Calcium-dependent changes in structure of calmodulin with substance P.

Solution x-ray scattering using synchrotron radiation as an x-ray source has been used to study the solution structure of calmodulin complexed with substance P, a undecapeptide neurotransmitter. The x-ray data indicate that the complex has a compact globular structure, the formation of which is dependent upon the binding of Ca2+ to calmodulin. In the Ca(2+)-saturated condition, the radius of gyration of complexed calmodulin was 4.2 A smaller than that of uncomplexed calmodulin. The Ca(2+)-dependent change in radius of gyration of calmodulin with substance P is complete by the third and fourth Ca2+ binding. The behavior of the Guinier plot at small-to-moderate angles for uncomplexed calmodulin corresponds to a dumbbell shape. The Guinier plot for complexed calmodulin, however, corresponds to a non-dumbbell shape. The susceptibility of calmodulin to proteolytic attack with trypsin was used to examine the nature of the calmodulin complexed with substance P. In the presence of equimolar substance P, the first and second Ca2+ binding to calmodulin was enough to form a trypsin-resistant complex. These biochemical and x-ray data suggest that the binding of substance P to calmodulin is completed when the C-terminal half of calmodulin is occupied by Ca2+, while a significant structural change of calmodulin in the complex is still induced by successive Ca2+ occupancy on the N-terminal half of this molecule.