Cryo-EM structures show the mechanistic basis of pan-peptidase inhibition by human α2-macroglobulin

Abstract

Humanα2-macroglobulin (hα2M) is a multidomain protein with a plethoraof essential functions, including transport of signaling molecules and endopeptidaseinhibition in innate immunity. Here, we dissected the molecular mechanism of theinhibitory function of the∼720-kDa hα2M tetramer through eight cryo–electronmicroscopy (cryo-EM) structures of complexes from human plasma. In the native com-plex, the hα2M subunits are organized in twoflexible modules in expanded conforma-tion, which enclose a highly porous cavity in which the proteolytic activity ofcirculating plasma proteins is tested. Cleavage of bait regions exposed inside the cavitytriggers rearrangement to a compact conformation, which closes openings and entrapsthe prey proteinase. After the expanded-to-compact transition, which occurs indepen-dently in the four subunits, the reactive thioester bond triggers covalent linking of theproteinase, and the receptor-binding domain is exposed on the tetramer surface forreceptor-mediated clearance from circulation. These results depict the molecular mecha-nism of a unique suicidal inhibitory trap.