This is the MG2 (macroglobulin) domain of alpha-2-macroglobulin in eukaryotes [1]. 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. This domain is found in eukaryotic and bacterial proteins. In human A2Ms, this domain is termed macroglobulin-like (MG) domain 2 and in Salmonella enterica ser A2Ms, this is domain 4 [2] [3].

1: Structures of complement component C3 provide insights into the function and evolution of immunity. Janssen BJ, Huizinga EG, Raaijmakers HC, Roos A, Daha MR, Nilsson-Ekdahl K, Nilsson B, Gros P; Nature. 2005;437:505-511. PMID:16177781

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

3: The crystal structure of human alpha2-macroglobulin reveals a unique molecular cage. Marrero A, Duquerroy S, Trapani S, Goulas T, Guevara T, Andersen GR, Navaza J, Sottrup-Jensen L, Gomis-Ruth FX; Angew Chem Int Ed Engl. 2012;51:3340-3344. PMID:22290936