The short-lived MAT alpha 2 transcriptional regulator is ubiquitinated in vivo

The substrates of ubiquitin-dependent proteolytic pathways include both damaged or otherwise abnormal proteins and undamaged proteins that are naturally short-lived. Few specific examples of the latter class have been identified, however. Previous work has shown that the cell type-specific MAT alpha 2 repressor of the yeast Saccharomyces cerevisiae is an extremely short-lived protein. We now demonstrate that alpha 2 is conjugated to ubiquitin in vivo. More than one lysine residue of alpha 2 can be joined to ubiquitin, and some of the ubiquitin moieties form a Lys48-linked multiubiquitin chain. Overexpression of degradation-impaired ubiquitin variants was used to show that at least a significant fraction of alpha 2 degradation is dependent on its ubiquitination.