Expression, localization, and biochemical characterization of nicotinamide mononucleotide adenylyltransferase 2.

Nicotinamide mononucleotide (NMN) adenylyltransferase 2 (Nmnat2) catalyzes the synthesis of NAD from NMN and ATP. The Nmnat2 transcript is expressed predominately in the brain; we report here that Nmnat2 is a low abundance protein expressed in neurons. Previous studies indicate that Nmnat2 localizes to Golgi. As Nmnat2 is not predicted to contain a signal sequence, lipid-binding domain, or transmembrane domain, we investigated the nature of this interaction. These experiments reveal that Nmnat2 is palmitoylated in vitro, and this modification is required for membrane association. Surprisingly, exogenous Nmnat2 is toxic to neurons, indicating that protein levels must be tightly regulated. To analyze Nmnat2 localization in neurons (previous experiments relied on exogenous expression in HeLa cells), mouse brains were fractionated, showing that Nmnat2 is enriched in numerous membrane compartments including synaptic terminals. In HeLa cells, in addition to Golgi, Nmnat2 localizes to Rab7-containing late endosomes. These studies show that Nmnat2 is a neuronal protein peripherally attached to membranes via palmitoylation and suggest that Nmnat2 is transported to synaptic terminals via an endosomal pathway.