Calmodulin remains extended upon binding to smooth muscle caldesmon: A combined small-angle scattering and Fourier transform infrared spectroscopy study

We show that calmodulin (CaM) has an extended conformation in its complexes with sequences from the smooth muscle thin filament protein caldesmon (CaD ) by using small-angle X-ray and neutron scattering with contrast variation . The CaD sequences used in these experiments were a C-terminal fragment, 2 2kCaD, and a smaller peptide sequence within this fragment, MG56C, Each of these sequences contains the CaM-binding sites A and B previously shown to interact with the C- and N-terminal lobes of CaM, respectively [Wang ct al, (1997) Biochemistry 36, 15026]. By modeling the scattering data, we show t hat the majority of the MG56C sequence binds to the N-terminal domain of Ca M. FTIR data on CaM complexed with 22kCaD or with MG56C peptide show the 22 kCaD sequence contains unordered, helix, and extended structures, and that the extended structures reside primarily in the MG56C portion of the sequen ce. There are small changes in secondary structure, involving similar to 12 residues, induced by CaM binding to CaD, These changes involve a net decre ase in extended structures accompanied by an increase in alpha-helix, and t hey occur within the CaM and/or in the MG56C sequence.