Molecular role of NAA38 in thermostability and catalytic activity of the human NatC N-terminal acetyltransferase

N-terminal acetylation occurs on over 80% of human proteins and is catalyzed by a family of N-terminal ace-tyltransferases (NATs). All NATs contain a small catalytic subunit, while some also contain a large auxiliary subunit that facilitates catalysis and ribosome targeting for co-translational acetylation. NatC is one of the major NATs containing an NAA30 catalytic subunit, but uniquely contains two auxiliary subunits, large NAA35 and small NAA38. Here, we report the cryo-EM structures of human NatC (hNatC) complexes with and without NAA38, together with biochemical studies, to reveal that NAA38 increases the thermostability and broadens the substrate-specificity profile of NatC by ordering an N-terminal segment of NAA35 and re-orienting an NAA30 N-terminal peptide binding loop for optimal catalysis, respectively. We also note impor-tant differences in engagement with a stabilizing inositol hexaphosphate molecule between human and yeast NatC. These studies provide new insights for the function and evolution of the NatC complex.

Automated summary

N-terminal acetylation, catalyzed by N-terminal acetyltransferases (NATs), occurs on over 80% of human proteins. NatC is a major NAT that contains NAA30 catalytic subunit and uniquely has two auxiliary subunits, NAA35 and NAA38. The cryo-EM structures of human NatC complexes with and without NAA38, along with biochemical studies, reveal that NAA38 increases thermostability and broadens substrate-specificity of NatC by ordering an N-terminal segment of NAA35 and re-orienting an NAA30 N-terminal peptide binding loop for optimal catalysis. Differences in engagement with a stabilizing inositol hexaphosphate molecule between human and yeast NatC are also noted. These findings provide new insights into the function and evolution of the NatC complex.