Native chicken gizzard tropomyosin is predominantly a beta gamma-heterodimer

Unmodified chicken gizzard tropomyosin (TM) has been fractionated into its two major isoforms beta and gamma, by chromatofocussing in the presence of 9 M urea and dithiothrieitol. Treatment of the native protein with several bifunctional N-hydroxysuccinimide esters gave the beta gamma-heterodimer as the major cross-linked product. A comparison of the thermal transition profiles of the two homodimers and of the native unfractionated TM also indicated the predominance of the beta gamma-heterodimer in the native protein. This conclusion is consistent with the absence of excimer fluorescence in pyrene-labeled gizzard TM and the relative resistance of the molecule to intramolecular disulfide formation (Lehrer, S.S., Betteridge, D.R., Graceffa, P., Wong, S., and Seidel, J. C. (1984) Biochemistry 23, 1591-1595) since the single cysteines on each of the two isoforms are widely separated. We conclude that further experimental evidence is required to assess the possibility that the gizzard TM is more rigid in its conformation than are those of the skeletal and cardiac proteins.