Binding of Both Ca2+ and Mastoparan to Calmodulin Induces a Large Change in the Tertiary Structure

The technique of small-angle X-ray scattering has been employed to examine the solution conformation of calmodulin and its complexes with Ca2+ alone, and with both Ca2+ and mastoparan. The radius of gyration decreased by 3.1 ± 0.3 Å upon binding of both 4 mol Ca2+/mol of protein and 1 mol mastoparan/mol of protein to form the ternary complex. A smaller increase was found for the separate binding of 4 mol Ca2+/mol of protein in the absence of mastoparan (0.6±0.3 Å). The analyses of pair distance distribution function showed that the maximal pair distance in calmodulin complex with both Ca2+ and mastoparan decreased by 20–30% in comparison with calmodulin or its complex with Ca2+, and a shoulder near 40 Å, which characterizes the dumbbell-shaped molecule of calmodulin, disappeared. These results indicate that the two globular domains of the calmodulin complex with Ca2+ and mastoparan come close together by 8.0–9.5 Å on average, if the size and the overall shape of the globular domains are the same in Ca2+ -calmodulin-mastoparan complex as in calmodulin or Ca2+-calmodulin complex.