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

Abstract

<div><p>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 <i>rbp7a</i> function and regulation. We find that early embryonic <i>rbp7a</i> expression is negatively regulated by the Nodal/FoxH1-signaling pathway and we show that Nodal/FoxH1 activity has the opposite effect on <i>aldh1a2</i>, 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 <i>nmnat1-rbp7</i> transcript encoding a fusion of Rbp7 and the NAD⁺ (<i>Nicotinamide adenine dinucleotide</i>) synthesizing enzyme Nmnat1. We show that <i>nmnat1-rbp7</i> is conserved in fish, mouse and chicken, and that in zebrafish regulation of <i>nmnat1-rbp7a</i> is distinct from that of <i>rbp7a</i> and <i>nmnat1</i>. 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.</p></div