Nicotinamide/nicotinate-nucleotide adenylyltransferase that acts as an axon maintenance factor (PubMed:20126265, PubMed:23082226). Catalyzes the formation of NAD(+) from nicotinamide mononucleotide (NMN) and ATP (By similarity). Can also use the deamidated form; nicotinic acid mononucleotide (NaMN) as substrate but with a lower efficiency (By similarity). Cannot use triazofurin monophosphate (TrMP) as substrate (By similarity). Also catalyzes the reverse reaction, i.e. the pyrophosphorolytic cleavage of NAD(+) (By similarity). For the pyrophosphorolytic activity prefers NAD(+), NADH and NaAD as substrates and degrades nicotinic acid adenine dinucleotide phosphate (NHD) less effectively (By similarity). Fails to cleave phosphorylated dinucleotides NADP(+), NADPH and NaADP(+) (By similarity). Axon survival factor required for the maintenance of healthy axons: acts by delaying Wallerian axon degeneration, an evolutionarily conserved process that drives the loss of damaged axons (PubMed:20126265, PubMed:23082226, PubMed:25818290$). ATP + beta-nicotinamide D-ribonucleotide + H(+) = diphosphate + NAD(+) ATP + H(+) + nicotinate beta-D-ribonucleotide = deamido-NAD(+) + diphosphate Divalent metal cations. Mg(2+) confers the highest activity. Inhibited by P1-(adenosine-5')-P3-(nicotinamide-riboside-5')-triphosphate (Np3AD) and P1-(adenosine-5')-P4-(nicotinamide-riboside-5')-tetraphosphate (Np4AD). Cofactor biosynthesis; NAD(+) biosynthesis; NAD(+) from nicotinamide D-ribonucleotide: step 1/1. Cofactor biosynthesis; NAD(+) biosynthesis; deamido-NAD(+) from nicotinate D-ribonucleotide: step 1/1. Monomer. Delivered to axons with Golgi-derived cytoplasmic vesicles. Expressed predominantly in the brain and nervous system. Degraded in response to injured neurite (PubMed:20126265, PubMed:23665224, PubMed:23610559). Degradation is probably caused by ubiquitination by MYCBP2 (PubMed:23665224, PubMed:23610559). Ubiquitinated on threonine and/or serine residues by MYCBP2; consequences of threonine and/or serine ubiquitination are however unclear (By similarity). Palmitoylated; palmitoylation is required for membrane association. Perinatal lethality (PubMed:23082226). Mice show a greatly distended bladder, underdeveloped diaphragm and a reduction in total skeletal muscle mass (PubMed:23082226). Defects are caused by defects in innervation of major organs and tissues (PubMed:23082226). Mice lacking both Sarm1 and Nmnat2 are viable and survive: Sarm1 deficiency corrects axon outgrowth in mice lacking Nmnat2, independently of NMNAT metabolites, preventing perinatal lethality (PubMed:25818290). Belongs to the eukaryotic NMN adenylyltransferase family.