Novel Mechanism for GRB2-SOS1-Mediated Liquid-Liquid Phase Separation

Researchers at Tokyo Metropolitan University have uncovered new details about how the proteins GRB2 and SOS1 transmit signals from membrane receptors to cell nuclei. By using nuclear magnetic resonance (NMR), they explored the specific regions of GRB2 and SOS1 that bind to each other, focusing on how these interactions contribute to liquid-liquid phase separation (LLPS). Signal transduction issues are a primary cause of cancer, and a deeper understanding of these processes could lead to transformative treatments.

Biological cells operate through intricate signal pathways, where reactions in one part of the cell lead sequentially to others. This process relies on structural changes in proteins, forming a vast biomolecular relay where signals are “passed” via a cascade of protein interactions. This "signal transduction" is essential for cellular health, and mutations in the genes encoding these signal-passing proteins contribute to many cancers. Consequently, scientists are keen to understand how this cellular relay system functions and is regulated to develop potential treatments and preventive strategies.

A research team from Tokyo Metropolitan University, led by Associate Professor Teppei Ikeya, has been studying GRB2 and SOS1, two proteins critical for transmitting signals from specific membrane receptors to the RAS protein. RAS, in turn, plays a pivotal role in sending signals to the cell nucleus, where DNA resides. This signaling process eventually enables the cell to control protein synthesis based on the original signal. However, the precise mechanisms behind this pathway are still not fully understood.

This gap in understanding is partly due to the “floppiness” or flexibility of GRB2 and SOS1, which makes them difficult to study with traditional methods like cryo-transmission electron microscopy and X-ray crystallography.

Using NMR and advanced statistical tools, the team has now provided new insights into the roles of GRB2 and SOS1 in signal transduction. GRB2 is known to contain three domains (NSH3, SH2, and CSH3), with the two SH3 domains (NSH3 and CSH3) binding to SOS1. The team found that NSH3 has 10 to 20 times more affinity for SOS1 than CSH3, contradicting the previous assumption that both SH3 domains had similar binding strengths. They also discovered distinct dynamics between the two domains, with CSH3 exhibiting independent mobility from the other regions.

This research offers a far more detailed view of RAS signal transduction than previously available. Recent studies also link GRB2 and SOS1 to liquid-liquid phase separation (LLPS), where dense droplets form within cells, modulating the strength of signals sent to the RAS protein.

According to the team’s proposed mechanism, the flexible CSH3 domain attracts additional free SOS1 molecules, while SOS1 regions with high affinity for the SH3 domains can bind multiple NSH3 domains. This interaction creates large, flexible domains rich in GRB2 and SOS1, with GRB1 acting as a bridging protein. This is the first time a mechanism for LLPS involving GRB2 and SOS1 has been suggested.

These findings provide unprecedented insight into cell signaling mechanisms and illuminate how failures in these processes can lead to disease. The team hopes that their work will not only inspire further research but also pave the way for new cancer treatments.