Part of the translocation and assembly module (TAM) autotransporter assembly complex, which functions in translocation of autotransporters across the outer membrane (PubMed:22466966, PubMed:25341963). Allows substrate (Ag43, AC P39180) to initiate penetration into the outer membrane; TamB is not necessary but may regulate this activity (PubMed:25341963). Has anion selective channel-forming ability, but the physiological relevance of this activity is unclear (PubMed:22466966). Interacts with TamB to form the translocation and assembly module (TAM). Contains 3 N-terminal periplasmic polypeptide transport-associated (POTRA) domains and a C-terminal 16-stranded beta-barrel transmembrane region (PubMed:24056943, PubMed:25341963). The 3 POTRA domains seem to form a rigid body (unlike BamA where the 5 POTRA domains are able to move independently) (PubMed:26243377). Upon substrate binding (Ag43, AC P39180) the 3 POTRA domains move away from the inner surface of the outer membrane by about 30 Angstroms, which would deform either the outer membrane or TamB and may provide force to reset TAM (PubMed:25341963). POTRA1 is required for interaction with TamB (PubMed:26243377). Deletion of POTRA1 decreases Ag43 binding to this protein, whereas deletion of POTRA1 and 2 obviates binding to Ag43 (PubMed:26243377). The beta-barrel is closed on the extracellular face by loop 6 (residues 456-495), while the C-terminal beta-strand forms an inward kink which may act as a gate for substrate access from the periplasm. This kink weakens the beta-strand pair between the first and last strands which may contribute to gating and substrate translocation (PubMed:24056943). Not essential, cells grow more slowly than wild-type. No changes in permeability or outer membrane integrity. 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 TamA family.