NAA50

NAA50 (Naa50/NatE) is a co-translationally acting Nα-terminal acetyltransferase that modifies the N-termini of nascent proteins retaining their initiator methionine, with a defined substrate preference for iMet followed by small or hydrophobic residues (iMet-Lys/Val/Ala/Tyr/Phe/Leu/Ser/Thr; optimal Met-Met) (PMID:25886145, PMID:27484799). Crystallographic and kinetic analyses across yeast, plant, and human orthologs establish its GNAT active site and catalytic residues, and show that NAA50 acetyltransferase activity is required for its in vivo function (PMID:33400917, PMID:32461302). NAA50 docks onto the NatA complex via conserved contacts to both the Naa10 and Naa15 subunits to form NatE, an association that promotes catalytic crosstalk in the human complex (PMID:31155310). By acetylating iMet-starting termini, NAA50 kinetically competes with methionine aminopeptidases to determine iMet retention versus cleavage (PMID:25886145). Beyond protein-N-terminal acetylation, NAA50 controls sister-chromatid cohesion: it is required for the cohesin-sororin interaction in human cells and antagonizes NatA in this role, and in Drosophila it N-terminally acetylates nascent Scc1 to support proper Scc1-Smc3 cohesin assembly (PMID:27422821, PMID:27996020). NAA50 also stabilizes specific substrates by N-terminal acetylation, acetylating the splicing factor SLU7 to protect it from ubiquitin-proteasomal degradation and thereby driving cisplatin resistance in bladder cancer cells (PMID:42151695). Small-molecule inhibitors selective for NAA50 over Naa10 and Naa60 have been defined structurally and shown to engage the enzyme in cells (PMID:32550998).

1. 2015 High

   Established that human Naa50 has a distinct substrate logic among NATs by defining its preference for iMet-retaining N-termini and showing it kinetically gates methionine excision.

   Evidence Quantitative N-terminal acetylome profiling in yeast expressing human Naa50 plus in vitro MetAP cleavage assays

   PMID:25886145

   Open questions at the time

   • Did not resolve which physiological substrates depend on iMet retention
   • Cellular consequences of MetAP competition not measured
2. 2016 Medium

   Connected NAA50 to a chromosome-segregation function distinct from generic Nt-acetylation, showing it is required for cohesin-sororin interaction and antagonizes NatA in cohesion.

   Evidence siRNA knockdown, Co-IP, mitotic arrest assays, and co-depletion epistasis in HeLa cells with in vitro NAT assays

   PMID:27422821

   Open questions at the time

   • Direct acetylation substrate underlying cohesion defect not pinned down in human cells
   • Mechanism of NatA antagonism unresolved
3. 2016 Medium

   Provided a candidate substrate for the cohesion role by showing Naa50/San co-translationally acetylates nascent Scc1 to enable Scc1-Smc3 cohesin assembly.

   Evidence Genetic epistasis and Co-IP of cohesin subunits in Naa50/San-depleted Drosophila

   PMID:27996020

   Open questions at the time

   • Site of Scc1 acetylation not mapped
   • Whether the same mechanism operates in human cohesion not shown
4. 2016 High

   Defined the structural basis of human Naa50 substrate selection, identifying Met-Met as optimal and capturing CoA and acetylated peptide products.

   Evidence X-ray crystallography of human Naa50 with a peptide library screen and thermal stability assays

   PMID:27484799

   Open questions at the time

   • Did not address regulation within the NatE complex
   • Physiological substrate set not enumerated
5. 2019 High

   Resolved how Naa50 integrates with NatA, showing conserved contacts to both Naa10 and Naa15 that drive catalytic crosstalk in the human complex.

   Evidence X-ray crystallography of the yeast NatA/Naa50 complex with comparative in vitro activity assays of yeast versus human complexes

   PMID:31155310

   Open questions at the time

   • Functional consequence of crosstalk for substrate selection in cells not established
   • Why yeast Naa50 activity is compromised mechanistically incomplete
6. 2020 High

   Demonstrated that catalytic activity, not merely complex association, is required for NAA50 function in vivo and that NatE substrate specificity is conserved across kingdoms.

   Evidence In vitro acetyltransferase assays with recombinant AtNAA50, E. coli acetylome profiling, and genetic complementation in Arabidopsis naa50 mutants

   PMID:32461302

   Open questions at the time

   • Significance of the in vitro lysine-autoacetyltransferase activity unclear
   • Plant-specific substrates not defined
7. 2020 High

   Delivered selective chemical tools for NAA50 by identifying inhibitors with defined binding modes and cellular target engagement, distinguishing it from related NATs.

   Evidence DNA-encoded library screening, co-crystal structures, biochemical inhibition assays, and cellular target engagement

   PMID:32550998

   Open questions at the time

   • Phenotypic effects of inhibition on cohesion or substrate stability not characterized in this study
8. 2021 High

   Defined the active-site architecture and catalytic residues of Naa50 using a bisubstrate analog, while confirming yeast Naa50 is inactive yet retains CoA binding.

   Evidence X-ray crystallography of AtNaa50-AcCoA and bisubstrate complexes with kinetics and active-site mutagenesis

   PMID:33400917

   Open questions at the time

   • Structural basis of substrate scope variation between species not fully resolved
9. 2022 Medium

   Revealed lineage-specific adaptations, with fungal Naa50 carrying terminal extensions that mediate DLC1 binding and NatA-independent ribosome association without forming NatE.

   Evidence X-ray crystallography of CtNaa50 with biochemical binding and ribosome-association assays

   PMID:36142717

   Open questions at the time

   • Functional role of DLC1 interaction unknown
   • Whether human Naa50 has analogous NatA-independent ribosome recruitment not addressed
10. 2023 Low

    Reported a possible second enzymatic activity, showing recombinant human Naa50 acetylates serotonin in vitro.

    Evidence In vitro SNAT enzyme assay with purified recombinant hNaa50

    PMID:36829878

    Open questions at the time

    • Single in vitro assay without independent replication
    • Physiological relevance of SNAT activity in human cells not established
    • High Km raises questions of in vivo significance
11. 2024 Medium

    Distinguished NAA50 function from NatA in vivo, showing that despite ribosomal NatA association, NAA50 negatively regulates plant immunity independently of NatA activity and salicylic acid.

    Evidence Split-luciferase proximity assay, interactome, transcriptome/proteome profiling, and pathogen resistance assays in Arabidopsis

    PMID:38588051

    Open questions at the time

    • Immunity-relevant NAA50 substrates not identified
    • Whether the immune role is conserved in animals unknown
12. 2026 Medium

    Identified a substrate-stabilization mechanism with disease relevance, showing NAA50 acetylates SLU7 to block its degradation and drive chemoresistance.

    Evidence Co-IP, MS, ubiquitination assays, RIP-qPCR, fractionation, xenografts, and pharmacological inhibition in bladder cancer cells

    PMID:42151695

    Open questions at the time

    • Acetylation site on SLU7 not mapped
    • Generality of N-terminal acetylation as a stabilization mechanism for other substrates not established

• It remains unresolved which endogenous human substrates link NAA50's Nt-acetyltransferase activity to its cohesion, immunity, and substrate-stabilization phenotypes, and whether its candidate second activities (SNAT, lysine autoacetylation) are physiologically operative.
• No comprehensive human in-cell substrate map
• Mechanistic link between cohesion role and specific acetylated substrate in humans missing
• Physiological status of non-canonical activities unclear