低铁环境下白色念珠菌核质转运受体Nmd5调控转录因子Sef1异常核输出的分子机制研究

As a clinically important opportunistic human fungal pathogen, Candida albicans is able to adapt to the iron rich gastrointestinal tract and iron-depleted blood environment. Our previous studies have identified Sef1 as a core factor in iron-regulatory circuit and results showed that its post-transcriptional modification is of importance in C. albicans commensal-pathogen transition. However, mechanisms regulating nuclei-cytoplasm transportation of Sef1 remains unclear. We recently found that under low iron condition, the exportin Nmd5 mediates unusual nuclear export of Sef1, even when cells requires this factor to activate gene transcription in nuclei. In order to unravel the mechanism of how Nmd5 exports Sef1 and the related biological significance, we seek to propose this research with three specific aims: (1) Mechanisms of cytoplasmic export of Sef1 by exportin Nmd5; (2) Molecular insights and biological significance of Sef1 cytoplasmic export by Nmd5; (3) Role of Nmd5-mediated Sef1 cytoplasmic export in C. albicans commensalism and pathogenicity. This study is expected to provide important insights to better understand molecular mechanisms involved in C. albicans intracellular protein transportation. Moreover, we hope that our proposed study can provides bases on disease diagnosis and identification of new targets of anti-fungal drugs.

白色念珠菌是临床重要的条件性致病真菌，能同时适应富铁肠道环境及贫铁血液环境。我们之前的研究发现转录因子Sef1在铁调控回路中处于核心地位，其转录后修饰对白色念珠菌共生-致病转换具有重要作用。然而，调控Sef1核质转运这一转录后修饰的分子机制目前尚未阐明。申请人前期工作首次发现低铁条件下在细胞需要Sef1入核激活基因转录表达时，Nmd5作为核输出受体却异常往细胞质输出Sef1。为探索Nmd5核输出Sef1的分子机制及生物学重要性，本项目拟开展如下研究：1）探索低铁条件下Nmd5作为核输出受体介导Sef1从细胞核向细胞质异常转运的分子机制；2）揭示低铁条件下Nmd5核输出Sef1的具体原因及生物学重要性；3）阐明Nmd5蛋白转运功能在白色念珠菌共生-致病转换过程中的重要作用。本项目的研究预期加深对白色念珠菌蛋白核质转运机制的理解，并且为疾病诊断及抗真菌药物新作用靶点的发现提供理论依据。

诸如真菌之类的微生物已经进化出复杂的机制，用以在不同的病态条件下实现有效的铁稳态。我们以往研究发现铁稳态调控因子Sef1控制着铁吸收基因的表达，并通过核质转运这一转录后修饰调控白色念珠菌共生-致病转换。然而，负责调控Sef1核质转运的因子尚不清楚。本研究，我们鉴定发现Nmd5作为一个转运蛋白，调控Sef1在细胞内的转运。令我们惊讶的是，与酿酒酵母Nmd5主要作为核输入受体(Importin)功能不同，白色念珠菌中Nmd5作为一个核输出受体(Exportin)，在低铁条件下将细胞核的Sef1输出到细胞质中使其降解，即使低铁条件下需要高水平的Sef1激活基因的表达。更重要的是，Sef1的异常核输出对于在低铁条件下维持其核内水平和确保正常的基因转录至关重要。我们发现阻止Sef1的核输出会导致基因的异常表达。其中包括Sfu1，它在白色念珠菌中为负调控因子并与Sef1形成功能上保守的铁调控回路，在低铁条件下Sfu1错误进入细胞核导致异常基因激活表达。总之，我们的研究揭示了一种新的铁依赖性转录后调控机制，核输出受体Nmd5通过这种机制调控铁响应因子的核/质转运，防止对维持铁稳态所需基因的表达产生有害作用。