SCFGrr1-mediated ubiquitination of Gis4 modulates glucose response in yeast

The F box protein Grr1 is the substrate specificity-determinant of the SCF(Grr1) E3 ubiquitin ligase complex. Genetic analyses of Grr1 mutants have implicated Grr1 in glucose repression, specifically with regard to expression of the SUC2 transcript. To better understand Grr1, we screened for substrates using a mutant version of Grr1 that should not associate with the SCF complex. We identified Gis4 as a novel Grr1 substrate. Gis4 was originally isolated as a multi-copy suppressor of a Gal--phenotype in the triple mutant snf1 mig1 srb8. Here, we show that Gis4 binds Grr1 in vivo and that Grr1 protein levels positively affect the protein levels of Gis4. The Gis4 protein is stable in wild-type cells and in grr1Delta cells; however, Gis4 is ubiquitinated in a Grr1-dependent manner. Furthermore, we show that Gis4 interacts with Snf1 in a Grr1-dependent fashion, and that Gis4 is involved in de-repression of SUC2 and in transcription of other Snf1-dependent transcripts. Gis4 appears to connect the glucose repression and de-repression pathways. We suggest that Gis4 may explain the glucose repression defects in carbon source metabolism for the grr1 mutants.

The ISG15 isopeptidase UBP43 is regulated by proteolysis via the SCFSkp2 ubiquitin ligase

The Skp2 oncoprotein belongs to the family of F-box proteins that function as substrate recognition factors for SCF (Skp1, cullin, F-box protein) E3 ubiquitin-ligase complexes. Binding of the substrate to the SCFSkp2 complex catalyzes the conjugation of ubiquitin molecules to the bound substrate, resulting in multi-ubiquitination and rapid degradation by the 26 S proteasome. Using Skp2 as bait in a yeast two-hybrid screen, we have identified UBP43 as a novel substrate for Skp2. UBP43 belongs to the family of ubiquitin isopeptidases and specifically cleaves ISG15, a ubiquitin-like molecule that is induced by cellular stresses, such as type 1 interferons (IFN), nephrotoxic damage, and bacterial infection. UBP43 was originally identified as an up-regulated gene in knock-in mice expressing an acute myelogenous leukemia fusion protein, AML1-ETO, as well as in melanoma cell lines treated with IFN-beta. The phenotype of UBP43 knockout mice includes shortened life span, hypersensitivity to IFN, and neuronal damage, suggesting that tight regulation of ISG15 conjugation is critical for normal cellular function. In this study, we demonstrate that UBP43 is ubiquitinated in vivo and accumulates in cells treated with proteasome inhibitors. We also show that Skp2 promotes UBP43 ubiquitination and degradation, resulting in higher levels of ISG15 conjugates. In Skp2-/- mouse cells, levels of UBP43 are consistently up-regulated, whereas levels of ISG15 conjugates are reduced. Our results demonstrate that the SCFSkp2 is involved in controlling UBP43 protein levels and may therefore play an important role in modulating type 1 IFN signaling.

Transferable domain in the G(1) cyclin Cln2 sufficient to switch degradation of Sic1 from the E3 ubiquitin ligase SCF(Cdc4) to SCF(Grr1)

Degradation of Saccharomyces cerevisiae G(1) cyclins Cln1 and Cln2 is mediated by the ubiquitin-proteasome pathway and involves the SCF E3 ubiquitin-ligase complex containing the F-box protein Grr1 (SCF(Grr1)). Here we identify the domain of Cln2 that confers instability and describe the signals in Cln2 that result in binding to Grr1 and rapid degradation. We demonstrate that mutants of Cln2 that lack a cluster of four Cdc28 consensus phosphorylation sites are highly stabilized and fail to interact with Grr1 in vivo. Since one of the phosphorylation sites lies within the Cln2 PEST motif, a sequence rich in proline, aspartate or glutamate, serine, and threonine residues found in many unstable proteins, we fused various Cln2 C-terminal domains containing combinations of the PEST and the phosphoacceptor motifs to stable reporter proteins. We show that fusion of the Cln2 domain to a stabilized form of the cyclin-dependent kinase inhibitor Sic1 (Delta N-Sic1), a substrate of SCF(Cdc4), results in degradation in a phosphorylation-dependent manner. Fusion of Cln2 degradation domains to Delta N-Sic1 switches degradation of Sic1 from SCF(Cdc4) to SCF(Grr1). Delta N-Sic1 fused with a Cln2 domain containing the PEST motif and four phosphorylation sites binds to Grr1 and is unstable and ubiquitinated in vivo. Interestingly, the phosphoacceptor domain of Cln2 binds to Grr1 but is not ubiquitinated and is stable. In summary, we have identified a small transferable domain in Cln2 that can redirect a stabilized SCF(Cdc4) target for SCF(Grr1)-mediated degradation by the ubiquitin-proteasome pathway.