Overexpression of Arabidopsis thaliana SKP1 homologues in yeast inactivates the Mig1 repressor by destabilising the F-box protein Grr1

The timed destruction of cell cycle regulatory proteins is of key importanc e in controlling cell cycle progression in eukaryotes. Recently, Skp1 from yeast (Saccharomyces cerevisiae) was shown to play an important role in the ubiquitin-mediated proteolysis of these proteins via the Skp1-Cdc53-F-box (SCF) pathway. Here we describe the fortuitous cloning of cDNAs for two Skp 1 homologues from the plant Arabidopsis thaliana on account of their abilit y to activate reporter gene expression in yeast directed by the cyt-1 eleme nt from the promoter of the Agrobacterium tumefaciens T-cyt gene, which is essential for expression of the gene in plants. This element is strikingly similar in sequence to the binding site for the yeast Mig1 protein, a trans criptional repressor of genes involved in the utilisation of carbohydrates other than glucose. We report that Mig1 protein binds to the cyt-1 element with similar specificity as a previously described plant nuclear protein fa ctor, and that the cyt-1 element is a target for an unknown yeast transcrip tional activator when Mig1 itself is inactivated. Interestingly, our data f urther indicate that A. thaliana Skp1 inactivates Mig1 by destabilising the yeast F-box protein Grr1, which is required for cyclin degradation and is thus involved in control of the cell cycle, and for glucose-regulated gene repression. Our results suggest that the plant counterpart of yeast Skp1 is probably also instrumental in ubiquitin-mediated proteolysis of specific p roteins via an SCF-like pathway.