GEF mechanism revealed by the structure of SmgGDS-558 and farnesylated RhoA complex and its implication for a chaperone mechanism-猫眼课题宝

DOI：

10.1073/pnas.1804740115

发表时间：

2018-09

期刊：

Proceedings of the National Academy of Sciences

影响因子：

--

通讯作者：

中科院分区：

其他

文献类型：

--

作者：

Hikaru Shimizu;S. Toma-Fukai;K. Kontani;T. Katada;Toshiyuki Shimizu

SmgGDS在细胞中发挥双重作用，不仅是Rho家族的GEF特异性蛋白，而且是具有c端多基区并伴有CaaX基序的小gtpase的伴侣蛋白。SmgGDS折叠成犰狳重复基序，在结构上不同于其他gef和戊烯基结合蛋白。SmgGDS结合诱导两个开关区域的构象变化，导致核苷酸结合位点暴露。CaaX基序上的烯丙基被插入到SmgGDS的隐口袋中，这个隐口袋是在结合时新产生的，屏蔽了脂质修饰的c端尾部不受环境影响。我们的结构表征提供了SmgGDS作为GEF如何工作的详细图像以及对伴侣机制的含义。SmgGDS在细胞中具有双重功能，既可以作为Rho家族的鸟嘌呤核苷酸交换因子（GEF）调节小gtpase，也可以作为小gtpase的分子伴侣，该小gtpase具有一个c端多基区，后面是四个c端残基CaaX基序，其翻译后在其半胱氨酸残基上被戊烯化。我们最近的结构研究表明，SmgGDS折叠成其他gef中不存在的犰狳重复基序（ARMs）的串联拷贝。然而，由于SmgGDS不具有典型的GEF催化结构域，并且缺乏容纳戊基的口袋，因此GEF活性的确切机制和戊基化CaaX基序的识别机制仍然未知。在这里，我们的目的是确定SmgGDS/法酰化RhoA复合物的晶体结构。我们发现SmgGDS在打开核苷酸结合位点的开关I和II区诱导了显着的构象变化，使戊烯基适合于n端ARMs的隐袋。综上所述，我们的发现可以促进对SmgGDS作用的理解，并使针对SmgGDS和小GTPases的药物设计策略成为可能。

Significance SmgGDS plays a dual role in the cell and acts as not only a GEF specific for the Rho family but also a chaperone protein for small GTPases possessing a C-terminal polybasic region accompanied by the CaaX motif. SmgGDS folds into armadillo-repeat motifs, structurally distinct from the other GEFs and prenyl group-binding proteins. SmgGDS binding induces conformational changes in both switch regions, resulting in exposure of the nucleotide binding site. The prenyl group at the CaaX motif is inserted into the cryptic pocket of SmgGDS, which is newly created upon binding, shielding the lipid-modified C-terminal tail from the environment. Our structural characterization provides a detailed picture of how SmgGDS works as a GEF and implication for a chaperone mechanism. SmgGDS has dual functions in cells and regulates small GTPases as both a guanine nucleotide exchange factor (GEF) for the Rho family and a molecular chaperone for small GTPases possessing a C-terminal polybasic region followed by four C-terminal residues called the CaaX motif, which is posttranslationally prenylated at its cysteine residue. Our recent structural work revealed that SmgGDS folds into tandem copies of armadillo-repeat motifs (ARMs) that are not present in other GEFs. However, the precise mechanism of GEF activity and recognition mechanism for the prenylated CaaX motif remain unknown because SmgGDS does not have a typical GEF catalytic domain and lacks a pocket to accommodate a prenyl group. Here, we aimed to determine the crystal structure of the SmgGDS/farnesylated RhoA complex. We found that SmgGDS induces a significant conformational change in the switch I and II regions that opens up the nucleotide-binding site, with the prenyl group fitting into the cryptic pocket in the N-terminal ARMs. Taken together, our findings could advance the understanding of the role of SmgGDS and enable drug design strategies for targeting SmgGDS and small GTPases.