In bacteria, K+ is used to maintain cell volume and osmotic potential. Homeostasis normally involves a network of constitutively expressed transport systems, but in K+ deficient environments, the KdpFABC complex uses ATP to pump K+ into the cell. This complex appears to be a hybrid of two types of transporters, with KdpA descending from the superfamily of K+ transporters and KdpB belonging to the superfamily of P-type ATPases. Studies of enzymatic activity documented a catalytic cycle with hallmarks of classical P-type ATPases and studies of ion transport indicated that K+ import into the cytosol occurred in the second half of this cycle in conjunction with hydrolysis of an aspartyl phosphate intermediate. Atomic structures of the KdpFABC complex from X-ray crystallography and cryo-EM have recently revealed conformations before and after formation of this aspartyl phosphate that appear to contradict the functional studies. Specifically, structural comparisons with the archetypal P-type ATPase, SERCA, suggest that K+ transport occurs in the first half of the cycle, accompanying formation of the aspartyl phosphate. Further controversy has arisen regarding the path by which K+ crosses the membrane. The X-ray structure supports the conventional view that KdpA provides the conduit, whereas cryo-EM structures suggest that K+ moves from KdpA through a long, intramembrane tunnel to reach canonical ion binding sites in KdpB from which they are released to the cytosol. This review discusses evidence supporting these contradictory models and identifies key experiments needed to resolve discrepancies and produce a unified model for this fascinating mechanistic hybrid.

中文翻译：

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KdpFABC复合物-K +克服一切困难。

在细菌中，K +用于维持细胞体积和渗透潜力。体内平衡通常涉及组成性表达运输系统的网络，但是在K +缺乏的环境中，KdpFABC复合物使用ATP将K +泵入细胞。该复合物似乎是两种类型转运蛋白的混合体，KdpA属于K +转运蛋白的超家族，而KdpB属于P型ATPase的超家族。酶活性的研究记录了具有经典P型ATP酶特征的催化循环，离子迁移的研究表明，在该循环的后半段，随着天冬氨酰磷酸酯中间体的水解，钾离子进入细胞质。X射线晶体学和cryo-EM的KdpFABC复合物的原子结构最近揭示了这种天冬氨酰磷酸形成前后的构象，这些构象似乎与功能研究相矛盾。具体而言，与原型P型ATPase SERCA的结构比较表明，K +转运发生在周期的前半段，伴随着天冬氨酰磷酸的形成。关于K +穿过膜的路径还引起了更多争议。X射线结构支持传统观点，即KdpA提供导管，而冷冻EM结构则表明K +从KdpA穿过长的膜内通道到达KdpB中的规范离子结合位点，然后从那里被释放到细胞质中。