冷冻电子显微镜（Cryo-EM）使得可以确定大小超过50 kDa的蛋白质的结构，其中包括许多其他方法难以解决的蛋白质，然而在较小蛋白质的结构确定中效果较差。在这里，我们通过将小蛋白质与基于设计的蛋白质笼子的刚性分子支架结合，获得了分辨率达到2.9 Å的原子细节的结构。我们将这个系统应用于关键的癌症信号蛋白KRAS（大小为19 kDa），通过Cryo-EM获得了四个癌基因突变变异体的结构。重要的是，关键的G12C突变体与抑制剂药物（AMG510）结合的结构与之前在晶体状态下的数据相比显示出显著的构象差异。这些发现强调了Cryo-EM分子支架在癌症和其他人类疾病的小型治疗蛋白靶标设计药物分子研究中前景广阔。

Cryoelectron microscopy (Cryo-EM) has enabled structural determination of proteins larger than about 50 kDa, including many intractable by any other method, but it has largely failed for smaller proteins. Here, we obtain structures of small proteins by binding them to a rigid molecular scaffold based on a designed protein cage, revealing atomic details at resolutions reaching 2.9 Å. We apply this system to the key cancer signaling protein KRAS (19 kDa in size), obtaining four structures of oncogenic mutational variants by cryo-EM. Importantly, a structure for the key G12C mutant bound to an inhibitor drug (AMG510) reveals significant conformational differences compared to prior data in the crystalline state. The findings highlight the promise of cryo-EM scaffolds for advancing the design of drug molecules against small therapeutic protein targets in cancer and other human diseases.