Electrostatically optimized Ras-binding Ral guanine dissociation stimulator mutants increase the rate of association by stabilizing the encounter complex

Association of two proteins can be described as a two-step process, with the formation of an encounter complex followed by desolvation and establishment of a tight complex. Here, by using the computer algorithm PARE, we designed a set of mutants of the Ras effector protein Ral guanine nucleotide dissociation stimulator (RalGDS) with optimized electrostatic steering. The fastest binding RalGDS mutant, M26K,D47K,E54K, binds Ras 14-fold faster and 25-fold tighter compared with WT. A linear correlation was found between the calculated and experimental data, with a correlation coefficient of 0.97 and a slope of 0.65 for the 24 mutants produced. The data suggest that increased electrostatic steering specifically stabilizes the encounter complex and transition state. This conclusion is backed up by Phi analysis of the encounter complex and transition state of the RalGDS(M26K,D47K,ES4K)/Ras complex, with both values being close to 1. Upon further formation of the final complex, the increased Coulombic interactions are probably counterbalanced by the cost of desolvation of charges, keeping the dissociation rate constant almost unchanged. This mechanism is also reflected by the mutual compensation of enthalpy and entropy changes quantified by isothermal titration calorimetry. The binding constants of the faster binding RalGDS mutants toward Ras are similar to those of Raf, the most prominent Ras effector, suggesting that the design methodology may be used to switch between signal transduction pathways.