N-Terminal Acetylation Inhibits Protein Targeting to the Endoplasmic Reticulum

Abstract

Amino-terminal acetylation is probably the most common protein modification in eukaryotes with as many as 50%–80% of proteins reportedly altered in this way. Here we report a systematic analysis of the predicted N-terminal processing of cytosolic proteins versus those destined to be sorted to the secretory pathway. While cytosolic proteins were profoundly biased in favour of processing, we found an equal and opposite bias against such modification for secretory proteins. Mutations in secretory signal sequences that led to their acetylation resulted in mis-sorting to the cytosol in a manner that was dependent upon the N-terminal processing machinery. Hence N-terminal acetylation represents an early determining step in the cellular sorting of nascent polypeptides that appears to be conserved across a wide range of species.

Author Summary

The eukaryotic cell comprises several distinct compartments, called organelles, required to perform specific functions. The proteins in these compartments are almost always synthesised in the cytoplasm and so require complex sorting mechanisms to ensure their delivery to the appropriate organelle. Of course, not all proteins need to leave the cytoplasm since many remain there to perform cytoplasmic functions. It is well known that many proteins are modified by acetylation of their amino-terminus at a very early stage in their synthesis. We have discovered a profound difference between the likelihood of such a modification on cytoplasmic proteins and on those destined for one of the major organelles, the endoplasmic reticulum (ER): whereas cytoplasmic proteins are typically acetylated, those bound for the ER are largely unmodified. Moreover, when specific ER proteins were engineered to induce their acetylation we found that their targeting to the ER was inhibited. Our data suggest that N-terminal acetylation is a major determinant in protein sorting in eukaryotes.