Alpha-2-macroglobulins (A2Ms) are plasma proteins that trap and inhibit a broad range of proteases and are major components of the eukaryotic innate immune system. However, A2M-like proteins were identified in pathogenically invasive bacteria and species that colonize higher eukaryotes. Bacterial A2Ms are located in the periplasm where they are believed to provide protection to the cell by trapping external proteases through a covalent interaction with an activated thioester. This domain is found on the N-terminal region in A2Ms in bacteria. Structure analysis of Salmonella enterica ser A2Ms (SA-A2Ms) show that they are composed of 13 domains, all of which fold as variants of beta sandwiches with the exception of the TED, which consists of 14 alpha helices. Most of the beta sandwich domains appear to serve a structural role and are referred to as the macroglobulin-like (MG) domains. This is the MG5 domain [1].

1: Structure of a bacterial alpha2-macroglobulin reveals mimicry of eukaryotic innate immunity. Wong SG, Dessen A; Nat Commun. 2014;5:4917. PMID:25221932