This conserved domain is called ESPR for Extended Signal Peptide Region. It is present at the N-terminus of the signal peptides of proteins belonging to the Type V secretion systems, including the autotransporters (T5aSS), TpsA exoproteins of the two-partner system (T5bSS) and trimeric autotransporters (TAAs). So far, the ESPR is present only in Gram-negative bacterial proteins originating from the classes Beta- and Gamma-proteobacteria. ESPR severely impairs inner membrane translocation, suggesting that it adopts a particular conformation or it interacts with a cytoplasmic or inner membrane co-factor, prior to exportation. Deletion of ESPR causes mis-folding of the TAAs passenger domain in the periplasm , substantially impairing its translocation across the outer membrane [3].

1: The unusual extended signal peptide region of the type V secretion system is phylogenetically restricted. Desvaux M, Cooper LM, Filenko NA, Scott-Tucker A, Turner SM, Cole JA, Henderson IR; FEMS Microbiol Lett. 2006;264:22-30. PMID:17020545

2: A conserved extended signal peptide region directs posttranslational protein translocation via a novel mechanism. Desvaux M, Scott-Tucker A, Turner SM, Cooper LM, Huber D, Nataro JP, Henderson IR; Microbiology. 2007;153:59-70. PMID:17185535

3: From self sufficiency to dependence: mechanisms and factors important for autotransporter biogenesis. Leyton DL, Rossiter AE, Henderson IR; Nat Rev Microbiol. 2012;10:213-225. PMID:22337167