Ap. Loh et al., An increase in side chain entropy facilitates effector binding: NMR characterization of the side chain methyl group dynamics is Cdc42Hs, BIOCHEM, 40(15), 2001, pp. 4590-4600
Cdc42Hs is a signal transduction protein that is involved in cytoskeletal g
rowth 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 measur
ements of deuterium T-1 and T-1p relaxation times. A wide variation in flex
ibility was observed throughout the protein, with methyl axis order paramet
ers (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 si
de chain dynamics of the GDP and GMPPCP forms are similar, with methyl grou
ps 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 incre
ase 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 favora
ble contribute to the overall entropy of effector binding and partially off
sets unfavorable entropy losses such as those that occur in the backbone.