N-alpha-acetyltransferase 15, NatA auxiliary subunit also known as gastric cancer antigen Ga19 (GA19), NMDA receptor-regulated protein 1 (NARG1), and Tbdn100 is a protein that in humans is encoded by the NAA15 gene. NARG1 is the auxiliary subunit of the NatA (Nα-acetyltransferase A) complex. This NatA complex can associate with the ribosome and catalyzes the transfer of an acetyl group to the Nα-terminal amino group of proteins as they emerge from the exit tunnel. Gene and transcripts Human NAA15 is located on chromosome 4q31.1 and contains 23 exons. Initially, 2 mRNA species were identified, of size 4.6 and 5.8 kb, both harboring the same open reading frame encoding a putative protein of 866 amino acids (~105 kDa) protein that can be detected in most human adult tissues. According to RefSeq/NCBI, only one human transcript variant exists, although 2 more isoforms are predicted. In addition to full length Naa15, an N-terminally truncated variant of Naa15 (named tubedown-1), Naa15273-865 has been described; however, in mouse only full length Naa15 is widely expressed, whereas smaller transcripts seem to visualized only in heart and testis. In addition to this, a NAA15 gene duplication, NAA16, has been identified, and the encoded protein shares 70% sequence identity to hNaa15 and is expressed in a variety of human cell lines, but is generally less abundant as compared to hNaa15. Three isoforms of Naa16 are validated so far (NCBI RefSeq). Mouse NAA15 is located on chromosome 2 D and contains 20 exons, whereas mouse NAA16 is located on chromosome 14 D3 and consists of 21 exons. In principle, NatA can assemble from all the Naa10 and Naa15 isoforms in human and mouse, creating a more complex and flexible system for Nα-terminal acetylation as compared to lower eukaryotes. Structure The X-ray crystal structure of the holo-NatA complex (Naa10/Naa15) from S. pombe revealed that Naa15 is composed of 13 conserved helical bundle tetratricopeptide repeat (TPR) motifs and adopts a ring-like topology that wraps around the catalytic subunit of NatA, Naa10. This interaction induces conformational changes in the catalytic center of Naa10 that allows the acetylation of conventional NatA substrates. The crystal structure of human NatA bound to the protein HYPK has also been solved. Because TPR motifs mediate protein–protein interactions, it has been postulated that this domain may facilitate the interaction with other NatA-binding partners such as the ribosome and Naa50/NatE. Naa15 harbors a putative NLS between residues 612-628 (KKNAEKEKQQRNQKKKK); however,