3.A.1.122.11
The hemin resistance transporter, HrtAB. Expression is activated by hemin or hemoglobin via the ChrAS transmembrane sensor kinase/response regulator system (Bibb and Schmitt 2010). HrtBA extracts heme from the membrane and expells it.  HrtBA consists of two cytoplasmic HrtA ATPase subunits and two transmembrane HrtB permease subunits. A heme-binding site is formed in the HrtB dimer and is laterally accessible to heme in the outer leaflet of the membrane. The heme-binding site captures heme from the membrane using a glutamate residue of either subunit as an axial ligand and sequesters the heme within the rearranged transmembrane helix bundle. By ATP-driven HrtA dimerization, the heme-binding site is squeezed to extrude the bound heme (Nakamura et al. 2022).