KAIST

<(From left) Professor Won Do Heo (KAIST), Postdoctoral Researcher Heeyoung Lee (KAIST, First Author), Professor Kwang-Hyun Cho (KAIST), Professor Kapsang Lee (Johns Hopkins University, USA), Dr. Sangkyu Lee (IBS), Dr. Dongsan Kim (LIBD), Dr. Yeaji Seo (Hulux) (Co-First Authors)>

Cell movement is an essential biological process, whether it's cancer cells metastasizing to other parts of the body or immune cells migrating to heal a wound. However, the principle by which cells autonomously determine their direction of movement without external stimuli has remained unknown until now.

Through this research, KAIST and an international joint research team have elucidated the principle by which cells decide their direction and move on their own, offering a crucial clue for identifying the causes of cancer metastasis and immune diseases and establishing new treatment strategies.

KAIST announced on the 10^th of November that the research team led by Endowed Chair Professor Won Do Heo of the Department of Biological Sciences, in collaboration with the research team of Endowed Chair Professor Kwang-Hyun Cho of the Department of Bio and Brain Engineering, and Professor Kapsang Lee's research team at Johns Hopkins University in the US, has for the first time in the world identified the 'autonomous driving mechanism' by which cells determine their direction of movement without external signals.

The research team developed a new imaging technique called 'INSPECT (INtracellular Separation of Protein Engineered Condensation Technique)' that allows direct visualization of how proteins interact within living cells. Using this technology, they revealed the principle of the cell's internal program for autonomously deciding its direction of movement.

The team newly analyzed the operation of the key proteins that regulate cell movement, the Rho family proteins (Rac1, Cdc42, RhoA). The results showed that these proteins do not merely divide the front and back of the cell, as previously theorized, but that the cell's decision to move straight or change direction depends on which protein it binds with.

The INSPECT technology artificially implements the phenomenon of 'phase separation,' where proteins, upon binding, naturally form segregated regions that do not mix well. This technique allows for the direct visualization of how proteins actually bind within the cell using a fluorescent signal.

<Figure 1. INSPECT: A technique for visualizing Intracellular Protein-Protein Interactions">

The research team used the proteins ferritin and the fluorescent protein DsRed to make the clusters, or 'condensates,' visible to the eye when proteins bind together like small droplets.

Using this technology, the team analyzed a total of 285 pairs of interactions by combining 15 types of Rho proteins with 19 types of binding proteins, confirming actual binding in 139 pairs. Specifically, they identified that the Cdc42–FMNL protein combination is the core circuit responsible for the cell's 'straight movement,' while the Rac1–ROCK protein combination is responsible for the cell's 'change of direction.'

The research team slightly modified a part of the Rac1 protein (the 37th amino acid), which is crucial for cell direction control, to prevent it from binding well with the 'steering wheel' protein, ROCK. As a result, the cells could not change direction and continued to move in a straight line.

In contrast, in normal cells, Rac1 and ROCK bind well, forming a structure called 'arc stress fiber' at the front of the cell. This fiber enables the cell to make near-perpendicular turns when changing direction.

Furthermore, in an experiment where the environment the cells were attached to was changed, normal cells adjusted their moving speed according to the surrounding environment, but the Rac1F37W cells (cells with a broken 'steering wheel') maintained the same speed regardless of environmental changes. This demonstrates that the Rac–ROCK protein axis subtly controls the cell's ability to recognize and adapt to its surrounding environment.