RELAXATION STUDY OF THE BACKBONE DYNAMICS OF HUMAN PROFILIN BY 2-DIMENSIONAL H-1-N-15 NMR

The dynamic properties of 111 backbone HN sites in uncomplexed human profilin, a protein of 139 residues, have been characterized by two-dimensional inverse-detected H-1-N-15 NMR spectroscopy. Heteronuclear {H-1}-N-15 nuclear Overhauser effects and N-15 longitudinal and transverse relaxation rates have been analyzed in terms of model-free spectral density functions and exchange contributions to transverse relaxation rates. Relatively high mobilities on the nanosecond timescale are observed for Asp(26) and Ser(27), which form part of a loop connecting beta-strands A and B, and for Thr(92) through Ala(95), which are in a loop connecting beta-strands E and F. Significant exchange contributions, indicative of motions on the microsecond to millisecond timescale, have been obtained for 30 residues. These include Leu(77), Asp(80) and Gly(81) of a loop between beta-strands D and E, Ser(84) and Met(85) of beta-strand E, Gly(121) of a loop connecting beta-strand G and the C-terminal helix, and Gln(138), which is next to the C-terminal residue Tyr(139). Some of the regions showing high flexibility in profilin are known to be involved in poly-L-proline binding.