The short-lived MATα2 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 MATα2 repressor of the yeast Saccharomyces cerevisiae is an extremely short-lived protein. We now demonstrate that α2 is conjugated to ubiquitin in vivo. More than one lysine residue of α2 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 α2 degradation is dependent on its ubiquitination.