Oligomeric state, hydrodynamic properties and target recognition of human Calcium and Integrin Binding protein 2 (CIB2)

Calcium- and Integrin-Binding protein 2 (CIB2) is a small and ubiquitously expressed protein with largely unknown biological function but ascertained role in hearing physiology and disease. Recent studies found that CIB2 binds Ca(2+) with moderate affinity and dimerizes under conditions mimicking the physiological ones. Here we provided new lines of evidence on CIB2 oligomeric state and the mechanism of interaction with the α7B integrin target. Based on a combination of native mass spectrometry, chemical cross-linking/mass spectrometry, analytical gel filtration, dynamic light scattering and molecular dynamics simulations we conclude that CIB2 is monomeric under all tested conditions and presents uncommon hydrodynamic properties, most likely due to the high content of hydrophobic solvent accessible surface. Surface plasmon resonance shows that the interaction with α7B occurs with relatively low affinity and is limited to the cytosolic region proximal to the membrane, being kinetically favored in the presence of physiological Mg(2+) and in the absence of Ca(2+). Although CIB2 binds to an α7B peptide in a 1:1 stoichiometry, the formation of the complex might induce binding of another CIB2 molecule.