ExeA binds to peptidoglycan and forms a multimer for assembly of the type II secretion apparatus in Aeromonas hydrophila

Abstract

Aeromonas hydrophila uses the type II secretion system (T2SS) to transport protein toxins across the outer membrane. The inner membrane complex ExeAB is required for assembly of the ExeD secretion channel multimer, called the secretin, into the outer membrane. A putative peptidoglycan-binding domain (Pfam number PF01471) conserved in many peptidoglycan-related proteins is present in the periplasmic region of ExeA (P-ExeA). In this study, co-sedimentation analysis revealed that P-ExeA was able to bind to highly pure peptidoglycan. The protein assembled into large multimers in the presence of peptidoglycan fragments, as shown in native PAGE, gel filtration and cross-linking experiments. The requirement of peptidoglycan for multimerization was abrogated when the protein was incubated at 30 degrees C and above. These results provide evidence that the putative peptidoglycan-binding domain of ExeA is involved in physical contact with peptidoglycan. The interactions facilitate the multimerization of ExeA, favouring a model in which the protein forms a multimeric structure on the peptidoglycan during the ExeAB-dependent assembly of the secretin multimer in the outer membrane.