Physicochemical evidence that Treponema pallidum TroA is a zinc-containingmetalloprotein that lacks porin-like structure

Although TroA (Tromp1) was initially reported to be a Treponema pallidum ou ter membrane protein with porin-like properties, subsequent studies have su ggested that it actually is a periplasmic substrate-binding protein involve d in the transport of metals across the treponemal cytoplasmic membrane. He re we conducted additional physicochemical studies to address the divergent viewpoints concerning this protein. Triton X-114 phase partitioning of rec ombinant TroA constructs with or without a signal sequence corroborated our prior contention that the native protein's amphiphilic behavior is due to its uncleaved lender peptide. Whereas typical porins are trimers with exten sive beta-barrel structure, size exclusion chromatography and circular dich roism spectroscopy revealed that TroA was a monomer and predominantly alpha -helical. Neutron activation, atomic absorption spectroscopy, and anomalous X-ray scattering all demonstrated that TroA binds zinc in a 1:1 molar stoi chiometric ratio. TroA does not appear to possess structural features consi stent with those of bacterial porins.