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.

SmgGDS在细胞中具有双重功能，并调节小GTP酶，既是Rho家族的鸟嘌呤核苷酸交换因子（GEF），又是小GTP酶的分子伴侣，该小伴侣具有C端多碱基区域，其后是四个C端残基，称为CaaX基序，其半胱氨酸残基被翻译后异戊二烯化。我们最近的结构研究表明，SmgGDS可以折叠成串联复制的犰狳重复基序（ARMs），而其他GEF则不存在。但是，由于SmgGDS没有典型的GEF催化结构域，并且缺少容纳异戊二烯基的口袋，因此GEF活性的精确机制和对异戊二烯基CaaX基序的识别机制仍然未知。在这里，我们旨在确定SmgGDS /法呢基RhoA复合物的晶体结构。我们发现，SmgGDS会在开关I和II区域中诱导显着的构象变化，从而打开核苷酸结合位点，异戊二烯基团会进入N末端ARM的隐秘口袋。综上所述，我们的发现可能会增进对SmgGDS作用的理解，并使针对SmgGDS和小GTP酶的药物设计策略成为可能。