TpsB

TpsB is an outer membrane protein that functions as the secretion gateway in the two-partner secretion (TPS) system found in many Gram-negative bacteria. In TPS, a separate protein TpsA is synthesized with an N-terminal signal peptide for Sec-dependent export to the periplasm; after crossing the periplasm, TpsA engages with its cognate TpsB at the outer membrane and is translocated out of the cell. TpsB thus determines substrate specificity and forms the conduit through which the mature TpsA passenger is exported.

Structural features: TpsB proteins typically comprise one or more periplasmic POTRA (polypeptide-transport-associated) domains that mediate interactions with TpsA, followed by a C-terminal β-barrel that inserts into the outer membrane to form the secretion channel. They belong to the Omp85 superfamily, and the number of POTRA domains can vary among species.

Examples and biology: The best-characterized TPS pair is FhaC/FhaB in Bordetella pertussis, where FhaB encodes filamentous haemagglutinin, a fimbrial-like adhesin. Other TPS systems include HMWB/HMWA in Haemophilus influenzae and multiple TpsA substrates in Neisseria and related bacteria. In these systems, secretion of the TpsA passenger contributes to adhesion, immune interaction, or cytotoxic activities, linking TpsB function to virulence.

Genetic and practical notes: TpsA and TpsB genes are often co-located in operons encoding the TPS pair, with TpsB serving as the essential outer-membrane receptor for export. Because of their role in pathogenicity, TPS components have been considered targets for vaccines or therapeutics.