Ras regulates signal transduction pathway function by dynamically interacting with various effectors. TO understand the basis for Ras function, its conformational dynamics were measured in the absence and presence of effectors using single molecule fluorescence resonance energy transfer (FRET) between probes located on the Switch II region and GTP. The time trajectories of FRET efficiency from GTP-bound Ras showed that this conformation spontaneously varies among Multiple states. Among them, a low FRET state was identified as an inactive state. The transition involving the inactive conformational state occurred in the time range of seconds. In contrast, fluctuation occurring most probably between multiple active high FRET conformational states lasted similar to 30 ins but converged to a specific conformational state upon binding to an effector. Thus, Ras conformation spontaneously fluctuates to readily interact with various effectors. (c) 2006 Elsevier Inc. All rights reserved.