Ubiquitin as a degradation signal.

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Abstract
For many short‐lived eukaryotic proteins, conjugation to ubiquitin, yielding a multiubiquitin chain, is an obligatory pre‐degradation step. The conjugated ubiquitin moieties function as a ‘secondary’ signal for degradation, in that their posttranslational coupling to a substrate protein is mediated by amino acid sequences of the substrate that act as a primary degradation signal. We report that the fusion protein ubiquitin‐‐proline‐‐beta‐galactosidase (Ub‐P‐beta gal) is short‐lived in the yeast Saccharomyces cerevisiae because its N‐terminal ubiquitin moiety functions as an autonomous, primary degradation signal. This signal mediates the formation of a multiubiquitin chain linked to Lys48 of the N‐terminal ubiquitin in Ub‐P‐beta gal. The degradation of Ub‐P‐beta gal is shown to require Ubc4, one of at least seven ubiquitin‐conjugating enzymes in S.cerevisiae. Our findings provide the first direct evidence that a monoubiquitin moiety can function as an autonomous degradation signal. This generally applicable, cis‐acting signal can be used to manipulate the in vivo half‐lives of specific intracellular proteins.