Identification and characterization from Candida glycerinogenes of hexose transporters having high efficiency at high glucose concentrations.

During high gravity fermentation, a set of hexose transporters in yeasts plays an important role in efficient sugar transport. However, hexose transporters have been studied mainly in the Saccharomyces cerevisiae model and at low or moderate sugar concentrations. The hexose transporters are still poorly understood in the industrial glycerol producer Candida glycerinogenes, which assimilates sugar efficiently at high glucose concentration. To explore these hexose transporters, 14 candidates were identified using a hidden Markov model and characterized. Five of these functioned as hexose transporters when expressed in S. cerevisiae. In particular, CgHxt4 showed the highest efficiency of glucose transport at elevated glucose concentration among a group of transporters including Hxt1 and Hxt7 from S. cerevisiae. qRT-PCR in C. glycerinogenes revealed that transcription of CgHXT4 was induced by high glucose concentrations while fluorescence localization analysis indicated that CgHxt4 remained relatively stable on the membrane under these conditions. In addition, site-directed mutagenesis revealed that the asparagine 329 from CgHxt4, located in the YYX(T/P) conserved motif of hexose transporters, promoted an increased glucose transport. Overexpressing CgHXT4 in S. cerevisiae enhanced glucose consumption and ethanol production more effectively at high glucose concentrations than ScHXT1, the most significant native transporter from S. cerevisiae. These results indicate that CgHxt4 plays an important role in the fermentation process as a hexose transporter with strong transport activity and efficient expression regulation at high glucose concentrations.