Methionine or not methionine at the beginning of a protein

Abstract

SummaryMethionine aminopeptidases with a universal specif city have been revealed from the sequences of the amino-terminal region of mutant forms of yeast iso-lcytochrome c and from a systematic examination of the literature for aminoterminal sequences formed at initiation sites. The aminopeptidase removes aminoterminal residues of methionine when they precede certain amino acids, with a specijicity that appears to be determined mainly by the residue adjacent to the methionine residue at the amino terminus. The result with the mutationally altered iso-1-cytochromes c and the results from published sequences of other proteins from a wide range of prokaryotes and eukaryotes suggest that the aminopeptidase usually cleaves amino-terminalmethionine when it precedes residues of alanine, cysteine, giycine, proline, serine, threonine and valine but not when it precedes residues of arginine, asparagine, aspartic acid, glutamine, glutamic acid, isoleucine, leucine, lysine or methionine. We suggest that the specy5city is almost always determined simply by the size of the side chain of the penultimate residue; methionine is mually cleavedfrom residues with a side chain having a radius of gyration of 1-29,d or less, but is not cleaved from residues with larger side chains.

Plastid Proteome Assembly without Toc159: Photosynthetic Protein Import and Accumulation of N-Acetylated Plastid Precursor Proteins

“…This modification mechanism is used for 60 to 90% of all proteins in eukaryotes (Goetze et al, 2009), and it is conceivable that it also modifies plastid precursor proteins under normal wild-type conditions during their cytosolic synthesis and transition. The sequence context around the identified N-acetylation sites is consistent with described requirements for this type of modification (i.e., small, uncharged residues like Ala follow the start Met) (Sherman et al, 1985;Meinnel et al, 2006). Our observation may therefore indicate a general modification pathway for plastid proteins in the cytosol.…”

“…The accumulated proteins are not restricted to abundant photosynthetic proteins but also include minor constituents of the chloroplast proteome, such as a 29-kD ribonucleoprotein, the chaperonin 20, and a thiazole-requiring enzyme. Notably, the sequence context around the identified N-acetylation sites is consistent with established requirements for the cytosolic NME/acetylation pathway (i.e., small, uncharged residues, such as Ala following the initiator Met) ( Figures 5C and 5D) (Sherman et al, 1985;Meinnel et al, 2006).…”

“…Following transit peptide removal, the newly generated protein N terminus may be acetylated (Ferro et al, 2003;Kleffmann et al, 2007;Zybailov et al, 2008). N-acetylation is catalyzed by N-terminal acetyltransferases in the cytosol and in the plastid (Sherman et al, 1985;Meinnel et al, 2006;Goetze et al, 2009). It is therefore possible that a chloroplast-targeted protein may be acetylated as a precursor in the cytosol and after import and processing as the mature protein.…”

The FliP and FliR proteins of Salmonella typhimurium, putative components of the type III flagellar export apparatus, are located in the flagellar basal body

“…Mature FliP has no Cys and 12 Met residues. FliR has one Cys residue and either 16 or 15 Met residues, depending on whether or not the N-terminal Met is cleaved; as position 2 contains a bulky amino acid, Ile, cleavage probably does not occur (Sherman et al, 1985). Conversion of the non-normalized data (Jones, 1989) then yields a stoichiometry of 4.5 subunits per basal body for mature FliP and 0.5 subunits per basal body for FliR.…”

N‐terminal‐methionylated interleukin‐1β has reduced receptor‐binding affinity

“…The purified protein was found to be a mixture of 80%o correctly processed protein (N-terminus = Ala) and 20% of an N-terminal-methionylated form. The presence of N-terminal Met in a mature polypeptide is not uncommon and may be due to the difficulty which the processing enzyme methionine aminopeptidase experiences in removing Met when it is followed by a residue with large radius of gyration [3]. For recombinant-derived proteins produced in E. coli the removal of Met may also be compromised by the very high levels of accumulated protein which may simply saturate the processing enzyme(s).…”