New member of the trefoil factor family of proteins is an alpha-macroglobulin protease inhibitor

Cryo-EM structures of human A2ML1 elucidate the protease-inhibitory mechanism of the A2M family

A2ML1 is a monomeric protease inhibitor belonging to the A2M superfamily of protease inhibitors and complement factors. Here, we investigate the protease-inhibitory mechanism of human A2ML1 and determine the structures of its native and protease-cleaved conformations. The functional inhibitory unit of A2ML1 is a monomer that depends on covalent binding of the protease (mediated by A2ML1’s thioester) to achieve inhibition. In contrast to the A2M tetramer which traps proteases in two internal chambers formed by four subunits, in protease-cleaved monomeric A2ML1 disordered regions surround the trapped protease and may prevent substrate access. In native A2ML1, the bait region is threaded through a hydrophobic channel, suggesting that disruption of this arrangement by bait region cleavage triggers the extensive conformational changes that result in protease inhibition. Structural comparisons with complement C3/C4 suggest that the A2M superfamily of proteins share this mechanism for the triggering of conformational change occurring upon proteolytic activation.

A2ML1 is a human protease inhibitor belonging to the A2M protein family. In this study, the authors determine structures of A2ML1 before and after protease inhibition and investigate its mechanism of action.

Structural and functional insight into pan-endopeptidase inhibition by α2-macroglobulins

Peptidases must be exquisitely regulated to prevent erroneous cleavage and one control is provided by protein inhibitors. These are usually specific for particular peptidases or families and sterically block the active-site cleft of target enzymes using lock-and-key mechanisms. In contrast, members of the +1400-residue multi-domain α2-macroglobulin inhibitor family (α2Ms) are directed against a broad spectrum of endopeptidases of disparate specificities and catalytic types, and they inhibit their targets without disturbing their active sites. This is achieved by irreversible trap mechanisms resulting from large conformational rearrangement upon cleavage in a promiscuous bait region through the prey endopeptidase. After decades of research, high-resolution structural details of these mechanisms have begun to emerge for tetrameric and monomeric α2Ms, which use 'Venus-flytrap' and 'snap-trap' mechanisms, respectively. In the former, represented by archetypal human α2M, inhibition is exerted through physical entrapment in a large cage, in which preys are still active against small substrates and inhibitors that can enter the cage through several apertures. In the latter, represented by a bacterial α2M from Escherichia coli, covalent linkage and steric hindrance of the prey inhibit activity, but only against very large substrates.

CD109 represents a novel branch of the alpha2-macroglobulin/complement gene family

We report here the genomic organization and phylogenic relationships of CD109, a member of the the alpha2-macroglobulin/complement (AMCOM) gene family. CD109 is a GPI-linked glycoprotein expressed on endothelial cells, platelets, activated T-cells, and a wide variety of tumors. We cloned full-length CD109 cDNA from the mammalian U373 cell line by RT-PCR and performed analysis of its corresponding genomic sequence. The CD109 cDNA spans 128 kb of chromosome 6q with its 33 exons constituting approximately 3.3% of the total CD109 genomic sequence. Sequence analysis revealed that CD109 contains specific motifs in its N-terminus, that are highly conserved in all AMCOM members. CD109 also shares motifs with certain other AMCOM members including: (1) a thioester 'GCGEQ" motif, (2) a furin site of four positively charged amino acids, and (3) a double tyrosine near the C-terminus. Based on a phylogenic analysis of human CD109 with other human homologs as well as orthologs from other mammalian species, C. elegans (ZK337.1) and E. coli homologs, we propose CD109 represents a novel and independent branch of the alpha2-macroglobulin/complement gene family (AMCOM) and may be its oldest member.