Abstract
Cellular heme is thought to be distributed between a pool of sequestered heme that is tightly bound within hemeproteins and a labile heme pool required for signaling and transfer into proteins. A heme chaperone that can hold and allocate labile heme within cells has long been proposed but never been identified. Here, we show that the glycolytic protein glyceraldehyde-3-phosphate dehydrogenase (GAPDH) fulfills this role by acting as an essential repository and allocator of bioavailable heme to downstream protein targets. We identified a conserved histidine in GAPDH that is needed for its robust heme binding both in vitro and in mammalian cells. Substitution of this histidine, and the consequent decreases in GAPDH heme binding, antagonized heme delivery to both cytosolic and nuclear hemeprotein targets, including inducible nitric-oxide synthase (iNOS) in murine macrophages and the nuclear transcription factor Hap1 in yeast, even though this GAPDH variant caused cellular levels of labile heme to rise dramatically. We conclude that by virtue of its heme-binding property, GAPDH binds and chaperones labile heme to create a heme pool that is bioavailable to downstream proteins. Our finding solves a fundamental question in cell biology and provides a new foundation for exploring heme homeostasis in health and disease.
Highlights
Cellular heme is thought to be distributed between a pool of sequestered heme that is tightly bound within hemeproteins and a labile heme pool required for signaling and transfer into proteins
We considered three highly-conserved His in human glyceraldehyde3-phosphate dehydrogenase (GAPDH)
Intracellular heme trafficking is a fundamental process in biology, but the transport and delivery pathways and their reg
Summary
That GAPDH binds heme that is naturally generated by mitochondria in cells and that its heme-binding function is the essential aspect that enables downstream heme deliveries, rather than any other property of GAPDH itself. Our work reveals that GAPDH acts as a middleman to shuttle labile heme, in effect making it bioavailable to target proteins that reside outside the mitochondria in mammalian cells and yeast
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