Nα-terminal Acetylation of Eukaryotic Proteins

Abstract

NH2-terminal, Nα-terminal N-terminal acetyltransferase The two cotranslational processes, cleavage of N-terminal methionine residues and N-terminal1 acetylation, are by far the most common modifications, occurring on the vast majority of eukaryotic proteins. Studies with the yeast Saccharomyces cerevisiae revealed three N-terminal acetyltransferases, NatA, NatB, and NatC, that acted on groups of substrates, each containing degenerate motifs. Orthologous genes encoding the three N-terminal acetyltransferases and the patterns of N-terminal acetylation suggest that eukaryotes generally use the same systems for N-terminal acetylation. The biological significance of this N-terminal modification varies with the particular protein, with some proteins requiring acetylation for function, whereas others do not. Cleavage of N-terminal methionine residues is by far the most common modification, occurring on the vast majority of proteins. Proteins from prokaryotes, mitochondria, and chloroplasts initiate with formylmethionine, whereas proteins from the cytosol of eukaryotes initiate with methionine. The formyl group is usually removed from prokaryotic proteins by a deformylase, resulting in methionine at N termini. The methionine at N termini is cleaved from nascent chains of most prokaryotic and eukaryotic proteins. Results with altered iso-1-cytochromes c from yeast (1Tsunasawa S. Stewart J.W. Sherman F. J. Biol. Chem. 1985; 260: 5382-5391Abstract Full Text PDF PubMed Google Scholar) were the basis for the hypothesis that methionine is cleaved from penultimate residues having radii of gyration of 1.29 Å or less (glycine, alanine, serine, cysteine, threonine, proline, and valine) (2Sherman F. Stewart J.W. Tsunasawa S. Bioessays. 1985; 3: 27-31Crossref PubMed Scopus (230) Google Scholar), a hypothesis that was confirmed from the results of a complete set of altered iso-1 having all possible amino acids at the penultimate position (3Moerschell R.P. Hosokawa Y. Tsunasawa S. Sherman F. J. Biol. Chem. 1990; 265: 19638-19643Abstract Full Text PDF PubMed Google Scholar). A similar pattern of cleavage also was observed with prokaryotic systems in vivo (4Hirel H-P. Schmitter J-M. Dessen P. Fayat G. Blanquet S. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 8247-8251Crossref PubMed Scopus (668) Google Scholar, 5Dalbı̈ge H. Bayen S. Pedersen J. FEBS Lett. 1990; 266: 1-3Crossref PubMed Scopus (71) Google Scholar) and in vitro (6Ben-Bassat A. Bauer K. Chang S.Y. Myambo K. Boosman A. Chang S. J. Bacteriol. 1987; 169: 751-757Crossref PubMed Google Scholar, 7Wingfield P. Graber P. Turcatti G. Movva N.R. Pelletier M. Craig S. Rose K. Miller C.G. Eur. J. Biochem. 1989; 180: 23-32Crossref PubMed Scopus (32) Google Scholar), and other eukaryotic systems in vivo (8Huang S. Elliott R.C. Liu P.S. Koduri R.K. Weickmann J.L. Lee J.H. Blair L.C. Gosh-Dastidar P. Bradshaw R.A. Bryan K.M. Einarson B. Kendall R.L. Kolacs K.H. Saito K. Biochemistry. 1987; 26: 8242-8246Crossref PubMed Scopus (151) Google Scholar, 9Boissel J.P. Kasper T.J. Bunn H.F. J. Biol. Chem. 1988; 263: 8443-8449Abstract Full Text PDF PubMed Google Scholar) and in vitro (10Chang Y.-H. Teichert U. Smith J.A. J. Biol. Chem. 1990; 265: 19892-19897Abstract Full Text PDF PubMed Google Scholar). Only minor differences were observed between the quantitative results obtained in vivo with yeast iso-1-cytochrome cand with the two proteins, TimJp and TimLp, from Escherichia coli (3Moerschell R.P. Hosokawa Y. Tsunasawa S. Sherman F. J. Biol. Chem. 1990; 265: 19638-19643Abstract Full Text PDF PubMed Google Scholar, 4Hirel H-P. Schmitter J-M. Dessen P. Fayat G. Blanquet S. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 8247-8251Crossref PubMed Scopus (668) Google Scholar, 5Dalbı̈ge H. Bayen S. Pedersen J. FEBS Lett. 1990; 266: 1-3Crossref PubMed Scopus (71) Google Scholar). The lack of action of methionine aminopeptidase on proteins with large penultimate residues can now be explained by steric hindrance, as deduced from the high resolution crystal structure of the inhibitor complex of methionine aminopeptidase from E. coli(11Lowther W.T. Orville A.M. Madden D.T. Lim S. Rich D.H. Matthews B.W. Biochemistry. 1999; 38: 7678-7688Crossref PubMed Scopus (137) Google Scholar). However, N-terminal methionine is completely or partially retained on certain exceptional proteins having penultimate residues with intermediate sizes of side chains. The mature forms of the S27A, S27B, and L42B ribosomal proteins have Met-Val-Leu or Met-Val-Asn terminal regions (12Arnold R. Polevoda B. Reilly J.P. Sherman F. J. Biol. Chem. 1999; 274: 37035-37040Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). Furthermore, Moerschell et al. (3Moerschell R.P. Hosokawa Y. Tsunasawa S. Sherman F. J. Biol. Chem. 1990; 265: 19638-19643Abstract Full Text PDF PubMed Google Scholar) demonstrated antepenultimate (the third residue) proline residues can inhibit methionine cleavage from certain residues with intermediate sizes