Six-transmembrane helical domain (6-TMD) of the ATP-binding cassette subunit in the type 1 secretion systems, and similar proteins
This group represents the six-transmembrane helical domain (6-TMD) of the ABC subunit in the type 1 secretion systems (T1SS) and similar proteins. These transporter subunits include HylB, PrtD, CyaB, CvaB, RsaD, HasD, LipB, and LapB, among many others. T1SS are found in pathogenic Gram-negative bacteria (such as Escherichia coli, Vibrio cholerae or Bordetella pertussis) to export proteins (often proteases) across both inner and outer membranes to the extracellular medium. This is one of three proteins of the type I secretion apparatus. In the case of the Escherichia coli HlyA T1SS, these three proteins are HlyB (a dimeric ABC transporter), HlyD (MFP, oligomeric membrane fusion protein) and TolC (OMP, a trimeric oligomeric outer membrane protein). Most targeted proteins are not cleaved at the N terminus, but rather carry signals located toward the extreme C terminus to direct type I secretion. However, the 10 kDa Escherichia coli colicin V (CvaB) targets the ABC transporter using a cleaved, N-terminal signal sequence. Almost all transport substrates of the type I system have critical functions in attacking host cells either directly or by being essential for host colonization. The ABC-dependent T1SS transports various molecules, from ions, drugs, to proteins of various sizes up to 900 kDa. The molecules secreted vary in size from the small Escherichia coli peptide colicin V, (10 kDa) to the Pseudomonas fluorescens cell adhesion protein LapA of 520 kDa. The best characterized are the RTX toxins such as the adenylate cyclase (CyaA) toxin from Bordetella pertussis, the causative agent of whooping cough, and the lipases such as LipA. Type I secretion is also involved in export of non-protein substrates such as cyclic beta-glucans and polysaccharides.
Comment:All members of this ABC transporter family contain 6 alpha-helices in each transmembrane domain (TMD) for a total of 12 TM segments in the complete transporter.
Jiyao W et al.(2023). "The conserved domain database in 2023.",