Nmnat1-Rbp7 Is a Conserved Fusion-Protein That Combines NAD+ Catalysis of Nmnat1 with Subcellular Localization of Rbp7

Retinol binding proteins (Rbps) are known as carriers for transport and targeting of retinoids to their metabolizing enzymes. Rbps are also reported to function in regulating the homeostatic balance of retinoid metabolism, as their level of retinoid occupancy impacts the activities of retinoid metabolizing enzymes. Here we used zebrafish as a model to study rbp7a function and regulation. We find that early embryonic rbp7a expression is negatively regulated by the Nodal/FoxH1-signaling pathway and we show that Nodal/FoxH1 activity has the opposite effect on aldh1a2, which encodes the major enzyme for early embryonic retinoic acid production. The data are consistent with a Nodal-dependent coordination of the allocation of retinoid precursors to processing enzymes with the catalysis of retinoic acid formation. Further, we describe a novel nmnat1-rbp7 transcript encoding a fusion of Rbp7 and the NAD+ (Nicotinamide adenine dinucleotide) synthesizing enzyme Nmnat1. We show that nmnat1-rbp7 is conserved in fish, mouse and chicken, and that in zebrafish regulation of nmnat1-rbp7a is distinct from that of rbp7a and nmnat1. Injection experiments in zebrafish further revealed that Nmnat1-Rbp7a and Nmnat1 have similar NAD+ catalyzing activities but a different subcellular localization. HPLC measurements and protein localization analysis highlight Nmnat1-Rbp7a as the only known cytoplasmic and presumably endoplasmic reticulum (ER) specific NAD+ catalyzing enzyme. These studies, taken together with previously documented NAD+ dependent interaction of RBPs with ER-associated enzymes of retinal catalysis, implicate functions of this newly described NMNAT1-Rbp7 fusion protein in retinol oxidation.