This family represents the ABC component of the protease secretion system PrtD, a 60-kDa integral membrane protein sharing 37% identity with HlyB, the ABC component of the alpha-hemolysin secretion pathway, in the C-terminal domain. They export degradative enzymes by using a type I protein secretion system and lack an N-terminal signal peptide, but contain a C-terminal secretion signal. The Type I secretion apparatus is made up of three components, an ABC transporter, a membrane fusion protein (MFP), and an outer membrane protein (OMP). For the HlyA transporter complex, HlyB (ABC transporter) and HlyD (MFP) reside in the inner membrane of E. coli. The OMP component is TolC, which is thought to interact with the MFP to form a continuous channel across the periplasm from the cytoplasm to the exterior. HlyB belongs to the family of ABC transporters, which are ubiquitous, ATP-dependent transmembrane pumps or channels. The spectrum of transport substrates ranges from inorganic ions, nutrients such as amino acids, sugars, or peptides, hydrophobic drugs, to large polypeptides, such as HlyA.
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