Bipartite Topology of Treponema pallidum Repeat Proteins C/D and I: OUTER MEMBRANE INSERTION, TRIMERIZATION, AND PORIN FUNCTION REQUIRE A C-TERMINAL -BARREL DOMAIN

Background:Treponema pallidum contains a paucity of outer membrane proteins. Results: The C-terminal domains of TprC/D and TprI are -barrels with porin function; their N-terminal domains provide periplasmic anchors for the -barrels. Conclusion: Full-length TprC subfamily proteins possess a dual domain topology. Significance: TprC/D are bona fide rare outer membrane proteins and a new class of dual function, bipartite, bacterial outer membrane protein. We previously identified Treponemapallidumrepeat proteins TprC/D, TprF, and TprI as candidate outer membrane proteins (OMPs) and subsequently demonstrated that TprC is not only a rare OMP but also forms trimers and has porin activity. We also reported that TprC contains N- and C-terminal domains (TprC(N) and TprC(C)) orthologous to regions in the major outer sheath protein (MOSPN and MOSPC) of Treponema denticola and that TprC(C) is solely responsible for -barrel formation, trimerization, and porin function by the full-length protein. Herein, we show that TprI also possesses bipartite architecture, trimeric structure, and porin function and that the MOSPC-like domains of native TprC and TprI are surface-exposed in T. pallidum, whereas their MOSPN-like domains are tethered within the periplasm. TprF, which does not contain a MOSPC-like domain, lacks amphiphilicity and porin activity, adopts an extended inflexible structure, and, in T. pallidum, is tightly bound to the protoplasmic cylinder. By thermal denaturation, the MOSPN and MOSPC-like domains of TprC and TprI are highly thermostable, endowing the full-length proteins with impressive conformational stability. When expressed in Escherichia coli with PelB signal sequences, TprC and TprI localize to the outer membrane, adopting bipartite topologies, whereas TprF is periplasmic. We propose that the MOSPN-like domains enhance the structural integrity of the cell envelope by anchoring the -barrels within the periplasm. In addition to being bona fide T. pallidum rare outer membrane proteins, TprC/D and TprI represent a new class of dual function, bipartite bacterial OMP.