Solution X-ray scattering reveals a novel structure of calmodulin complexed with a binding domain peptide from the...

The solution structures of complexes between calcium-saturated calmodulin (Ca (2+)/CaM) and a CaM-binding domain of the HIV-1 matrix protein p17 have been determined by small-angle X-ray scattering with use of synchrotron radiation as an intense and stable X-ray source. We used three synthetic peptides of residues 11-28, 26-47, and 11-47 of p17 to demonstrate the diversity of CaM-binding conformation. Ca (2+)/CaM complexed with residues 11-28 of p17 adopts a dumbbell-like structure at a molar ratio of 1:2, suggesting that the two peptides bind each lobe of CaM, respectively. Ca (2+)/CaM complexed with residues 26-47 of p17 at a molar ratio of 1:1 adopts a globular structure similar to the NMR structure of Ca (2+)/CaM bound to M13, which adopted a compact globular structure. In contrast to these complexes, Ca (2+)/CaM binds directly with both CaM-binding sites of residues 11-47 of p17 at a molar ratio of 1:1, which induces a novel structure different from known structures previously reported between Ca (2+)/CaM and peptide. A tertiary structural model of the novel structure was constructed using the biopolymer module of Insight II 2000 on the basis of the scattering data. The two domains of CaM remain essentially unchanged upon complexation. The hinge motions, however, occur in a highly flexible linker of CaM, in which the electrostatic residues 74Arg, 78Asp, and 82Glu interact with N-terminal electrostatic residues of the peptide (residues 12Glu, 15Arg, and 18Lys). The acidic residues in the N-terminal domain of CaM interact with basic residues in a central part of the peptide, thereby enabling the central part to change the conformations, while an acidic residue in the C-terminal domain interacts with two basic residues in the two helical sites of the peptide. The overall structure of the complex adopts an extended structure with the radius of gyration of 20.5 A and the interdomain distance of 34.2 A. Thus, the complex is principally stabilized by electrostatic interactions. The hydrophobic patches of Ca (2+)/CaM are not responsible for the binding with the hydrophobic residues in the peptide, suggesting that CaM plays a role to sequester the myristic acid moiety of p17.