Expression and activity of the Hxt7 high‐affinity hexose transporter of Saccharomyces cerevisiae

Abstract

High-affinity hexose transport is required for efficient utilization of low hexose concentrations by the baker's yeast Saccharomyces cerevisiae. These low concentrations occur during the late exponential phase of batch growth on hexoses, during hexose-limited chemostat or fed-batch culture, or during growth on sugars such as sucrose and raffinose that are hydrolysed to hexoses outside the cell. The expression of the Hxt7 high-affinity glucose transporter of S. cerevisiae was examined during batch growth on glucose medium in a wild-type strain and a strain expressing only HXT7 (i.e. with null mutations in HXT1-HXT6). In the wild-type strain, HXT7 transcription was repressed at high glucose and was detected when the glucose in the culture approached depletion. In the HXT7-only strain, transcription of HXT7 was constitutive throughout the glucose growth phase and was increased further at low glucose concentrations. After glucose depletion, the levels of HXT7 mRNA declined rapidly in both strains. In contrast, the Hxt7 protein was relatively stable after glucose depletion. By monitoring the subcellular localization of an Hxt7::GFP fusion protein it was observed that Hxt7 was localized in the plasma membrane, even when expressed at high glucose concentrations in the HXT7-only strain. After glucose depletion Hxt7 was gradually endocytosed and targeted to the vacuole for degradation. The Hxt7::GFP fusion protein was a fully functional hexose transporter with a catalytic centre activity of approximately 200/sec. It is concluded that repression of HXT7 and degradation of Hxt7 at high glucose concentrations is dependent on a high glucose transport capacity.

The amino‐terminal tail of Hxt11 confers membrane stability to the Hxt2 sugar transporter and improves xylose fermentation in the presence of acetic acid

“…For instance, yeast expresses the high‐affinity glucose transporter encoding HXT2 and HXT7 genes at low‐glucose conditions (Diderich et al, 1999). Importantly, transcription of these genes is repressed at high glucose concentration, while under those conditions, Hxt2 and Hxt7 proteins at the membrane are targeted to the vacuole for degradation (Kruckeberg et al, 1999; Ye et al, 2001). Hxt7 and Hxt2 can be ubiquitinated and phosphorylated at N‐terminal residues (Swaney et al, 2013) and this likely contributes to their degradation.…”

Homeostatic Adjustment and Metabolic Remodeling in Glucose-limited Yeast Cultures

“…In contrast, the many genes subject to glucose repression are expressed minimally (Carlson, 1999). When the diauxic shift is completed, the expression pattern has changed to favor the high-affinity transporters and kinases (Herrero et al, 1995;Ye et al, 2001) and the genes of the high-efficiency, low-flux oxidative respiratory pathway. As glucose in the growth medium becomes depleted, this physiological switch into the oxidative respiratory mode theoretically maximizes the efficiency of energy extraction.…”

Derepression of a baker’s yeast strain for maltose utilization is associated with severe deregulation of HXT gene expression

“…The same is true for all other strains expressing solely each of the Hxt1 –six transporters (Reifenberger et al. 1997; Ye et al. 2001).…”

“…Efficient expression of HXT6/7 during growth on high glucose concentrations might be a direct consequence of the mutation present in PYCC 5334, but it could also be an indirect effect of the inability to express the other HXT genes, considering that a strain expressing HXT7 alone in a hxt‐ null background is partially derepressed (Reifenberger et al. 1997; Ye et al. 2001).…”