Schizosaccharomyces pombe Git7p, a Member of the Saccharomyces cerevisiae Sgt1p Family, Is Required for Glucose and Cyclic AMP Signaling, Cell Wall Integrity, and Septation

Abstract

The Schizosaccharomyces pombe fbp1 gene, encoding fructose-1,6-bisphosphatase, is transcriptionally repressed by glucose. Mutations that confer constitutive fbp1 transcription identify git (glucose-insensitive transcription) genes that encode components of a cyclic AMP (cAMP) signaling pathway required for adenylate cyclase activation. Four of these genes encode the three subunits of a heterotrimeric G protein (gpa2, git5, and git11) and a G protein-coupled receptor (git3). Three additional genes, git1, git7, and git10, act in parallel to or downstream from the G protein genes. Here, we describe the cloning and characterization of the git7 gene. The Git7p protein is a member of the Saccharomyces cerevisiae Sgt1p protein family. In budding yeast, Sgt1p associates with Skp1p and plays an essential role in kinetochore assembly, while in Arabidopsis, a pair of SGT1 proteins have been found to be involved in plant disease resistance through an interaction with RAR1. Like S. cerevisiae Sgt1p, Git7p is essential, but this requirement appears to be due to roles in septation and cell wall integrity, which are unrelated to cAMP signaling, as S. pombe cells lacking either adenylate cyclase or protein kinase A are viable. In addition, git7 mutants are sensitive to the microtubule-destabilizing drug benomyl, although they do not display a chromosome stability defect. Two alleles of git7 that are functional for cell growth and septation but defective for glucose-triggered cAMP signaling encode proteins that are altered in the highly conserved carboxy terminus. The S. cerevisiae and human SGT1 genes both suppress git7-93 but not git7-235 for glucose repression of fbp1 transcription and benomyl sensitivity. This allele-specific suppression indicates that the Git7p/Sgt1p proteins may act as multimers, such that Git7-93p but not Git7-235p can deliver the orthologous proteins to species-specific targets. Our studies suggest that members of the Git7p/Sgt1p protein family may play a conserved role in the regulation of adenylate cyclase activation in S. pombe, S. cerevisiae, and humans.

The Interaction between Sgt1p and Skp1p Is Regulated by HSP90 Chaperones and Is Required for Proper CBF3 Assembly

“…These observations are consistent with the finding that in Schizosaccharomyces pombe, mutations in the SGT1 homologue git7 produce defects in cell wall integrity and cytokinesis (31). It is tempting to speculate that changes in the levels of CBF3 or other kinetochore complexes may have important regulatory consequences, possibly affecting cell cycle advance and the completion of mitosis.…”

Molecular Genetic Evidence for the Role of SGT1 in the Intramolecular Complementation of Bs2 Protein Activity in Nicotiana benthamiana

“…The Bs2 RAR1 independence may simply be the result of high Bs2 expression, similar to the RAR1 independence that results for high RPS2 expression in Arabidopsis (Tornero et al, 2002). SGT1 is a highly conserved eukaryotic protein involved in such diverse pathways as the regulation of cell division, SCF (Skip1p/ Cdc53p-Cullin-F-box)-mediated ubiquitination of proteins, and the regulation of the cAMP pathway in yeast (Kitagawa et al, 1999;Schadick et al, 2002). One element these pathways seem to have in common is the involvement of multimeric protein complexes.…”

RAR1 Positively Controls Steady State Levels of Barley MLA Resistance Proteins and Enables Sufficient MLA6 Accumulation for Effective Resistance

“…In yeast, SGT1 is required for proper functioning of multiple nuclear and cytoplasmic targets, including the nuclear kinetochore complex, the SCF ubiquitination complex, and the LRR containing adenylate cyclase Cyr1p (Kitagawa et al, 1999;Dubacq et al, 2002;Schadick et al, 2002). In Arabidopsis, absence of SGT1b enhances defects in auxin signaling in plants containing a partially inactive TIR1 F-box protein, indicating an SGT1 function unrelated to disease resistance (Gray et al, 2003).…”