Cryo-EM structures show the mechanistic basis of pan-peptidase inhibition by human alpha(2)-macroglobulin

Human alpha(2)-macroglobulin (h alpha M-2) is a multidomain protein with a plethora of essential functions, including transport of signaling molecules and endopeptidase inhibition in innate immunity. Here, we dissected the molecular mechanism of the inhibitory function of the similar to 720-kDa h alpha M-2 tetramer through eight cryo-electron microscopy (cryo-EM) structures of complexes from human plasma. In the native complex, the h alpha M-2 subunits are organized in two flexible modules in expanded conformation, which enclose a highly porous cavity in which the proteolytic activity of circulating plasma proteins is tested. Cleavage of bait regions exposed inside the cavity triggers rearrangement to a compact conformation, which closes openings and entraps the prey proteinase. After the expanded-to-compact transition, which occurs independently in the four subunits, the reactive thioester bond triggers covalent linking of the proteinase, and the receptor-binding domain is exposed on the tetramer surface for receptor-mediated clearance from circulation. These results depict the molecular mechanism of a unique suicidal inhibitory trap.