Two glucose sensing/signaling pathways stimulate glucose-induced inactivation of maltose permease in Saccharomyces

Glucose is a global metabolic regulator in Saccharomyces. It controls the expression of many genes involved in carbohydrate utilization at the level of transcription, and it induces the inactivation of several enzymes by a posttranslational mechanism. SNF3, RGT2, GRR1 and RGT1 are known to be involved in glucose regulation of transcription. We tested the roles of these genes in glucose-induced inactivation of maltose premease. Our results suggest that at least two signaling pathways are used to monitor glucose levels. One pathway requires glucose transport and the second pathway is independent of glucose transport. Rgt2p, which along with Snf3p monitors extracellular glucose levels, appears to be the glucose sensor for the glucose-transport-independent pathway. Transmission of the Rgt2p-dependent signal requires Grr1p. RGT2 and GRR1 also play a role in regulating the expression of the HXT genes, which appear to be the upstream components of the glucose-transport-dependent pathway regulating maltose permease inactivation. XGT2-1, which was identified as a dominant mutation causing constitutive expression of several HXT genes, causes constitutive proteolysis of maltose permease, that is, in the absence of glucose. A model of these glucose sensing/signaling pathways is presented.