GAPDH heme delivery to Indoleamine 2,3-dioxygenase 1 involves complex formation and complementary charge pairing at their protein-protein interface.

In eukaryotes the last steps of heme biosynthesis occur in mitochondria and so heme must be transported to reach many heme-dependent proteins that mature and function outside this organelle. Although the enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH) has emerged as a key intracellular heme chaperone, how it performs heme deliveries to its numerous clients is poorly understood. It is unknown if handoffs of the GAPDH-bound heme require that it make direct contact with its clients or instead involve GAPDH passing its heme to middlemen proteins to execute the final heme transfers. To address this question, we studied GAPDH heme transfer to the client protein indoleamine dioxygenase 1 (IDO1), whose activity is heme-dependent and regulates mammalian immune responses and cancer progression. A chemical crosslinking-mass spectrometry approach identified two Lys residues that formed an inter-protein crosslink across a previously uncharacterized GAPDH-IDO1 interface. This guided our building a model of the GAPDH-IDO1 complex so we could interrogate by point mutagenesis the role of the GAPDH-IDO1 contact in enabling delivery of GAPDH heme to IDO1. We characterized behaviors of the GAPDH and IDO1 variants in purified form and when expressed in the HEK293T human cell line. This revealed GAPDH heme transfer to IDO1 in cells requires that they make a direct contact which relies on a specific Lys-Asp charge pairing interaction forming across the complex interface. These findings illuminate a key step in the maturation of functional IDO1 and improve our understanding of how GAPDH may perform its heme trafficking function in mammals.