New findings about GRB2 and SOS1 could pave the way for cancer treatments

Researchers from Tokyo Metropolitan University have discovered new insights into how the proteins GRB2 and SOS1 in cells pass signals from membrane receptors to nuclei. They used nuclear magnetic resonance (NMR) to study how and which specific regions of GRB2 and SOS1 bind together, specifically how they trigger liquid-liquid phase separation (LLPS). Signal transduction issues are a major cause of cancers: understanding how it works could lead to radical new treatments.

Biological cells function through a complex network of signal pathways, where reactions in specific parts of the cell lead sequentially to others through structural changes in proteins, a vast biomolecular relay where “batons” are passed through a cascade of proteins that bind to and modify each other another. This ‘signaling’ process is key to healthy cell function. Mutations in the genes that code for these signal-transducing proteins are responsible for many tumors and cancers. In the search for new treatments and prevention methods, scientists have focused their efforts on understanding how the relay works and how the whole process is regulated.

A team of researchers led by Associate Professor Teppei Ikeya from Tokyo Metropolitan University are studying the role played by GRB2 and SOS1, two proteins known to play an important role in relaying information from certain membrane receptors to the RAS protein, which is a key factor in receiving signals in the cell nucleus, where DNA is located. This eventually leads to the cell being able to use the initial signal to regulate the production of more proteins. However, the exact function of this pathway is not fully understood. A big reason for this was the softness or “floppyness” of GRB2 and SOS1, making them difficult to study with tools such as X-ray crystallography and cryo-transmission electron microscopy.

Now, the team used nuclear magnetic resonance (NMR) techniques and state-of-the-art statistical tools to reveal new details about how GRB2 and SOS1 participate in signal transduction. GRB2 is known to have three domains (NSH3, SH2, CSH3), where the two SH3 domains (NSH3, CSH3) bind to the SOS1 protein. While it was long believed that both bound to SOS1 with equal strength, the team found that NSH3 had ten to twenty times more affinity for SOS1 than CSH3. Not only that, they also discovered key differences in their dynamics. CSH3 exhibited free mobility independent of the other domains.

The picture it provided was far more detailed than anything previously imagined for RAS signal transduction. It was also linked to recent research suggesting that GRB2 and SOS1 participate in the liquid-liquid phase separation (LLPS), where they form dense droplets in cells and regulate how strong signals are passed to the RAS. In the team’s new mechanism, parts of SOS1 that stick to SH3 domains will be able to bind multiple NSH3 domains due to their strong affinity, while the flexible CSH3 domain can attract other free SOS1 molecules. This leads to the GRB1 protein acting as a bridge, leading to large, flexible regions rich in GRB2 and SOS1. It is the first time that a mechanism for LLPS of GRB2 and SOS1 has been proposed.

This unprecedented level of detail provides new insights into how cell signaling works and can help us understand how pathologies take root when it doesn’t work as it should. The team hopes their findings will inspire not only new research, but also pathways to new cancer treatments.

This work was supported by the Funding Program for Basic Research in Evolutionary Science and Technology (CREST JPMJCR13M3 and JPMJCR21E5) from the Japan Science and Technology Agency (JST), Grants-in-Aid for Scientific Research (JP15K06979, JP19H05645) and Scientific Research for Innovative Areas (JP15H01645, JP16H00847, JP17H05887, JP19H05773, JP26102538, JP25120003, JP16H00779 and JP21K06114, the Japan Foundation, ProScience Foundation for the Advancement of Measurement Technology. The NMR experiments were performed using the platform NMR supported by the Ministry of Education, Culture, of Sports, Science and Technology (MEXT), Program Grant Number JPMXS0450100021.