Folding of homologous proteins: Conservation of the folding mechanism of the .alpha. subunit of tryptophan synthase from Escherichia coli, Salmonella typhimurium, and five interspecies hybrides
The eqilibrium and kinetic properties for the urea-induced unfolding of the .alpha. subunit of tryptophan synthase from Escherichia coli, Salmonella typhimurium, and five interspecies hybrids were compared to determine the role of protein folding in evolution. The parent proteins differ at 40 positions in the sequence of 268 amino acids, and the hybrids differ by up to 15 amino acids from the Escherichia coli .alpha. subunit. The results show that all the proteins follow the same folding mechanism and are consistent with a previously proposed hypothesis [Hollecker, M., and Creighton, T. E. (1983) J. Mol. Biol. 168, 409; Krebs, H., Schmid, F. X., and Jaenicke, R. (1983) J. Mol. Biol. 169, 619] that the folding mechanisms are conserved in homologous proteins. Analysis of the kinetic data suggests that the 15 positions at which the parent proteins differ in the amino folding unit, residue 1-88, do not play a role in a rate-limiting step in folding that has been previously identified as the association of the amino and carboxyl folding units [Beasty, A. M., Hurle, M. R., Manz, J. T., Stackhouse, T. S., Onuffer, J. J., and Matthews, C. R. (1986) Biochemistry 25, 2965]. One or more of the 25 positions at which the parent proteins differ in the carboxyl folding unit, residues 189-268, do appear to play a role in this same rate-limiting step.