A combination of ion mobility and mass spectrometry methods was used to characterize the molecular shape of the protein calmodulin (CaM) and its complexes with calcium and a number of peptide ligands. CaM, a calcium-binding protein composed of 148 amino acid residues, was found by X-ray crystallography to occur both in a globular shape and in the shape of an extended dumbbell. Here, it was found, as solutions of CaM and CaM complexes were sprayed into the solvent-free environment of the mass spectrometer, that major structural features of the molecule and the stoichiometry of the units constituting a complex in solution were preserved in the desolvation process. Two types of CaM structures were observed in our experiments: a compact and an extended form of CaM with measured cross sections in near-perfect agreement with those calculated for the known globular and extended dumbbell X-ray geometries. Calcium-free solutions yielded predominantly an extended CaM conformation. Ca n2+-CaM complexes were observed in calcium-containing solutions, n = 0-4, with the population of the compact conformation increasing relative to the elongated conformation as n increases. For n = 4, a predominantly compact globular conformation was observed. Solutions containing the peptide CaMKII 290-309, the CaM target domain of the Ca2+/calmodulin-dependent protein kinase II (CaMKII) enzyme, yielded predominantly globular Ca 42+-CaM-CaMKII290-309 complexes. Similar results were obtained with the 26-residue peptide melittin. For the 14-residue C-terminal melittin fragment, on the other hand, formation of both a 1:1 and a 1:2 CaM-peptide complex was detected. On the basis of the entirety of our results, we conclude that the collapse of extended (dumbbell-like) CaM structures into more compact globular structures occurs upon specific binding of four calcium ions. Furthermore, this calcium-induced structural collapse of CaM appears to be a prerequisite for formation of a particularly stable CaM- peptide complex involving peptides long enough to be engaged in interactions with both lobes of CaM.