Might be part of the outer membrane protein assembly complex, which is involved in assembly and insertion of beta-barrel proteins into the outer membrane. Both rabbit immune serum and rabbit antiserum specific for extracytoplasmic loop L4 promote bacteria internalization by rabbit peritoneal macrophages. Pools of human syphilitic sera from the USA and Columbia recognize both the N-terminal POTRA-containing and C-terminal beta-barrel domains as well as loop L4, showing this protein stimulates the immune system in both humans and rabbits (PubMed:25825429). Part of 2 complexes of about 239 and 164 kDa. The C-terminal beta-barrel domain is exposed on the outer surface of the outer membrane (PubMed:21488980). Antibody labeling by antisera to either the periplasmic or membrane domain occurs in a punctate fashion over the length of the spirochete; detection by periplasmic POTRA-domain-specific antibodies only occurs after mild detergent solubilization (PubMed:25825429). Transcribed at low levels, estimated to be present at about 100 molecules per cell (at protein level). The N-terminus (residues 22-369) partitions into an aqueous phase and is predicted to be periplasmic, while the C-terminus (residues 426-837) partitions into Triton X-114 and has the hallmarks of a beta-stranded protein, suggesting it is membraneous. Recognized by sera from 42/43 syphilis patients, it shows promise as a diagnostic antigen (PubMed:12904373). In further diagnostic tests for syphilis this protein exhibits a sensitivity of 86% and a specificity of 99%. A fusion with TP0453 (residues 32-287 of TP0453 fused N-terminally to residues 38-450 of TP0326) exhibits a sensitivity of 98% and a specificity of 99%, and exhibits superior accuracy in classifying analytical false-positive samples than a commericially available test, making it a better candiate for diagnosis (PubMed:23100335). The outer membrane of T.pallidum has no lipopolysaccharides, few proteins and is a fluid and very fragile bilayer. Its lack of surface antigenicity is thought to contribute to the ability of the pathogen to evade the human immune system. Belongs to the BamA family. Extended N-terminus. Extended N-terminus.