Grr1 of Saccharomyces cerevisiae is connected to the ubiquitin proteolysis machinery through Skp1: coupling glucose sensing to gene expression and the cell cycle

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Abstract
Grr1 protein of the yeast Saccharomyces cerevisiae is a central component of a glucose signal transduction mechanism responsible for glucose‐induced gene expression. It is required for glucose‐stimulated regulation of Rgt1, a repressor of several glucose‐induced HXT genes. Grr1 also plays a role in regulating the cell cycle, because it is required for degradation of the G1 cyclins Cln1 and Cln2. We discovered that Grr1 physically interacts with Skp1, a protein that has been implicated in a ubiquitin‐conjugating enzyme complex that targets for degradation the cell cycle regulators Cln1 and Cln2, and the cyclin‐dependent kinase inhibitor Sic1. Thus, Grr1 may regulate the cell cycle and glucose‐induced gene expression via ubiquitin‐mediated protein degradation. Consistent with this idea, Skp1, like Grr1, was found to be required for glucose‐induced HXT gene expression. Two functional domains of Grr1 are required for its interaction with Skp1: 12 leucine‐rich repeats (LRR) and an adjacent F‐box. The Grr1‐Skp1 interaction is enhanced by high levels of glucose. This could provide yeast with a mechanism for coupling nutrient availability to gene expression and cell cycle regulation.