The N-end rule of selective protein turnover and its implications [abstract only]

When a chimeric gene encoding a ubiquitin: β-galactosidase fusion protein is expressed in the yeast Saccharomyces cerevisiae , ubiquitin is efficiently cleaved off the nascent fusion protein, yielding a deubiquitinated β-galactosidase (βgal). With one exception, this cleavage takes place irrespective of the nature of the amino acid residue of βgal at the ubiquitin-βgal junction. This result, in effect, allows one to expose different residues at the N-termini of the otherwise identical βgal proteins produced in vivo . The βgal proteins thus designed exhibit a striking diversity of in vivo half-lives, from more than 10h to less than 3 min, depending on the nature of the amino acid exposed at the N-terminus of βgal. The N-terminal location of an amino acid is essential for its effect on βgal half-life. The set of individual amino acids can thus be ordered with respect to the half-lives that they confer on βgal when present at its N-terminus (the ‘N-end rule’). The known N-terminal residues in long-lived intracellular proteins from both prokaryotes and eukaryotes are exclusively of the stabilizing class as predicted by the N-end rule. In contrast, a majority of the N-terminal residues in compartmentalized (e.g. secreted) proteins are of the destabilizing class. The N-end rule may thus underlie both the diversity of protein half-lives in vivo and the selective destruction of otherwise normal but miscompartmentalized proteins. The N-end may also account for the function of the previously described post-translational addition of single amino acids to protein N-termini. Thus the recognition of an N-terminal residue in a protein may mediate both the metabolic stability of the protein and the potential for regulation of its stability.