Effects of varying the local propensity to form secondary structure on the stability 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 betaalphabeta structure with a long C-terminal helix. The folding transitions of a 56 residue version of this protein have previously been characterized, here we report the 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 judged by CD and two-dimensional NMR, but it is less stable as judged by chemical and thermal denaturation experiments. Studies with synthetic peptides demonstrate that the C-terminal helix of the 51 residue version has very little propensity to fold in isolation in contrast to the C-terminal helix of the 56 residue variant. The folding rates of the two proteins, as measured 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.