Protects the bacterial cell from host peptidases (PubMed:18697741, PubMed:26143919, PubMed:26100869). Acts by a 'trapping' mechanism. Cleavage of the bait-region domain by host peptidases leads to a global conformational change, which results in entrapment of the host peptidase and activation of the thioester bond that covalently binds the attacking host peptidase (PubMed:26143919, PubMed:26100869). Trapped peptidases are still active except against very large substrates (PubMed:26100869). May protect the entire periplam, including the lipoproteins anchored to the periplasmic side of the outer membrane, against intruding endopeptidases (PubMed:26100869). May form homooligomers. Cleavage causes major structural rearrangement of more than half the 13-domain structure from a native to a compact induced form. Bacterial alpha-2-macroglobulins were probably acquired one or more times by horizontal gene transfer from metazoan hosts. Belongs to the protease inhibitor I39 (alpha-2-macroglobulin) family. Bacterial alpha-2-macroglobulin subfamily.