dPob/EMC is essential for biosynthesis of rhodopsin and other multi-pass membrane proteins in Drosophila photoreceptors.
dPob/EMC is essential for biosynthesis of rhodopsin and other multi-pass membrane proteins in Drosophila photoreceptors.
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DOI：
10.7554/elife.06306
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发表时间：
2015-02-26
期刊：
影响因子：
7.7
通讯作者：
Satoh AK
中科院分区：
文献类型：
--
作者：
Satoh T;Ohba A;Liu Z;Inagaki T;Satoh AK
在真核生物中，大多数整合膜蛋白是合成的，整合到膜中，并在内质网（ER）中正确折叠。我们筛选了影响果蝇光感受器中视紫红质1（Rh 1）横纹体表达的突变体，发现dPob/EMC 3，EMC 1和EMC 8/9，果蝇内质网膜蛋白复合物（EMC）亚基的同源物，对于在较早的步骤中稳定未成熟的Rh 1比另一个Rh 1特异性伴侣（NinaA）的作用是必不可少的。dPob/EMC 3定位于ER并与EMC 1和钙连接蛋白结合。此外，EMC是所需的其他多通道跨膜蛋白，如次要视紫红质Rh 3和Rh 4，瞬时受体电位，和Na+K+-ATP酶的稳定表达，但不是分泌蛋白或I型单通道跨膜蛋白。此外，我们发现dPob/EMC 3缺陷以光非依赖性方式诱导横纹肌变性。这些结果共同表明，EMC是包括Rh 1在内的多次跨膜蛋白生物发生的关键因素，其缺失会导致视网膜变性。DOI：http://dx.doi.org/10.7554/eLife.06306.001围绕细胞的膜含有许多蛋白质，那些跨越整个膜宽度的蛋白质被称为跨膜蛋白。视紫红质是一种这样的跨膜蛋白，它存在于眼睛的感光细胞中，在视觉中起着重要作用。跨膜蛋白质在细胞内产生，并插入到细胞膜上，包围着一个叫做内质网的隔室。在这里，它们成熟并在伴侣蛋白的帮助下“折叠”成正确的三维形状。一旦正确折叠，跨膜蛋白可以被转运到细胞膜。蛋白质的错误折叠可能会产生严重的后果;如果视紫红质被错误折叠，感光细胞可能会死亡，导致人类和其他动物失明。在斑马鱼中进行的实验表明，伴侣蛋白Pob是感光细胞存活所必需的。Pob是内质网中一组或“复合体”伴侣蛋白的一部分，称为EMC复合体。这表明EMC复合物可能参与折叠视紫红质，但细节仍不清楚。在这里，Satoh等人研究了EMC复合物在果蝇视紫红质折叠中的作用。这涉及到检查数百只携带各种基因突变的果蝇，这些果蝇的视紫红质水平也很低。实验表明，dPob-苍蝇版本的Pob-和EMC复合物中的其他两种蛋白质是新制备的视紫红质稳定所必需的。如果感光细胞缺少复合体中的蛋白质，眼睛中的感光结构就会退化。视紫红质被称为“多通道”膜蛋白，因为它多次穿过膜。Satoh等人发现，EMC复合物也是感光细胞中其他多通道膜蛋白折叠所必需的。下一个挑战将是揭示EMC复合物如何能够特异性靶向这种类型的跨膜蛋白。DOI：http://dx.doi.org/10.7554/eLife.06306.002
In eukaryotes, most integral membrane proteins are synthesized, integrated into the membrane, and folded properly in the endoplasmic reticulum (ER). We screened the mutants affecting rhabdomeric expression of rhodopsin 1 (Rh1) in the Drosophila photoreceptors and found that dPob/EMC3, EMC1, and EMC8/9, Drosophila homologs of subunits of ER membrane protein complex (EMC), are essential for stabilization of immature Rh1 in an earlier step than that at which another Rh1-specific chaperone (NinaA) acts. dPob/EMC3 localizes to the ER and associates with EMC1 and calnexin. Moreover, EMC is required for the stable expression of other multi-pass transmembrane proteins such as minor rhodopsins Rh3 and Rh4, transient receptor potential, and Na+K+-ATPase, but not for a secreted protein or type I single-pass transmembrane proteins. Furthermore, we found that dPob/EMC3 deficiency induces rhabdomere degeneration in a light-independent manner. These results collectively indicate that EMC is a key factor in the biogenesis of multi-pass transmembrane proteins, including Rh1, and its loss causes retinal degeneration. DOI: http://dx.doi.org/10.7554/eLife.06306.001 The membranes that surround cells contain many proteins, and those that span the entire width of the membrane are known as transmembrane proteins. Rhodopsin is one such transmembrane protein that is found in the light-sensitive ‘photoreceptor’ cells of the eye, where it plays an essential role in vision. Transmembrane proteins are made inside the cell and are inserted into the membrane surrounding a compartment called the endoplasmic reticulum. Here, they mature and ‘fold’ into their correct three-dimensional shape with help from chaperone proteins. Once correctly folded, the transmembrane proteins can be transported to the cell membrane. Incorrect folding of proteins can have severe consequences; if rhodopsin is incorrectly folded the photoreceptor cells can die, leading to blindness in humans and other animals. Experiments carried out in zebrafish have shown that the chaperone protein Pob is required for the survival of photoreceptor cells. Pob is part of a group or ‘complex’ of chaperone proteins in the endoplasmic reticulum called the EMC complex. This suggests that the EMC complex may be involved in folding rhodopsin, but the details remain unclear. Here, Satoh et al. studied the role of the EMC complex in the folding of rhodopsin in fruit flies. This involved examining hundreds of flies that carried a variety of genetic mutations and that also had low levels of rhodopsin. The experiments show that dPob—the fly version of Pob—and two other proteins in the EMC complex are required for newly-made rhodopsin to be stabilized. If photoreceptor cells are missing proteins from the complex, the light-sensitive structures in the eye degenerate. Rhodopsin is known as a ‘multi-pass’ membrane protein because it crosses the membrane multiple times. Satoh et al. found that the EMC complex is also required for the folding of other multi-pass membrane proteins in photoreceptor cells. The next challenge will be to reveal how the EMC complex is able to specifically target this type of transmembrane protein. DOI: http://dx.doi.org/10.7554/eLife.06306.002