Autotransporters are synthesized as precursor proteins with three functional domains, namely, an N-terminal signal peptide, an internal passenger domain, and a C-terminal pore-forming translocator domain. The C-terminal translocator domain is embedded in the outer membrane and facilitates delivery of the internal passenger domain to the bacterial surface. In conventional autotransporters, the C-terminal translocator domain contains approximately 300 amino acids and is monomeric. In contrast, in trimeric autotransporters, the translocator domain contains 60 to 70 amino acids and forms trimers in the outer membrane [1]. This entry represents a Trp-ring domain which is found in the translocator region of H. influenzae Hia autotransporter, an adhesive protein that promotes adherence to respiratory epithelial cells [2]. Trp-ring domains appear to be crucial repeated modular units in Hia, both in the general architecture of the passenger domain and in the structure of the binding domains [1].

1: Repetitive architecture of the Haemophilus influenzae Hia trimeric autotransporter. Meng G, St Geme JW 3rd, Waksman G; J Mol Biol. 2008;384:824-836. PMID:18948113

2: Structural basis for host recognition by the Haemophilus influenzae Hia autotransporter. Yeo HJ, Cotter SE, Laarmann S, Juehne T, St Geme JW 3rd, Waksman G; EMBO J. 2004;23:1245-1256. PMID:15029242