Relaxation study of the backbone dynamics of human profilin by two‐dimensional 1H‐15N 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 1H-15N NMR spectroscopy. Heteronuclear {1H}-15N nuclear Overhauser effects and 15N 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 Asp26 and Ser27, which form part of a loop connecting β-strands A and B, and for Thr92 through Ala95, which are in a loop connecting β-strands E and F. Significant exchange contributions, indicative of motions on the microsecond to millisecond timescale, have been obtained for 30 residues. These include Leu77, Asp80 and Gly81 of a loop between β-strands D and E, Ser84 and Met85 of β-strand E, Gly121 of a loop connecting β-strand G and the C-terminal helix, and Gln138, which is next to the C-terminal residue Tyr139. Some of the regions showing high flexibility in profilin are known to be involved in poly-L-proline binding.