This is C-terminal domain (CTD) of M60-peptidases Pfam:PF13402 [1]. It Can also be found at the C-terminal region of gingipain B (RgpB) from P. gingivalis. It was found to possess a typical Ig-like fold encompassing seven antiparallel beta-strands organized in two beta-sheets, packed into a beta-sandwich structure that can spontaneously dimerise through C-terminal strand swapping. Translocation of gingipains from the periplasm across the OM is dependent on the conserved CTD, which appears to be important for secretion of the proteins and in particular, truncation of the last few C-terminal residues of this domain leads to accumulation of gingipains in the periplasm. Subsequently, the T9SS targeting signal was demonstrated to reside within the last 22 residues at the C-terminus of the CTD. During gingipain translocation across the OM, the CTD is cleaved off by PorU [2].

1: Recognition of protein-linked glycans as a determinant of peptidase activity. Noach I, Ficko-Blean E, Pluvinage B, Stuart C, Jenkins ML, Brochu D, Buenbrazo N, Wakarchuk W, Burke JE, Gilbert M, Boraston AB; Proc Natl Acad Sci U S A. 2017;114:E679. PMID:28096352

2: The outer-membrane export signal of Porphyromonas gingivalis type IX secretion system (T9SS) is a conserved C-terminal beta-sandwich domain. de Diego I, Ksiazek M, Mizgalska D, Koneru L, Golik P, Szmigielski B, Nowak M, Nowakowska Z, Potempa B, Houston JA, Enghild JJ, Thogersen IB, Gao J, Kwan AH, Trewhella J, Dubin G, Gomis-Ruth FX, Nguyen KA, Potempa J; Sci Rep. 2016;6:23123. PMID:27005013