Topology of the retinal cone NCKX2 Na/Ca-K exchanger
Topology of the retinal cone NCKX2 Na/Ca-K exchanger
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DOI：
10.1021/bi0270788
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发表时间：
2003-03-04
期刊：
影响因子：
2.9
通讯作者：
Schnetkamp, PPM
中科院分区：
文献类型：
--
作者：
Kinjo, TG;Szerencsei, RT;Schnetkamp, PPM
Na/Ca-K交换器（NCKX）是一种多位膜蛋白，在视网膜视杆细胞和视锥细胞的Ca 2+稳态中起关键作用。NCKX 1亚型存在于视杆细胞中，而NCKX 2亚型存在于视锥细胞、视网膜神经节细胞和大脑的各个部分中。Na/Ca-K交换剂的拓扑结构被认为由两个大的亲水环和两组跨膜段（TM）组成。第一个大的亲水环位于细胞外的N-末端;另一个是细胞质的，并将两组TM分开。TM由五个和五个跨膜螺旋或五个和六个跨膜螺旋组成，这取决于所使用的预测算法。关于各种跨膜螺旋的取向和连接这些螺旋的短环的定位，还没有什么具体的信息。在这项研究中，我们已经确定了哪些连接环暴露于细胞外环境使用两种不同的方法：取代的半胱氨酸残基和插入N-糖基化位点的可及性。这两种方法产生了一致的NCKX拓扑结构，其中两组TM各自含有五个跨膜螺旋。我们的新模型将以前的跨膜螺旋6置于细胞质中，将C末端置于细胞外表面。令人惊讶的是，该NCKX拓扑模型与目前的NCX拓扑模型在C-末端三个膜螺旋方面不同。
The Na/Ca-K exchanger (NCKX) is a polytopic membrane protein that plays a critical role in Ca2+ homeostasis in retinal rod and cone photoreceptors. The NCKX1 isoform is found in rods, while the NCKX2 isoform is found in cones, in retinal ganglion cells, and in various parts of the brain. The topology of the Na/Ca-K exchanger is thought to consist of two large hydrophilic loops and two sets of transmembrane spanning segments (TMs). The first large hydrophilic loop is located extracellularly at the N-terminus; the other is cytoplasmic and separates the two sets of TMs. The TMs consist of either five and five membrane spanning helices or five and six membrane spanning helices, depending upon the predictive algorithm used. Little specific information is yet available on the orientation of the various membrane spanning helices and the localization of the short loops connecting these helices. In this study, we have determined which of the connecting loops are exposed to the extracellular milieu using two different methods: accessibility of substituted cysteine residues and insertion of N-glycosylation sites. The two methods resulted in a consistent NCKX topology in which the two sets of TMs each contain five membrane spanning helices. Our new model places what was previously membrane spanning helix six in the cytoplasm, which places the C-terminus on the extracellular surface. Surprisingly, this NCKX topology model is different from the current NCX topology model with respect to the C-terminal three membrane helices.