrium with higher molecular weight species. Molecular weight determinations at high ionic strength by low speed equilibrium sedimentation and osmotic pressure indicate dissociation on dilution to give a value close to 68,000 for the molecular weight at zero concentration. Estimates of the size of the smallest macromolecular species present at both low and high ionic strength were made by the high speed equilibrium technique, and these were in accord with a molecular weight of 68,000 for the native tropomyosin monomer. Determination of molecular weights in 8 m urea by equilibrium sedimentation and osmotic pressure indicate further dissociation to two subunits of molecular weight 33,500. The subunits appear to be very similar in mass and charge, and the results are consistent with a two-chain structure for the native monomer. On the hypothesis that tropomyosin consists of two adjacent α helices, the length of the molecule is approximately 437 A. Although the existence of material other than tropomyosin can be detected in both aqueous buffer and 8 m urea, the results of gel filtration, chromatography on diethylaminoethyl cellulose, and gel electrophoresis suggest that the amounts are insufficient to affect the conclusions of this study.