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N-terminal acetylome analyses reveals the substrate specificity profile of Naa50 (Nat5) and implicate NATs in influencing initiator Methionine processing

ABSTRACT: Co-translational N-terminal (Nt-) acetylation of nascent polypeptides is catalyzed by N-terminal acetyltransferases (NATs). The very N-terminal amino acid sequence is the major factor determining whether or not a given protein is Nt-acetylated. In humans, six different NATs, denoted NatA-NatF, are identified. In the current study we used N-terminal COFRADIC analysis to define the in vivo substrate specificity of Naa50 (Nat5)/NatE as this has long remained elusive. Three yeast strains were generated; a control strain endogenously expressing yNaa50, a deletion strain depleted of yNaa50 and a strain deleted of yNaa50 but ectopically expressing human Naa50. When comparing the Nt-acetylation status of different N-termini in the control strain with the deletion strain, a reduction in Nt-acetylation for several yeast proteins was observed. To our surprise, these substrates were not of the predicted NatE-type substrates, but rather canonical NatA substrates. Further, ectopic expression of hNaa50 mainly resulted in the Nt-acetylation of a selected class of otherwise Nt-free yeast N-termini besides increasing the degree of Nt-acetylation of several other yeast proteins, and as such (partially) complementing those N-termini displaying reduced Nt-acetylation upon yNAA50-deletion. The preferred substrates of hNaa50 were predominantly Met-starting N-termini including Met-Lys-, Met-Val-, Met-Ala-, Met-Tyr-, Met-Phe-, Met-Leu-, Met-Ser- and Met-Thr, and highly overlapped with the previously identified human Naa60/NatF substrate specificity profile. Identification of several hNaa50 substrates with a small amino acid in the second position also revealed a potential interplay between the NATs and methionine aminopeptidases (MetAPs). The initiator Methionine (iMet) is normally cleaved off by MetAPs when the second amino acid is small, but our in vitro data suggest that in contrast to a free iMet, an Nt-acetylated iMet is not hydrolyzed by MetAPs. Thus, Naa50-mediated Nt-acetylation may potentially act as a mechanism to retain the iMet of proteins with a small amino acid at the second position that normally would be hydrolyzed.

INSTRUMENT(S): LTQ Orbitrap

ORGANISM(S): Saccharomyces Cerevisiae (baker's Yeast)

SUBMITTER: Petra Van Damme

LAB HEAD: Petra Van Damme

PROVIDER: PXD001437 | Pride | 2015-05-29

REPOSITORIES: Pride

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N-terminal acetylome analysis reveals the specificity of Naa50 (Nat5) and suggests a kinetic competition between N-terminal acetyltransferases and methionine aminopeptidases.

Van Damme Petra P Hole Kristine K Gevaert Kris K Arnesen Thomas T

Proteomics 20150605 14

Cotranslational N-terminal (Nt-) acetylation of nascent polypeptides is mediated by N-terminal acetyltransferases (NATs). The very N-terminal amino acid sequence largely determines whether or not a given protein is Nt-acetylated. Currently, there are six distinct NATs characterized, NatA-NatF, in humans of which the in vivo substrate specificity of Naa50 (Nat5)/NatE, an alternative catalytic subunit of the human NatA, so far remained elusive. In this study, we quantitatively compared the Nt-acet ...[more]

PMID: 25886145

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Project description:Abstract: N-terminal acetylation (Nt-acetylation) occurs on the majority of eukaryotic proteins and is catalysed by N-terminal acetyltransferases (NATs). Nt- acetylation is increasingly recognized as a vital modification with functional implications ranging from protein degradation to protein localization. Very recently, the first human X-linked genetic disorder caused by a mutation in a NAT gene was reported; Ogden syndrome boys harbour a p.Ser37Pro variant in the gene encoding Naa10, the catalytic subunit of the NatA complex, and suffer from global developmental delays and lethality during infancy. Here, we developed a Saccharomyces cerevisiae model by introducing the human wildtype or mutant NatA complex into yeast lacking NatA (NatA-∆). The human NatA complex phenotypically complemented the NatA-∆ strain, while only a partial rescue was observed for the Ogden mutant NatA complex suggesting that hNaa10-S37P is only partially functional in vivo. Furthermore, we performed quantitative Nt-acetylome analyses on a control yeast strain (yNatA), a yeast NatA deletion strain (yNatA-∆), a yeast NatA deletion strain expressing human NatA (hNatA), and a yeast NatA deletion strain expressing mutant human NatA (hNatA-hNaa10-S37P). Interestingly, a reduced degree of Nt-acetylation was specifically observed among a large group of NatA substrates in the yeast expressing mutant hNatA as compared to yeast expressing wildtype hNatA. Furthermore, immunoprecipitated mutant NatA complex displayed a reduced catalytic activity in vitro as compared to the wildtype NatA complex. Combined, these data provide strong support for the functional impairment of hNaa10-S37P in vivo and suggest that reduced Nt-acetylation of one or more target substrates contributes to the pathogenesis of Ogden syndrome. Comparative analysis between human and yeast NatA also provided novel insights into the co-evolution of the NatA complexes and their substrates. For instance, (Met-)Ala- N- termini are more prevalent in the human proteome as compared to the yeast proteome, and hNatA displays a relative preference towards these N-termini as compared to yNatA. Methods: The obtained peptide mixtures were introduced into an LC-MS/MS system, the Ultimate 3000 (Dionex, Amsterdam, The Netherlands) in-line connected to an LTQ Orbitrap XL (Thermo Fisher Scientific, Bremen, Germany). Samples were first loaded on a trapping column (made in-house, 100 µm internal diameter (I.D.) x 20 mm, 5 µm beads C18 Reprosil-HD, Dr. Maisch). After back-flushing from the trapping column, the sample was loaded on a reverse-phase column (made in- house, 75 µm I.D. x 150 mm , 5 µm beads C18 Reprosil-HD, Dr. Maisch). Peptides were loaded with solvent A (0.1% trifluoroacetic acid, 2% acetonitrile), and were separated with a linear gradient from 2% solvent A’ (0.05% formic acid) to 55% solvent B’ (0.05% formic acid and 80% acetonitrile) at a flow rate of 300 nl/min followed by a wash reaching 100% solvent B’. The mass spectrometer was operated in data-dependent mode, automatically switching between MS and MS/MS acquisition for the six most abundant peaks in a given MS spectrum. Full scan MS spectra were acquired in the Orbitrap at a target value of 1E6 with a resolution of 60,000. The six most intense ions were then isolated for fragmentation in the linear ion trap, with a dynamic exclusion of 60 s. Peptides were fragmented after filling the ion trap at a target value of 1E4 ion counts. From the MS/MS data in each LC run, Mascot Generic Files were created using the Mascot Distiller software (version 2.3.01, Matrix Science). While generating these peak lists, grouping of spectra was allowed with a maximum intermediate retention time of 30 s and a maximum intermediate scan count of 5 was used where possible. Grouping was done with 0.005 Da precursor tolerance. A peak list was only generated when the MS/MS spectrum contained more than 10 peaks. There was no de-isotoping and the relative signal to noise limit was set at 2. These peak lists were then searched with the Mascot search engine (Matrix Science) using the Mascot Daemon interface (version 2.3, Matrix Science). Spectra were searched against the yeast (S. cerevisiae) Swiss-Prot database (version 2011_03 of the UniProtKB/Swiss-Prot protein database containing 7,320 yeast sequence entries (525997 sequences in total)). )). 13C2D3- [(13C2)-trideutero-acetylation] at lysines, carbamidomethylation of cysteine and methionine oxidation to methionine-sulfoxide were set as fixed modifications for the N-terminal COFRADIC analyses. Variable modifications were 13C2D3- acetylation and acetylation of protein N-termini. Pyroglutamate formation of N-terminal glutamine was additionally set as a variable modification. Mass tolerance on precursor ions was set to 10 ppm (with Mascot’s C13 option set to 1) and on fragment ions to 0.5 Da. Endoproteinase semi-Arg-C/P (Arg-C specificity with arginine-proline cleavage allowed) was set as enzyme allowing no missed cleavages. The peptide charge was set to 1+, 2+, 3+ and instrument setting was put on ESI-TRAP. Only peptides that were ranked one and scored above the threshold score, set at 99% confidence, were withheld. The estimated false discovery rate by searching decoy databases was typically found to lie between 2 and 4% on the spectrum level [33]. Quantification of the degree of Nt-Ac was performed as described previously (5). All data management was done in ms_lims ([49]). Please note!! the pride result files will show a lot of delta m/z deviations. The modification for the heavy acetylation (+47 Da) used in this experiment is not in Pride. So another heavy acetylation (+45 Da) modification was annotated when creating the xml files.

Dataset Information

N-terminal acetylome analyses reveals the substrate specificity profile of Naa50 (Nat5) and implicate NATs in influencing initiator Methionine processing

Publications

N-terminal acetylome analysis reveals the specificity of Naa50 (Nat5) and suggests a kinetic competition between N-terminal acetyltransferases and methionine aminopeptidases.

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