TP0326, a Treponema pallidum β-Barrel Assembly Machinery A (BamA) Ortholog and Rare Outer Membrane Protein

• Published in: Molecular microbiology 2011-04, Vol.80 (6), p.1496-1515
• Main Authors: Desrosiers, Daniel C., Anand, Arvind, Luthra, Amit, Dunham-Ems, Star M, LeDoyt, Morgan, Cummings, Michael A. D., Eshghi, Azad, Cameron, Caroline E., Cruz, Adriana R., Salazar, Juan C., Caimano, Melissa J., Radolf, Justin D.
• Format: Article
• Language: English
• ISSN: 0950-382X; 1365-2958
• DOI: 10.1111/j.1365-2958.2011.07662.x

Definitive identification of Treponema pallidum ( Tp ) rare outer membrane proteins (OMPs) has long eluded researchers. TP0326, the sole protein in Tp with sequence homology to a Gram-negative OMP, belongs to the BamA family of proteins essential for OM biogenesis. Structural modeling predicted that five polypeptide transport-associated (POTRA) domains comprise the N-terminus of TP0326, while the C-terminus forms an 18-stranded amphipathic β-barrel. Circular dichroism, heat-modifiability by SDS-PAGE, Triton X-114 phase partitioning and liposome incorporation supported these topological predictions and confirmed that the β-barrel is responsible for the native protein's amphiphilicity. Expression analyses revealed that native TP0326 is expressed at low abundance, while a protease-surface accessibility assay confirmed surface exposure. Size-exclusion chromatography and blue native polyacrylamide gel electrophoresis revealed a modular Bam complex in Tp considerably larger than that of E. coli . Non-orthologous ancillary factors and self-association of TP0326 via its β-barrel may both contribute to the Bam complex. Tp -infected rabbits mount a vigorous antibody response to both POTRA and β-barrel portions of TP0326, whereas humans with secondary syphilis respond predominantly to POTRA. The syphilis spirochete appears to have devised a stratagem for harnessing the Bam pathway while satisfying its need to limit surface antigenicity.