Part of the translocation and assembly module (TAM) autotransporter assembly complex, which functions in translocation of autotransporters across the outer membrane (PubMed:22466966). In reconstituted TAM this subunit (Ag43, AC P39180) is not necessary for substrate penetration in the outer membrane. Substrate binding to TamA moves its POTRA domains about 30 Angstroms into the periplasm, which would deform either the outer membrane or TamB and may provide force to reset TAM (PubMed:25341963). Interacts with TamA to form the translocation and assembly module (TAM). The periplasmic domain is elongated (up to 160 Angstroms long) and forms contacts with the N-terminal POTRA domains of TamA (PubMed:25341963). Loss of localization of an exogenous protein, P1121 (ROD_p1121 of Citrobacter rodentium), to outer membranes, loss of cell aggregation when adhesins are over-expressed in a double tamA-tamB deletion mutant. Belongs to the TamB family. Produces two separate ORFs.