Madrid R, Le Maout S, Barrault M B, Janvier K, Benichou S, Mérot J
Service de Biologie Cellulaire, Département de Biologie Cellulaire et Moléculaire, CEA/Saclay, F-91191 Gif-sur-Yvette, Cedex, France.
EMBO J. 2001 Dec 17;20(24):7008-21. doi: 10.1093/emboj/20.24.7008.
Aquaporin 4 (AQP4) is the predominant water channel in the brain. It is targeted to specific membrane domains of astrocytes and plays a crucial role in cerebral water balance in response to brain edema formation. AQP4 is also specifically expressed in the basolateral membranes of epithelial cells. However, the molecular mechanisms involved in its polarized targeting and membrane trafficking remain largely unknown. Here, we show that two independent C-terminal signals determine AQP4 basolateral membrane targeting in epithelial MDCK cells. One signal involves a tyrosine-based motif; the other is encoded by a di-leucine-like motif. We found that the tyrosine-based basolateral sorting signal also determines AQP4 clathrin-dependent endocytosis through direct interaction with the mu subunit of AP2 adaptor complex. Once endocytosed, a regulated switch in mu subunit interaction changes AP2 adaptor association to AP3. We found that the stress-induced kinase casein kinase (CK)II phosphorylates the Ser276 immediately preceding the tyrosine motif, increasing AQP4-mu 3A interaction and enhancing AQP4-lysosomal targeting and degradation. AQP4 phosphorylation by CKII may thus provide a mechanism that regulates AQP4 cell surface expression.
水通道蛋白4（AQP4）是大脑中主要的水通道。它定位于星形胶质细胞的特定膜结构域，在应对脑水肿形成时对脑水平衡起着关键作用。AQP4也在上皮细胞的基底外侧膜中特异性表达。然而，其极化靶向和膜运输所涉及的分子机制在很大程度上仍不清楚。在这里，我们表明两个独立的C端信号决定了上皮MDCK细胞中AQP4的基底外侧膜靶向。一个信号涉及基于酪氨酸的基序；另一个由类似双亮氨酸的基序编码。我们发现基于酪氨酸的基底外侧分选信号还通过与AP2衔接复合体的μ亚基直接相互作用来决定AQP4网格蛋白依赖性内吞作用。一旦被内吞，μ亚基相互作用的调节性切换会将AP2衔接蛋白的结合改变为AP3。我们发现应激诱导激酶酪蛋白激酶（CK）II使酪氨酸基序之前紧邻的Ser276磷酸化，增加AQP4-μ 3A相互作用并增强AQP4溶酶体靶向和降解。因此，CKII介导的AQP4磷酸化可能提供一种调节AQP4细胞表面表达的机制。
