Effects of varying the local propensity to form secondary structure on thestability and folding kinetics of a rapid folding mixed alpha/beta protein: Characterization of a truncation mutant of the N-terminal domain of the ribosomal protein L9

The N-terminal domain of the ribosomal protein L9 forms a split pap structu re with a long C-terminal helix. The folding transitions of a 56 residue ve rsion of this protein have previously been characterized, here we report th e results of a study of a truncation mutant corresponding to residues 1-51. The 51 residue protein adopts the same fold as the 56 residue protein as j udged by CD and two-dimensional NMR, but it is less stable as judged by che mical and thermal denaturation experiments. Studies with synthetic peptides demonstrate that the C-terminal helix of the 51 residue version has very L ittle propensity to fold in isolation in contrast to the C-terminal helix o f the 56 residue variant. The folding rates of the two proteins, as measure d by stopped-flow fluorescence, are essentially identical, indicating that formation of local structure in the C-terminal helix is not involved in the rate-limiting step of folding. (C) 1999 Academic Press.