TP0326, a Treponema pallidum β‐barrel assembly machinery A (BamA) orthologue and rare outer membrane protein

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Abstract
Definitive identification of Treponema pallidum rare outer membrane proteins (OMPs) has long eluded researchers. TP0326, the sole protein in T. pallidum with sequence homology to a Gram‐negative OMP, belongs to the BamA family of proteins essential for OM biogenesis. Structural modelling 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 T. pallidum larger than that of Escherichia coli. Non‐orthologous ancillary factors and self‐association of TP0326 via its β‐barrel may both contribute to the Bam complex. T. pallidum‐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 spirochaete appears to have devised a stratagem for harnessing the Bam pathway while satisfying its need to limit surface antigenicity.