Unlocking the mysteries of Na+-K+-ATPase endocytosis: phosphorylation is the key.
Unlocking the mysteries of Na+-K+-ATPase endocytosis: phosphorylation is the key.
复制标题
解开Na-K-ATP酶内吞作用的奥秘：磷酸化是关键。
DOI：
10.1165/rcmb.f317
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发表时间：
2006
影响因子：
6.4
通讯作者：
Collawn,JamesF
中科院分区：
文献类型：
--
作者：
Collawn,JamesF
对受体和转运蛋白内吞作用的任何理解都始于Brown和Goldstein对LDL受体和家族性高胆固醇血症患者的经典研究（综述见参考文献1）。他们证明LDL受体通过网格蛋白包被的凹坑快速内化，因为它在受体的50个氨基酸胞质结构域中含有基于酪氨酸的信号（2）。很快，其他受体（如转铁蛋白受体）中的相关信号被发现，因此似乎有一种共同的机制可以解释所有细胞表面蛋白质是如何被内吞的。对于像LDL受体和转铁蛋白受体这样的转运蛋白，细胞在其中摄取货物（即胆固醇和铁），除了在细胞分裂期间发生的中断外，该过程似乎是组成性的。有趣的是，后来的研究表明网格蛋白介导的内吞途径受到有丝分裂磷酸化的抑制（3），说明经典的磷酸化-去磷酸化调节模式是内吞循环的一个不可或缺的调节组分。在确定胞质尾内的信号被网格蛋白包被的小窝的组分识别之后，该过程中的关键参与者被鉴定为被称为衔接蛋白-2或AP-2的组装蛋白（在参考文献4中综述）。这种蛋白质充当受体和网格蛋白之间的桥梁，并通过其识别基于酪氨酸的信号，促进两件事：相关受体的聚集和网格蛋白组装。因此，建立的模型是AP-2是一个共同的适配器，识别和促进所有细胞表面蛋白的内化。然而，这个简单的观点很快就被推翻了，因为很明显存在大量其他受体的“适配器”蛋白，包括介导某些G蛋白偶联受体（GPCR）内化的抑制蛋白（5）。然而，AP-2，无论是直接或间接，仍然是网格蛋白介导的细胞表面蛋白的内吞作用的中心组成部分。AP-2识别基于酪氨酸的信号YXX（其中X是任何氨基酸，并且是大体积疏水残基[6]）的机制涉及AP-2复合物的一个亚基2与细胞表面蛋白质尾区中的4-残基序之间的直接相互作用（7）。AP-2复合物是由两条100-kD链（和2）、一条50-kD链（2）和一条17-kD链（2）组成的细胞溶质异源四聚体。部分AP-2异源四聚体的晶体结构显示，2亚基的C-末端结构域可将基于酪氨酸的信号容纳到2亚基中的疏水口袋中（8）。然而，有趣的是，这种基于酪氨酸的基序的疏水结合口袋通常被掩埋，这表明与基于酪氨酸的信号相互作用需要构象变化（9）。随后的研究提供了证据，证明2亚基的苏氨酸156的磷酸化是高亲和力结合和受体内化所必需的（10，11），支持了原始模型。这种磷酸化开关提供了一种关键的调节机制，
Any understanding of endocytosis of receptors and transporters begins with the classic studies on the LDL receptor and patients with familial hypercholesterolemia by Brown and Goldstein (reviewed in Ref. 1). They demonstrated that the LDL receptor rapidly internalized via clathrin-coated pits because it contained a tyrosine-based signal within the receptor’s 50–amino acid cytoplasmic domain (2). Soon related signals in other receptors such as the transferrin receptor were identified, and thus it appeared that a common mechanism could explain how all cell surface proteins were endocytosed. For transport proteins like the LDL receptor and the transferrin receptor where cargo (ie, cholesterol and iron) are taken in by the cell, the process appeared to be constitutive except for the interruption that occurs during cell division. Interestingly, later studies demonstrated that the clathrin-mediated endocytic pathway is inhibited by mitotic phosphorylation (3), illustrating that the classic phosphorylation–dephosphorylation regulation paradigm is an integral regulatory component of the endocytic cycle. After establishing that signals within the cytoplasmic tails were recognized by components of the clathrin-coated pit, a key player in this process was identified as an assembly protein referred to as adaptor protein-2, or AP-2 (reviewed in Ref. 4). This protein served as a bridge between the receptors and clathrin and through its recognition of the tyrosine-based signal, promoted two things: clustering of the relevant receptors and clathrin assembly. Thus the model established was that AP-2 was a common adaptor that recognized and promoted the internalization of all cell surface proteins. This simple view, however, was soon dispelled when it became clear that a large number of other “adaptor” proteins existed for other receptors, including the-arrestins that mediate internalization of some G protein–coupled receptors (GPCR)(5). AP-2, however, either directly or indirectly, still remains a central component of clathrin-mediated endocytosis of cell surface proteins. The mechanism for recognition of the tyrosine-based signal, YXX (where X is any amino acid and is a bulky hydrophobic residue [6]) by AP-2 involves a direct interaction between one of the subunits of the AP-2 complex, 2, and the 4-residue motif in the cytoplasmic tail of the cell surface protein (7). The AP-2 complex is a cytosolic heterotetramer consisting of two 100-kD chains (and 2), one 50-kD chain (2), and a 17-kD chain (2). The crystal structure for part of the AP-2 heterotetramer revealed that the C-terminal domain of the 2 subunit could accommodate the tyrosine-based signal into a hydrophobic pocket in the 2 subunit (8). Interestingly, however, this hydrophobic binding pocket for the tyrosine-based motif is normally buried, suggesting that a conformation change would be required for interaction with the tyrosine-based signal (9). Subsequent studies provided evidence that phosphorylation of threonine 156 of the 2 subunit is required for high-affinity binding and receptor internalization (10, 11), supporting the original model. This phosphorylation switch provided a key regulatory