9.B.42 The ExeAB (ExeAB) Secretin Assembly/Export Complex Family
The ExeAB complex of Aeromonas hydrophila is required for the localization and assembly of the oligomeric ring structure of the ExeD secretin of the main terminal branch (MTB) of the type II protein secretion system which secretes toxins such as aerolysin (Ast et al., 2002). In the absence of ExeAB, ExeD remains in a monomeric form in the inner membrane, while expression of exeAB allows appearance of the ExeD secretin ring structure in the outer membrane. It is not known if ExeAB plays a direct or indirect role in transport of ExeD.
ExeA is 547 aas with 1 putative TMS. ExeB is 226 aas with 1 or 2 putative TMSs. The ExeAB complex is found in the cytoplasmic membrane. Only a few homologues were revealed by BLAST searches, and most are sequence divergent. However, ExeA binds to peptidoglycan and forms a multimer for assembly of the type II secretion apparatus in Aeromonas hydrophila (Li & Howard et al., 2010).