The action of N-terminal acetyltransferases on yeast ribosomal proteins

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometr y was used to determine the state of N-terminal acetylation of 68 ribosomal proteins from a normal strain of Saccharomyces cerevisiae and from the ard 1-Delta, nat3-Delta, and mak3-Delta mutants (4), each lacking a catalytic s ubunit of three different N-terminal acetyltransferases. A total 30 of the of 68 ribosomal proteins were N-terminal-acetylated, and 24 of these (80%) were NatA substrates, unacetylated in solely the ard1-Delta mutant and havi ng mainly Ac-Ser- termini and a few with Ac-Ala- or Ac-Thr- termini, Only 4 (13%) were NatB substrates, unacetylated in solely the nat3-Delta mutant, and having Ac-Met-Asp- or Ac-Met-Glu- termini. No NatC substrates were unco vered, e.g. unacetylated in solely mak3-Delta mutants, consistent with find ing that none of the ribosomal proteins had Ac-Met-Ile-, Ac-Met-Leu-, or Ac -Met-Phe- termini, Interestingly, two new types of the unusual Nato substra tes were uncovered, having either Ac-Ser-Asp-Phe- or Ac-Ser-Asp-Ala- termin i that were unacetylated in the ard1-Delta mutant, and only partially acety lated in the mak3-Delta mutant and, for one case, also only partially in th e nat3-Delta mutant. We suggest that the acetylation of Nato substrates req uires not only Ard1p and Nat1p, but also auxiliary factors that are acetyla ted by the Mak3p and Nat3p N-terminal acetyltransferases.