Abstract
Amide proton exchange rates of Ca2+-saturated calmodulin and Ca2+-saturated calmodulin-mastoparan complex were studied by 1H NMR spectroscopy. Exchange rates of G1y25, G1y61, G1y98, G1y134, 11e27, Ile100, and Asn137 were determined for Ca2+-saturated calmodulin and for Ca2+-saturated calmodulin-mastoparan complex, and were found to be less than 10-4 s-1. All these residues of which the amide proton resonances appear at lower fields were considered to form hydrogen bonds, based on the results of X-ray analysis. Exchange rates of I1e27 and Asn137 became an order of magnitude smaller when mastoparan bound to Ca2+-saturated calmodulin, while those of the four glycines and Ile100 did not change appreciably. The reduction in accessibility of Asn137 to water cased by mastoparan binding suggests that a part of the mastoparan binding site is probably located in or near the hydrophobic cluster of the C-terminal-half domain. The reduction in accessibility of 11e27 also suggests that another part of the mastoparan binding site is located in or near the hydrophobic cleft of the N-terminal-half domain.
