Structural and kinetic characterization of the simplified SH3 domain FP1

The simplified SH3 domain sequence, FP1, obtained in phage display selection experiments has an amino acid composition that is 95% Ile, Lys, Glu, Ala, Gly. Here we use NMR to investigate the tertiary structure of FP1. We find that the overall topology of FP1 resembles that of the src SH3 domain, the hydrogen-deuterium exchange and chemical shift perturbation profiles are similar to those of naturally occurring SH3 domains, and the N-15 relaxation rates are in the range of naturally occur-ring small proteins. Guided by the structure, we further simplify the FP1 sequence and compare the effects on folding kinetics of point mutations in FP1 and the wild-type src SH3 domain. The results suggest that the folding transition state of FP1 is similar to but somewhat less polarized than that of the wild-type src SH3 domain.