Hxt

Hxt is a term used for a family of hexose transporter proteins in yeasts, most notably Saccharomyces cerevisiae. Members of the HXT family are encoded by a set of HXT genes and belong to the major facilitator superfamily (MFS) of membrane transporters. They function to import hexose sugars across the plasma membrane, enabling yeast cells to take up glucose and, to varying extents, other hexoses such as fructose and galactose.

The HXT transporters show a range of kinetic properties. Some are high-affinity transporters that operate efficiently at low extracellular sugar concentrations, while others are low-affinity, high-capacity transporters that dominate when sugar is plentiful. This functional diversity allows yeast to adapt sugar uptake to changing environmental conditions. All HXT proteins are predicted to have the 12 transmembrane helices typical of MFS sugar porters and are thought to mediate transport via an alternating-access mechanism.

Expression of HXT genes is tightly controlled by the glucose-sensing regulatory network. The plasma membrane sensors Snf3 and Rgt2 detect extracellular glucose and signal through the transcription factor Rgt1, with corepressors Mth1 and Std1 modulating the response. In high glucose, low-affinity HXT transporters are typically induced or upregulated; in low glucose, high-affinity HXT transporters predominate to improve uptake efficiency. Regulation also involves glucose-responsive pathways that balance transporter expression with metabolic state.

Beyond S. cerevisiae, HXT-like transporters are found in related yeasts, often with altered substrate ranges and regulatory interactions. In research and biotechnology, HXT transporters are studied as models of membrane transport and have been engineered to optimize sugar uptake in industrial strains.