Fischer Ute, Schäuble Nico, Schütz Sabina, Altvater Martin, Chang Yiming, Faza Marius Boulos, Panse Vikram Govind
Institute of Biochemistry, Department of Biology, ETH Zurich, Zurich, Switzerland.
Elife. 2015 Apr 21;4:e05745. doi: 10.7554/eLife.05745.
The transport receptor Crm1 mediates the export of diverse cargos containing leucine-rich nuclear export signals (NESs) through complex formation with RanGTP. To ensure efficient cargo release in the cytoplasm, NESs have evolved to display low affinity for Crm1. However, mechanisms that overcome low affinity to assemble Crm1-export complexes in the nucleus remain poorly understood. In this study, we reveal a new type of RanGTP-binding protein, Slx9, which facilitates Crm1 recruitment to the 40S pre-ribosome-associated NES-containing adaptor Rio2. In vitro, Slx9 binds Rio2 and RanGTP, forming a complex. This complex directly loads Crm1, unveiling a non-canonical stepwise mechanism to assemble a Crm1-export complex. A mutation in Slx9 that impairs Crm1-export complex assembly inhibits 40S pre-ribosome export. Thus, Slx9 functions as a scaffold to optimally present RanGTP and the NES to Crm1, therefore, triggering 40S pre-ribosome export. This mechanism could represent one solution to the paradox of weak binding events underlying rapid Crm1-mediated export.
转运受体Crm1通过与RanGTP形成复合物，介导含有富含亮氨酸核输出信号（NESs）的多种货物的输出。为确保货物在细胞质中有效释放，NESs已进化为对Crm1表现出低亲和力。然而，克服低亲和力以在细胞核中组装Crm1-输出复合物的机制仍知之甚少。在本研究中，我们揭示了一种新型的RanGTP结合蛋白Slx9，它促进Crm1募集到与40S前核糖体相关的含NES的衔接蛋白Rio2上。在体外，Slx9结合Rio2和RanGTP，形成复合物。该复合物直接加载Crm1，揭示了一种非经典的逐步机制来组装Crm1-输出复合物。Slx9中一个损害Crm1-输出复合物组装的突变会抑制40S前核糖体输出。因此，Slx9作为一种支架，以最佳方式将RanGTP和NES呈现给Crm1，从而触发40S前核糖体输出。这种机制可能代表了快速Crm1介导的输出背后弱结合事件这一矛盾的一种解决方案。
