UvA-DARE ( Digital Academic Repository ) Ubiquitination in apoptosis signaling

More than 80% of all human proteins are modified by N-terminal (N-)acetylation, but the function of this modification is not well understood. N-acetylation has been suggested to affect protein stability, but in certain studies it inhibited protein degradation, while in others it acted as a degradation signal. To resolve this issue, we determined the contribution of N-acetylation to protein ubiquitination and turn-over. N-acetylation was modulated by substituting one or two N-terminal amino acids to generate good or poor substrates for N-acetyltransferases. Our data consistently indicate that N-acetylation stabilizes proteins by preventing their proteasomal degradation. It did so by two mechanisms: N-acetylation prevented N-terminal poly-ubiquitination on protein substrates and thereby prevented their degradation, and N-acetylation also inhibited proteasomal degradation regardless of its impact on substrate ubiquitination. In this way, it prevented proteasomal degradation of the naturally lysine-less cell cycle regulator p16 and it could even counteract degradation of ubiquitinated proteins. We conclude that in human cells, N-acetylation stabilizes protein substrates by inhibiting their ubiquitin-mediated targeting to the proteasome, and by inhibiting proteasomal processing of the substrate.