The Glucose Sensor-Like Protein Hxs1 Is a High-Affinity Glucose Transporter and Required for Virulence in Cryptococcus neoformans

Cryptococcus is a major fungal pathogen that frequently causes systemic infection in patients with compromised immunity. Glucose, an important signal molecule and the preferred carbon source for Cryptococcus, plays a critical role in fungal development and virulence. Cryptococcus contains more than 50 genes sharing high sequence homology with hexose transporters in Saccharomyces cerevisiae. However, there is no report on their function in glucose sensing or transport. In this study, we investigated two hexose transporter-like proteins (Hxs1 and Hxs2) in Cryptococcus that share the highest sequence identity with the glucose sensors Snf3 and Rgt2 in S. cerevisiae. The expression of HXS1 is repressed by high glucose, while the HXS2 expression is not regulated by glucose. Functional studies showed that Hxs1 is required for fungal resistance to oxidative stress and fungal virulence. The hxs1Δ mutant exhibited a significant reduction in glucose uptake activity, indicating that Hxs1 is required for glucose uptake. Heterologous expression of Cryptococcus HXS1 rendered the S. cerevisiae mutant lacking all 20 hexose transporters a high glucose uptake activity, demonstrating that Hxs1 functions as a glucose transporter. Heterologous expression of HXS1 in the snf3Δ rgt2Δ double mutant did not complement its growth in YPD medium containing the respiration inhibitor antimycin A, suggesting that Hxs1 may not function as a glucose sensor. Taken together, our results demonstrate that Hxs1 is a high-affinity glucose transporter and required for fungal virulence.

隐球菌（Cryptococcus）是一类主要的真菌病原体，常于免疫受损患者中引发全身性感染。葡萄糖作为重要的信号分子，亦是隐球菌首选的碳源，在真菌发育与毒力调控中发挥关键作用。隐球菌基因组中存在超过50个与酿酒酵母（Saccharomyces cerevisiae）己糖转运蛋白序列同源性极高的基因，但目前尚无关于这些基因在葡萄糖感知或转运中功能的相关报道。本研究针对隐球菌中两个与酿酒酵母葡萄糖传感器Snf3和Rgt2序列一致性最高的己糖转运蛋白样蛋白（Hxs1与Hxs2）展开了探究。实验结果显示，HXS1的表达会被高葡萄糖抑制，而HXS2的表达则不受葡萄糖调控。功能验证实验表明，Hxs1参与真菌对氧化应激的抗性以及真菌毒力的调控。hxs1Δ突变体的葡萄糖摄取活性显著降低，证实Hxs1具备葡萄糖摄取功能。将隐球菌HXS1进行异源表达，可使缺失全部20种己糖转运蛋白的酿酒酵母突变体恢复高葡萄糖摄取活性，证明Hxs1确实为葡萄糖转运蛋白。将HXS1在snf3Δ rgt2Δ双突变体中异源表达，无法互补该突变体在含呼吸抑制剂抗霉素A的YPD培养基中的生长缺陷，提示Hxs1可能不具备葡萄糖传感器的功能。综上，本研究结果证实Hxs1是一种高亲和力葡萄糖转运蛋白，且对真菌毒力至关重要。