An increase in side chain entropy facilitates effector binding: NMR characterization of the side chain methyl group dynamics is Cdc42Hs

Cdc42Hs is a signal transduction protein that is involved in cytoskeletal growth and organization. We describe here the methyl side chain dynamics of three forms of H-2, C-13, N-15-Cdc42Hs [GDP-bound (inactive), GMPPCP-bound (active), and GMPPCP/PBD46-bound (effector-bound)] from C-13-H-1 NMR measurements of deuterium T-1 and T-1p relaxation times. A wide variation in flexibility was observed throughout the protein, with methyl axis order parameters (S-axis(2)) ranging from 0.2 to 0.4 (highly disordered) in regions near the PBD46 binding site to 0.8-1.0 (highly ordered) in some helices. The side chain dynamics of the GDP and GMPPCP forms are similar, with methyl groups on the PBD46 binding surface experiencing significantly greater mobility (lower S-axis(2)) than those not on the binding surface. Binding of PBD46 results in a significant increase in the disorder and a corresponding increase in entropy for the majority of methyl groups. Many of the methyl groups that experience an increase in mobility are found in residues that are not part of the PBD46 binding interface. This entropy gain represents a favorable contribute to the overall entropy of effector binding and partially offsets unfavorable entropy losses such as those that occur in the backbone.