HlyD is a component of the prototypical alpha-haemolysin (HlyA) bacterial type I secretion system, along with the other components HlyB and TolC. HlyD and HlyB are inner-membrane proteins and specific components of the transport apparatus of alpha-haemolysin. HlyD is anchored in the cytoplasmic membrane by a single transmembrane domain and has a large periplasmic domain within the carboxy-terminal 100 amino acids, HlyB and HlyD form a stable complex that binds the recombinant protein bearing a C-terminal HlyA signal sequence and ATP in the cytoplasm. HlyD, HlyB and TolC combine to form the three-component ABC transporter complex that forms a trans-membrane channel or pore through which HlyA can be transferred directly to the extracellular medium. Cutinase has been shown to be transported effectively through this pore.
Lu S et al.(2020). "CDD/SPARCLE: the conserved domain database in 2020.",