Molecular kinetics. Ras activation by SOS: allosteric regulation by altered fluctuation dynamics

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Signaling

Background:

• The small guanosine triphosphatase H-Ras is a key regulator in cellular signal transduction.
• Son of Sevenless (SOS) is an exchange factor that activates H-Ras by catalyzing nucleotide exchange.
• SOS activity is tightly regulated by protein and membrane interactions.

Purpose of the Study:

• To characterize the specific activity of individual SOS molecules during H-Ras nucleotide exchange.
• To investigate the role of Ras-guanosine triphosphate (GTP) in modulating SOS activity at the single-molecule level.

Main Methods:

• Single-molecule kinetic analysis was employed to track the activity of individual SOS enzymes.
• Kinetic traces were analyzed to identify distinct functional states and their dwell times.

Main Results:

• Individual SOS molecules exhibit a broad distribution of turnover rates, fluctuating between long-lived functional states (over 100 seconds).
• Allosteric activation of SOS by Ras-GTP was not observed in the average rate.
• Ras-GTP was found to modulate fluctuations into highly active SOS states.

Conclusions:

• Enzyme function may be determined by the dynamic sampling of rate distributions by individual molecules, rather than ensemble averages.
• This study reveals a novel regulatory mechanism for SOS activity in H-Ras signaling pathways.
• The findings suggest a paradigm shift in understanding enzyme regulation, emphasizing dynamics over static averages.