Hemes are metalloporphyrins used by nearly all organisms as protein cofactors for energy production, binding and sensing gases, and as a catalyst for various reactions. While heme biosynthesis has been well-characterized, the pathways for transporting heme between cells and within a cell remain poorly understood. C. elegans serves as a unique animal model for uncovering these pathways, as it is unable to synthesize its own heme and depends on the uptake of dietary heme for growth and reproduction. Functional RNAi screens implicated
mrp-5 as a potential heme transporter in C. elegans. This gene encodes a membrane-bound ABC transporter that is highly expressed in the worm intestine and is transcriptionally regulated by heme. Worms lacking
mrp-5 were unable to lay viable eggs, a phenotype that was fully rescued by dietary heme supplementation. Depletion of
mrp-5 activated a number heme deprivation signals within the worm, including activation of the
hrg-1 and
hrg-2 promoters. Furthermore,
mrp-5 RNAi worms accumulated the fluorescent heme analog, zinc mesoporphyrin, in intestinal cells, indicating a defect in heme export from the intestine. Indeed, GFP-tagged MRP-5 localizes to the basolateral intestinal membrane, and intestinal-specific knockdown of
mrp-5 recapitulates the embryonic lethality of whole animal
mrp-5 RNAi. Functional assays in yeast support the hypothesis that MRP-5 is capable of exporting heme across cell membranes. Expression of MRP-5 in a yeast strain deficient for heme synthesis resulted in decreased growth in the presence of exogenous heme. Consistent with the reduced growth, MRP-5 expression in yeast lowered heme levels in the cytosol with a concomitant increase in heme levels in the secretory pathway, suggesting a role for MRP-5 in transporting heme from the cytosol into the secretory compartment. Altogether, our results implicate MRP-5 as a key regulator of systemic heme homeostasis in C. elegans that is required for heme export from the intestine to other tissues.