Conformational states of human rat sarcoma (Ras) protein complexed with its natural ligand GTP and their role for effector interaction and GTP hydrolysis

The guanine nucleotide-binding protein Ras exists in solution in two different conformational states when complexed with different GTP analogs such as GppNHp or GppCH(2)p. State 1 has only a very low affinity to effectors and seems to be recognized by guanine nucleotide exchange factors, whereas state 2 represents the high affinity effector binding state. In this work we investigate Ras in complex with the physiological nucleoside triphosphate GTP. By polarization transfer (31)P NMR experiments and effector binding studies we show that Ras(wt)·Mg(2+)·GTP also exists in a dynamical equilibrium between the weakly populated conformational state 1 and the dominant state 2. At 278 K the equilibrium constant between state 1 and state 2 of C-terminal truncated wild-type Ras(1-166) K(12) is 11.3. K(12) of full-length Ras is >20, suggesting that the C terminus may also have a regulatory effect on the conformational equilibrium. The exchange rate (k(ex)) for Ras(wt)·Mg(2+)·GTP is 7 s(-1) and thus 18-fold lower compared with that found for the Ras·GppNHp complex. The intrinsic GTPase activity substantially increases after effector binding for the switch I mutants Ras(Y32F), (Y32R), (Y32W), (Y32C/C118S), (T35S), and the switch II mutant Ras(G60A) by stabilizing state 2, with the largest effect on Ras(Y32R) with a 13-fold increase compared with wild-type. In contrast, no acceleration was observed in Ras(T35A). Thus Ras in conformational state 2 has a higher affinity to effectors as well as a higher GTPase activity. These observations can be used to explain why many mutants have a low GTPase activity but are not oncogenic.

Fundamental link between folding states and functional states of proteins

Folding and function of proteins are two aspects of proteins which are usually considered as basically unrelated phenomena that are optimized by evolution independently. From the funnel model of folding/unfolding and the associated energy landscape, we infer the paradigm that the minimum number of folding intermediates is determined by the number of all functional states of a protein ("essential" folding intermediates). Here, we demonstrate the supposed fundamental link using the Ras protein complexed with the GTP analogue GppNHp that occurs in two structural states coexisting in solution. State 2 was shown earlier to represent the effector interacting state, and the function of state 1 was hitherto unknown. By (31)P NMR spectroscopy, we demonstrate that state 1 represents the conformation interacting with guanine nucleotide exchange factors (GEFs). Denaturation experiments of the protein with a chaotropic reagent show that both functional states coexist during folding and unfolding. Application of high pressure represents another perturbation of the energy landscape, leading to an increased population of the state 1 as observed by NMR spectroscopy. The specific volume difference between the two states DeltaV(12) is 17.2 +/- 0.5 mL mol(-1), indicating that state 1 represents a more open conformation of the protein. The free energies of stabilization for state 1 and state 2 at 278 K can be determined as 8.3 and 9.8 kJ mol(-1), respectively.

Conformational states of Ras complexed with the GTP analogue GppNHp or GppCH2p: implications for the interaction with effector proteins

The guanine nucleotide-binding protein Ras occurs in solution in two different states, state 1 and state 2, when the GTP analogue GppNHp is bound to the active center as detected by (31)P NMR spectroscopy. Here we show that Ras(wt).Mg(2+).GppCH(2)p also exists in two conformational states in dynamic equilibrium. The activation enthalpy DeltaH(++)(12) and the activation entropy DeltaS(++)(12) for the transition from state 1 to state 2 are 70 kJ mol(-1) and 102 J mol(-1) K(-1), within the limits of error identical to those determined for the Ras(wt).Mg(2+).GppNHp complex. The same is true for the equilibrium constants K(12) = [2]/[1] of 2.0 and the corresponding DeltaG(12) of -1.7 kJ mol(-1) at 278 K. This excludes a suggested specific effect of the NH group of GppNHp on the equilibrium. The assignment of the phosphorus resonance lines of the bound analogues has been done by two-dimensional (31)P-(31)P NOESY experiments which lead to a correction of the already reported assignments of bound GppNHp. Mutation of Thr35 in Ras.Mg(2+).GppCH(2)p to serine leads to a shift of the conformational equilibrium toward state 1. Interaction of the Ras binding domain (RBD) of Raf kinase or RalGDS with Ras(wt) or Ras(T35S) shifts the equilibrium completely to state 2. The (31)P NMR experiments suggest that, besides the type of the side chain of residue 35, a main contribution to the conformational equilibrium in Ras complexes with GTP and GTP analogues is the effective acidity of the gamma-phosphate group of the bound nucleotide. A reaction scheme for the Ras-effector interaction is presented which includes the existence of two conformations of the effector loop and a weak binding state.