2.A.110 The Heme Transporter, Heme-Responsive Gene Protein (HRG) Family
Caenorhabditis elegans and human HRG-1-related proteins are conserved, membrane-bound permeases that bind and translocate heme in metazoan cells. Yuan et al. (2012) showed that cellular import of heme by HRG-1-related proteins from worms and humans require strategically located amino acids that are topologically conserved across species. The endosomal CeHRG-1 required both a specific histidine in the predicted second TMS and the FARKY motif in the C-terminal tail for heme transport. By contrast, the plasma membrane CeHRG-4 protein transports heme by utilizing a histidine in the exoplasmic (E2) loop and the FARKY motif. Optimal activity under heme limiting conditions, however, requires histidine in the E2 loop of CeHRG-1 and tyrosine in TMS2 of CeHRG-4. An analogous system exists in humans because mutation of the synonymous histidine in TMS2 of hHRG-1 eliminates heme transport activity, implying an evolutionarily conserved heme transport mechanism that predates vertebrate origins. These results support a model in which heme is translocated across membranes facilitated by conserved amino acids positioned on the exoplasmic, cytoplasmic, and transmembrane regions of HRG-1-related proteins (Yuan et al. 2012).
Adult humans have about 25 trillion red blood cells (RBCs), and each second they recycle about 5 million RBCs by erythrophagocytosis (EP) in macrophages of the reticuloendothelial system. White et al (2013) showed that the mammalian homolog of HRG1, a transmembrane heme permease in C. elegans, is essential for macrophage iron homeostasis and transports heme from the phagolysosome to the cytoplasm during EP. HRG1 is strongly expressed in macrophages of the reticuloendothelial system and specifically localizes to the phagolysosomal membranes during EP. Depletion of Hrg1 in mouse macrophages caused attenuation of heme transport from the phagolysosomal compartment, and missense polymorphisms in human HRG1 proved to be defective in heme transport. The results revealed that human HRG1 is the long-sought heme transporter for heme-iron recycling in macrophages and suggested that genetic variations in HRG1 could be modifiers of human iron metabolism.
The reaction catalyzed by HRG1 is:
heme (lysosome) → heme (cytoplasm)