Functional analysis of the hexose transporter homologue HXT5 in Saccharomyces cerevisiae

The HXT5 gene encodes a functional hexose transporter that has moderate affinity for glucose (K(m)=10 mM), moderate to low affinity for fructose (K(m)=40 mM) and low affinity for mannose (K(m)>100 mM). The sole presence of Hxt5p in an otherwise hexose transport null mutant is sufficient to sustain a flux through glycolysis from glucose to fermentative products. However, the presence of HXT5 as the sole hexose transporter gene results in extremely poor growth on glucose, which suggests the involvement of glucose repression in the transcriptional regulation of HXT5. From Northern blot analysis on the members of the HXT family and studies with HXT5 tagged with the green fluorescent protein (GFP), it is evident that HXT5 is transcribed and translated during conditions of relatively slow growth, during growth on non-fermentable carbon sources and in particular during sporulation. In wild-type batch cultivations on fermentable carbon sources, Hxt5p is abundant in stationary phase or after depletion of the fermentable carbon source, which seems independent of the carbon source. The deletion of HXT5 does not result in a clear phenotype. A shift of stationary phase cells to fresh glucose medium resulted in somewhat slower resumption of growth in the hxt5 deletion strain compared to the wild-type strain. The abundance of Hxt5p during stationary phase, sporulation and low glucose conditions suggests that HXT5 is a 'reserve' transporter, which might be involved in the initial uptake of glucose after the appearance of glucose. Other possible functions of the protein encoded by HXT5 will be discussed in the context of the results.