Functional analysis of the hexose transporter homologue HXT5 in Saccharomyces cerevisiae
Functional analysis of the hexose transporter homologue HXT5 in Saccharomyces cerevisiae
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作者：
Diderich, JA;Schuurmans, JM;Van Dam, K
HXT 5基因编码功能性己糖转运蛋白，其对葡萄糖具有中等亲和力（K-m = 10 mm），对果糖具有中等至低亲和力（K-m = 40 mm），对甘露糖具有低亲和力（K-m > 100 mm）。在其他己糖转运无效突变体中Hxt 5 p的单独存在足以维持从葡萄糖到发酵产物的糖酵解通量。然而，HXT 5作为唯一的己糖转运蛋白基因的存在导致在葡萄糖上极差的生长，这表明在HXT 5的转录调节中涉及葡萄糖阻遏。从对HXT家族成员的北方印迹分析和对用绿色荧光蛋白（GFP）标记的HXT 5的研究中，很明显，HXT 5在相对缓慢生长的条件下、在不可发酵的碳源上生长期间、特别是在孢子形成期间被转录和翻译。在可发酵碳源上的野生型分批培养中，Hxt 5 p在稳定期或可发酵碳源耗尽后是丰富的，这似乎与碳源无关。HXT 5的缺失不会导致明确的表型。与野生型菌株相比，固定期细胞向新鲜葡萄糖培养基的转移导致hxt 5缺失菌株中生长的恢复稍慢。Hxt 5 p在稳定期、孢子形成和低葡萄糖条件下的丰度表明，HXT 5是一种“储备”转运蛋白，可能参与葡萄糖出现后的初始摄取。HXT 5编码的蛋白质的其他可能功能将在结果的背景下讨论。版权所有（C）2001约翰威利父子有限公司
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. Copyright (C)2001 John Wiley Sons, Ltd.