1. Department of Animal & Avian Sciences and Department of Cell Biology & Molecular Genetics, University of Maryland, College Park, Maryland 20742, USA
2. Division of Hematology, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA
3. Department of Pathology, Children's Hospital Boston, Boston, Massachusetts 02115, USA
4. National Centre for Biological Sciences, Tata Institute of Fundamental Research, UAS-GKVK campus, Bangalore 560 065, India
5. Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA

Correspondence to: Iqbal Hamza¹ Correspondence and requests for materials should be addressed to I.H. (Email: hamza@umd.edu).

Haems are metalloporphyrins that serve as prosthetic groups for various biological processes including respiration, gas sensing, xenobiotic detoxification, cell differentiation, circadian clock control, metabolic reprogramming and microRNA processing^{1, 2, 3, 4}. With a few exceptions, haem is synthesized by a multistep biosynthetic pathway comprising defined intermediates that are highly conserved throughout evolution⁵. Despite our extensive knowledge of haem biosynthesis and degradation, the cellular pathways and molecules that mediate intracellular haem trafficking are unknown. The experimental setback in identifying haem trafficking pathways has been the inability to dissociate the highly regulated cellular synthesis and degradation of haem from intracellular trafficking events⁶. Caenorhabditis elegans and related helminths are natural haem auxotrophs that acquire environmental haem for incorporation into haemoproteins, which have vertebrate orthologues⁷. Here we show, by exploiting this auxotrophy to identify HRG-1 proteins in C. elegans, that these proteins are essential for haem homeostasis and normal development in worms and vertebrates. Depletion of hrg-1, or its paralogue hrg-4, in worms results in the disruption of organismal haem sensing and an abnormal response to haem analogues. HRG-1 and HRG-4 are previously unknown transmembrane proteins, which reside in distinct intracellular compartments. Transient knockdown of hrg-1 in zebrafish leads to hydrocephalus, yolk tube malformations and, most strikingly, profound defects in erythropoiesis—phenotypes that are fully rescued by worm HRG-1. Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1. These findings reveal conserved pathways for cellular haem trafficking in animals that define the model for eukaryotic haem transport. Thus, uncovering the mechanisms of haem transport in C. elegans may provide insights into human disorders of haem metabolism and reveal new drug targets for developing anthelminthics to combat worm infestations.

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