Degradation of Mth1p and Std1p is thought to inactivate the repressor function of Rgt1p and allow for its phosphorylation by the Protein kinase A homologues. Phosphorylation targets the corepressors for ubiquitination which results in their degradation. It is not known how glucose specifically activates the sensors, but activation requires the cytoplasmic domains which are thought to facilitate the phosphorylation of the homologous corepressor proteins Mth1p and Std1p by the Yeast Casein Kinase ( YCK1, YCK2). Rgt1p appears to regulate a limited set of genes including most of the HXT genes that are critical for growth on glucose. Mth1p and Std1p are the only known link between the cytoplasmic membrane bound sensors and the DNA binding repressor Rgt1p in the nucleus where they act to facilitate the repressor function of Rgt1p. Null alleles of snf3 result in the loss of fermentative growth on low levels of glucose (0.05%) while null alleles of both sensors result in the loss of fermentative growth on 2% glucose. In contrast to the HXT's, both Snf3p and Rgt2p appear to have lost the capacity to transport glucose and both proteins also contain an extra C terminal cytoplasmic domain that is important for the transmission of the signal. Snf3p and Rgt2p, which display homology to the HXT transporters, are thought to act as sensors of low and high glucose respectively. Mutations in several components of this pathway show extreme growth defects on glucose that result from the inability to induce high level expression of the major HXT genes.
Īlthough interpathway crosstalk is likely crucial to optimized HXT regulation, it is clear that the relief of HXT repression via the SNF3/RGT2 glucose induction pathway is the major on/off switch essential to transporter induction. These pathways include: glucose induction, glucose repression, High Osmolarity Glycerol (HOG), Target of Rapamycin (TOR) GCR1/GCR2 pathway and the RAS/PKA pathway. Many signaling pathways are known to converge at the promoters of the HXT genes to either positively or negatively regulate specific transporters in response to their respective inputs. cerevisiae and a complex regulatory network has evolved to maintain optimal transporter activity in response to external nutrient availability. Transport is the rate limiting step in glucose utilization by S. Glucose transport occurs via facilitated diffusion mediated by a group of homologous transmembrane hexose transporters, encoded by the HXT genes. Glucose is the preferred carbon source for many organisms including the budding yeast Saccharomyces cerevisiae. Further, the observation that HXT2 is the gene responsible for restoration of growth under these conditions suggests that the numbers of repressor binding domains found in the regulatory regions of members of the HXT family may have biological relevance and enable differential regulation. These findings support the model of relief of repression as the mechanism of restoration of growth on low glucose concentrations in the absence of functional Snf3p. Deletion of the HXT2 gene in a heterozygous diploid, RGT1/rgt1Δ MTH1/mth1Δ snf3Δ/snf3Δ hxt2Δ/hxt2Δ, prevented the suppression of snf3Δ.
HXT gene reporter fusion assays indicated that the presence of heterozygosity at the MTH1 and RGT1 alleles leads to increased expression of the HXT2 gene. Mutants of the STD1 homolog of MTH1 did not display haploinsufficiency singly or in combination with mutations in RGT1. This observed haploinsufficiency is consistent with the finding of repressor titration as a mechanism of suppression of snf3. Since null alleles in the heterozygous state lead to suppression, MTH1 and RGT1 display the phenomenon of combined haploinsufficiency. Strains heterozygous for both the RGT1 and MTH1 genes ( RGT1/rgt1Δ MTH1/mth1Δ snf3Δ/snf3Δ) but homozygous for the snf3∆ were found to grow on low glucose. The relief of repression allows expression of HXT transporter proteins, the resumption of glucose uptake and therefore of growth in the absence of a functional Snf3 sensor. The snf3 null mutation phenotype is suppressed by the loss of either one of the downstream co-repressor proteins Rgt1p or Mth1p.
Null mutations of snf3 result in a defect in growth on low glucose concentrations due to the inability to relieve repression of a subset of the HXT genes.
The SNF3 gene in the yeast Saccharomyces cerevisiae encodes a low glucose sensor that regulates expression of an important subset of the hexose transporter (HXT) superfamily.
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