Trafficking receptors control protein localization through the recognition of specific signal sequences that specify unique cellular locations. Differences in luminal pH are important for the vectorial trafficking of cargo receptors. The KDEL receptor is responsible for maintaining the integrity of the ER by retrieving luminally localized folding chaperones in a pH-dependent mechanism. Structural studies have revealed the end states of KDEL receptor activation and the mechanism of selective cargo binding. However, precisely how the KDEL receptor responds to changes in luminal pH remains unclear. To explain the mechanism of pH sensing, we combine analysis of X-ray crystal structures of the KDEL receptor at neutral and acidic pH with advanced computational methods and cell-based assays. We show a critical role for ordered water molecules that allows us to infer a direct connection between protonation in different cellular compartments and the consequent changes in the affinity of the receptor for cargo.

转运受体通过识别指定独特细胞位置的特定信号序列来控制蛋白质定位。腔内 pH 值的差异对于货物受体的载体运输很重要。 KDEL 受体通过以 pH 依赖性机制检索管腔局部折叠伴侣来负责维持 ER 的完整性。结构研究揭示了 KDEL 受体激活的最终状态和选择性货物结合的机制。然而，KDEL 受体如何准确响应管腔 pH 值的变化仍不清楚。为了解释 pH 传感的机制，我们将中性和酸性 pH 下 KDEL 受体的 X 射线晶体结构分析与先进的计算方法和基于细胞的测定结合起来。我们展示了有序水分子的关键作用，使我们能够推断不同细胞区室中的质子化与随后的受体对货物的亲和力的变化之间的直接联系。