The crystal structure of calcium-free human m-calpain suggests an electrostatic switch mechanism for activation by calcium

Calpains (calcium-dependent cytoplasmic cysteine proteinases) are implicate d in processes such as cytoskeleton remodeling and signal transduction. The 2.3-Angstrom crystal structure of full-length heterodimeric [80-kDa (dI-dI V)+30-kDa (dV+dVI)] human m-calpain crystallized in the absence of calcium reveals an oval disc-like shape, with the papain-like catalytic domain dII and the two calmodulin-like domains dIV+dVI occupying opposite poles, and t he tumor necrosis factor alpha-like beta-sandwich domain dill and the N-ter minal segments dI+dV located between. Compared with papain, the two subdoma ins dIIa+dIIb of the catalytic unit are rotated against one another by 50 d egrees, disrupting the active site and the substrate binding site, explaini ng the inactivity of calpains in the absence of calcium. Calcium binding to an extremely negatively charged loop of domain dill (an electrostatic swit ch) could release the adjacent barrel-like subdomain dIIb to move toward th e helical subdomain dIIa, allowing formation of a functional catalytic cent er. This switch loop could also mediate membrane binding, thereby explainin g calpains' strongly reduced calcium requirements in vivo. The activity sta tus at the catalytic center might be further modulated by calcium binding t o the calmodulin domains via the N-terminal linkers.