Catalyzes the formation of NAD(+) from nicotinamide mononucleotide (NMN) and ATP (PubMed:16118205, PubMed:17402747, PubMed:26616331). Can also use the deamidated form; nicotinic acid mononucleotide (NaMN) as substrate with the same efficiency. Can use triazofurin monophosphate (TrMP) as substrate. Can also use GTP and ITP as nucleotide donors. Also catalyzes the reverse reaction, i.e. the pyrophosphorolytic cleavage of NAD(+). For the pyrophosphorolytic activity, can use NAD(+), NADH, NaAD, nicotinic acid adenine dinucleotide phosphate (NHD), nicotinamide guanine dinucleotide (NGD) as substrates. Fails to cleave phosphorylated dinucleotides NADP(+), NADPH and NaADP(+). Protects against axonal degeneration following injury (PubMed:16118205, PubMed:17402747). May be involved in the maintenance of axonal integrity (By similarity). Also functions as a stress-response chaperone protein that prevents toxic aggregation of proteins; this function may be independent of its NAD(+) synthesis activity (PubMed:18344983). {ECO:0000250|UniProtKB:Q99JR6, ECO:0000269|PubMed:16118205, ECO:0000269|PubMed:17402747, ECO:0000269|PubMed:18344983, ECO:0000269|PubMed:26616331}. This is the function of ENSG00000163864 (NMNAT3, nicotinamide nucleotide adenylyltransferase 3).