1.B.45 The Treponema Porin (T-Por) Family
Treponema pallidum and T. denticola each possesses an outer membrane protein that is probably a porin. The protein in T. pallidum is TP0453 and is 287 aas. This protein, when inserted into artificial bilayers, confers enhanced permeability to the membrane (Hazlett et al., 2005). The authors suggested that this protein lacks extensive β-sheet structure and that it is a lipoprotein with an amphipathic polypeptide containing multiple membrane inserting, amphipathic α-helices. Insertion of the non-lipidated protein into artificial membranes resulted in bilayer destabilization and enhanced permeability. TP0453 is probably a novel type of bacterial outer membrane protein which may render the T. pallidum outer membrane permeable to nutrients while remaining inaccessible to antibody (Hazlett et al. 2005).
More recently, the 3-d structure of TP0453 has been solved (3K8G - J; Luthra et al. 2011). TP0453, the only lipoprotein associated with the inner leaflet of the Tp OM. Whereas polypeptides of other treponemal lipoproteins are hydrophilic, non-lipidated TP0453 can integrate into membranes, a property attributed to its multiple amphipathic helices (AHs). Membrane integration was found to increase membrane permeability, suggesting it is a porin. The crystal structure consists of an α/β/α-fold and includes five stably folded AHs. In high concentrations of detergent, TP0453 transitions from a closed to open conformation by lateral movement of two groups of AHs, exposing a large hydrophobic cavity. Two adjacent AHs are critical for membrane sensing/integration. TP0453 increased efflux of fluorophore only at acidic pH. One AH critical for membrane sensing/insertion also forms a dimeric interface. Based in part on structural dynamics and comparison with Mycobacterium tuberculosis lipoproteins LprG and LppX, TP0453 probably functions as a carrier of lipids, glycolipids, and/or derivatives during OM biogenesis (Luthra et al. 2011).