title: Topologically conserved residues direct heme transport in HRG-1-related proteins.
abstract: Caenorhabditis elegans and human HRG-1-related proteins are conserved, membrane-bound permeases that bind and translocate heme in metazoan cells via a currently uncharacterized mechanism. Here, we show that cellular import of heme by HRG-1-related proteins from worms and humans requires strategically located amino acids that are topologically conserved across species. We exploit a heme synthesis-defective Saccharomyces cerevisiae mutant to model the heme auxotrophy of C. elegans and demonstrate that, under heme-deplete conditions, the endosomal CeHRG-1 requires both a specific histidine in the predicted second transmembrane domain (TMD2) and the FARKY motif in the C terminus tail for heme transport. By contrast, the plasma membrane CeHRG-4 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 TMD2 of CeHRG-4. An analogous system exists in humans, because mutation of the synonymous histidine in TMD2 of hHRG-1 eliminates heme transport activity, implying an evolutionary conserved heme transport mechanism that predates vertebrate origins. Our 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. These findings may provide a framework for understanding the structural basis of heme transport in eukaryotes and human parasites, which rely on host heme for survival.
authors.affiliation: Department of Animal & Avian Sciences and Department of Cell Biology & Molecular Genetics, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Xiaojing
authors.initials: X
authors.lastname: Yuan
authors.forename: Olga
authors.initials: O
authors.lastname: Protchenko
authors.forename: Caroline C
authors.initials: CC
authors.lastname: Philpott
authors.forename: Iqbal
authors.initials: I
authors.lastname: Hamza
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D001693:Biological Transport, Active / Q000502:physiology; D017173:Caenorhabditis elegans / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D029742:Caenorhabditis elegans Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006420:Hemeproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008954:Models, Biological; D009154:Mutation; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: D029742:Caenorhabditis elegans Proteins; D002352:Carrier Proteins; D006420:Hemeproteins; C554828:Hrg-1 protein, C elegans; C546602:SLC48A1 protein, human; D006418:Heme