Glucose inhibits meiotic DNA replication through SCFGrr1p-dependent destruction of Ime2p kinase.

Sporulation in the yeast Saccharomyces cerevisiae is regulated by three principal nutritional signals: nonfermentable carbon sources and starvation for essential growth nutrients promote sporulation, whereas glucose inhibits sporulation. Sporulation in yeast transforms a diploid cell into four haploid spores (reviewed in reference 30) and consists of two sequential processes—meiosis and spore formation. Nutritional signals regulate both the initiation of meiosis and later stages of sporulation (reviewed in references 22 and 26). Glucose regulates initiation of meiosis through several mechanisms. At one level, glucose inhibits transcription of IME1. This gene encodes a “master regulator” transcription factor, which activates transcription of most early meiotic genes. At a second level, glucose inhibits assembly of Ime1p and other transcription factors into an active complex (33, 41, 50, 53). At a third level, glucose may inhibit the activity Ime2p kinase, which is encoded by an early meiotic gene. Ime2p kinase is required at multiple stages throughout sporulation, suggesting that Ime2p drives meiosis in the same way that cyclin/Cdk drives the cell division cycle (reviewed in reference 18). Although nutritional controls on Ime2p kinase have not been identified, this kinase is regulated by phosphorylation (3), association with other proteins (12), and stability (7). Glucose inhibits sporulation through several different signal transduction pathways. Both the protein kinase A pathway and the glucose-repression pathway mediate glucose controls on meiosis as well as many other cellular processes (reviewed in references 9 and 49). These pathways regulate both initiation of meiosis and later stages of sporulation (21, 34). A third pathway, the glucose induction pathway, may also regulate sporulation since a key regulator of this pathway, Grr1p, is required to prevent sporulation during late stages of growth (38). Grr1p activates the glucose induction pathway by targeting destruction of the Mth1p transcription factor, leading to the activation of hexose transporter genes (14, 25, 35). Grr1p is also required in an array of other cellular processes, including amino acid import (4), pseudohyphal differentiation (6), and G1 cyclin expression (reviewed in reference 48). Grr1p is the F-box protein component of the SCFGrr1p ubiquitin ligase complex, also termed an E3 (reviewed in reference 51). SCFGrr1p targets specific proteins for ubiquitination, recognizing a conserved PEST domain in these proteins. The target protein must also be phosphorylated before it can be recognized by the SCF. Once ubiquitinated, these targeted proteins, e.g., Cln2p and Gic2p, are degraded by the proteosome. In this study, we present evidence that SCFGrr1p promotes ubiquitination and turnover of Ime2p in response to moderate amounts of glucose, a condition commonly encountered during late stages of growth. We propose that destruction of Ime2p under these conditions stabilizes Sic1p, a cyclin-dependent kinase inhibitor, resulting in a block to meiotic DNA replication. In addition, GRR1 has separate roles in regulating late meiotic events.