This is the B protein from some operons of bacterial secretion systems of type II. The exact function of the B protein is not known, though in the case of Vibrio cholerae there is a fusion protein between proteins A and B that includes an AAA domain, a PG_binding domains well as this domain at the C-terminus [2]. Many of the other species have no A or B domain genes in this operon. The type II secretion pathway is conserved in Gram-negative bacteria that are prevalent in bacterial pathogens of plants (Pseudomonas fluorescens, Erwinia or Xanthomonas species), animals (Aeromonas hydrophila) and humans (Klebsiella oxytoca, Pseudomonas aeruginosa, Vibrio cholerae or Legionella pneumophila). Typical type II secretion systems (T2SSs) are encoded by a set of 12 to 16 gsp (general secretion pathway) genes organised into large operons including the conserved 'core' genes denoted C to O and in some bacterial species, as indicated above, extra gsp genes such as gspAB, gspN or gspS. A different nomenclature is used for Pseudomonas T2SSs, so the B gene is referred to as the P protein [1].

1: The type II secretion system: biogenesis, molecular architecture and mechanism. Korotkov KV, Sandkvist M, Hol WG; Nat Rev Microbiol. 2012;10:336-351. PMID:22466878

2: Structure of a periplasmic domain of the EpsAB fusion protein of the Vibrio vulnificus type II secretion system. Martynowski D, Grochulski P, Howard PS; Acta Crystallogr D Biol Crystallogr. 2013;69:142-149. PMID:23385451