Single-molecule superresolution imaging allows quantitative analysis of RAF multimer formation and signaling

Area of Science:

• Cell Biology
• Molecular Biology
• Biophysics

Background:

• RAF serine/threonine kinases regulate cell growth via the MAPK pathway.
• RAF inhibitors (RAFis) target these kinases in cancer treatment.
• Protein multimerization is implicated in RAF activation and tumor response to RAFis, but visualization has been limited.

Purpose of the Study:

• To visualize and quantify RAF protein multimerization in cells.
• To elucidate the role of RAF multimer stoichiometry and localization in RAF/MAPK pathway activation.
• To understand the mechanisms of RAF multimer formation and its implications for RAF inhibitor efficacy.

Main Methods:

• Photoactivated localization microscopy (PALM) for high-resolution imaging of protein multimers.
• Quantitative spatial analysis to determine multimer stoichiometry and localization.
• Utilized N-terminal truncated CRAF (CatC) and membrane-targeting constructs to investigate multimerization mechanisms.

Main Results:

• CRAF exists as monomers in the cytoplasm but forms dimers, trimers, and tetramers at the cell membrane upon active RAS binding.
• N-terminal truncated CRAF (CatC) forms constitutive dimers and tetramers in the cytoplasm.
• Disruption of the CRAF-CRAF dimer interface prevents multimer formation.
• Forcing CRAF to the membrane induces multimerization, but activation requires an intact dimer interface.

Conclusions:

• Directly confirms the existence and visualization of RAF dimers and higher-order multimers.
• RAF multimer formation is critical, but not sufficient, for RAF/MAPK pathway activation.
• Multimerization can occur through multiple mechanisms and is essential for RAF signaling in cancer.