 Penultimate residues affect the efficiency of acetylation, in that acetylation is promoted by acidic residues and inhibited by basic ones.The N-terminal amino acid sequences of p19Arf (Met-Gly-Arg) and p14ARF (Met-Val-Arg) would be processed by methionine aminopeptidase but would not be acetylated, allowing ubiquination to proceed.The sequence of smARF, however, predicts that the initiating methionine would not be cleaved by methionine aminopeptidase and would probably be acetylated, and so is degraded by the proteasome without ubiquination.