Cryo-EM Reveals Cancer-Causing Mutations in Protein Assembly

Scientists at St. Jude Children's Research Hospital have used cryo-electron microscopy to capture the first 3D structure of the entire SPOP assembly, a protein that plays a role in prostate, endometrial, uterine, and other cancers. The study, published in Molecular Cell, revealed previously unknown SPOP interfaces that harbor clusters of cancer-causing mutations.

The normal function of SPOP is to control the level of certain proteins within a cell. When SPOP is dysregulated through mutations, it can cause abnormal behaviors in the cell and trigger cancer. The researchers identified key interactions between protein regions that had not been seen before. These were precisely the regions that are mutated in endometrial cancer.

The researchers found that mutations in the MATH domain affected the propensity of the protein to assemble and hang around in cells. They also observed how a single mutation could dramatically affect how the protein forms filaments — for example, mutating the interface that locks the MATH domain in place completely changed the assembly of the proteins from a single to a double filament with intertwined MATH domains. This discovery helped further explain how SPOP mutations contribute to cancer.

The team hopes their findings will lead to new drugs targeting the mutant forms of SPOP in endometrial cancer. In addition, it may be possible to differentially target SPOP based on its functional form within the cell. This would be a significant advancement in cancer treatment as it would allow for more targeted and effective therapies.

Previous studies demonstrated that the SPOP protein assembles into long filaments. However, the researchers could tell that the complete picture had yet to be uncovered, as certain mutations unexpectedly affected this filament. Discovering these additional protein interfaces in the filament using cryo-EM helped further explain how SPOP mutations contribute to cancer.

"Taking a broader view to look at the full-length protein gave us a deeper understanding of how mutations affect SPOP," says co-first author Brian O'Flynn, Ph.D., St. Jude Department of Structural Biology. "The scale of the change just from one point mutation is huge. It was unexpected to see that."