Calcium-induced tripartite binding of intrinsically disordered calpastatin to its cognate enzyme, calpain

Abstract 

The activity of calpain is controlled by the free intracellular calcium level and by the protein’s intrinsically disordered endogenous inhibitor, calpastatin, mediated by short conserved segments: subdomains A–C. The exact binding mode of calpastatin to the enzyme has until now been unclear. Our NMR data of the 141 amino acid long inhibitor, with and without calcium and calpain, have revealed structural changes and a tripartite binding mode, in which the disordered inhibitor wraps around, and contacts, the enzyme at three points, facilitated by flexible linkers. This unprecedented binding mode permits a unique combination of specificity, speed and binding strength in regulation.

Structured summary

Abbreviations: IDP, intrinsically disordered protein, hCSd1, human calpastatin domain 1, hCSd1·Ca²⁺, human calpastatin domain 1 with equivalent CaCl₂, complex, human calpastatin domain 1 and m-calpain complex, cCS, combined chemical shift, A, subdomain A, B, subdomain B, C, subdomain C

Keywords: Intrinsically disordered protein, Supramolecular complex by NMR, Ligand binding specificity