Bipartite Topology of Treponema pallidum Repeat Proteins C/D and I: Outer Membrane Insertion and Porin Function Require a C-terminal {beta}-barrel Domain [Microbiology]
March 24th, 2015 by Anand, A., LeDoyt, M., Karanian, C., Luthra, A., Koszelak-Rosenblum, M., Malkowski, M. G., Puthenveetil, R., Vinogradova, O., Radolf, J. D.
We previously identified T. pallidum repeat 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 (TprCN and TprCC) orthologous to regions in the major outer sheath protein (MOSPN and MOSPC) of T. denticola and that TprCC is solely responsible for β-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 while 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 E. coli with PelB signal sequences, TprC and TprI localize to the OM, 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 β-barrels within the periplasm. In addition to being bona fide T. pallidum rare outer membrane proteins, TprC/D and I represent a new class of dual function, bipartite bacterial OMP.