Functional Sub-states by High-pressure Macromolecular Crystallography

High Pressure Promotes Binding of the Allosteric Inhibitor Zn2+-Cyclen in Crystals of Activated H-Ras

In this work, we experimentally investigate the potency of high pressure to drive a protein toward an excited state where an inhibitor targeted for this state can bind. Ras proteins are small GTPases cycling between active GTP-bound and inactive GDP-bound states. Various states of GTP-bound Ras in active conformation coexist in solution, amongst them, state 2 which binds to effectors, and state 1, weakly populated at ambient conditions, which has a low affinity for effectors. Zn2+-cyclen is an allosteric inhibitor of Ras protein, designed to bind specifically to the state 1. In H-Ras(wt).Mg2+.GppNHp crystals soaked with Zn2+-cyclen, no binding could be observed, as expected in the state 2 conformation which is the dominant state at ambient pressure. Interestingly, Zn2+-cyclen binding is observed at 500 MPa pressure, close to the nucleotide, in Ras protein that is driven by pressure to a state 1 conformer. The unknown binding mode of Zn2+-cyclen to H-Ras can thus be fully characterized in atomic details. As a more general conjunction from our study, high pressure x-ray crystallography turns out to be a powerful method to induce transitions allowing drug binding in proteins that are in low-populated conformations at ambient conditions, enabling the design of specific inhibitors.

Pushed to extremes: distinct effects of high temperature versus pressure on the structure of STEP

Protein function hinges on small shifts of three-dimensional structure. Elevating temperature or pressure may provide experimentally accessible insights into such shifts, but the effects of these distinct perturbations on protein structures have not been compared in atomic detail. To quantitatively explore these two axes, we report the first pair of structures at physiological temperature versus. high pressure for the same protein, STEP (PTPN5). We show that these perturbations have distinct and surprising effects on protein volume, patterns of ordered solvent, and local backbone and side-chain conformations. This includes interactions between key catalytic loops only at physiological temperature, and a distinct conformational ensemble for another active-site loop only at high pressure. Strikingly, in torsional space, physiological temperature shifts STEP toward previously reported active-like states, while high pressure shifts it toward a previously uncharted region. Altogether, our work indicates that temperature and pressure are complementary, powerful, fundamental macromolecular perturbations.

Exploring the structural dynamics of proteins by pressure perturbation using macromolecular crystallography

High pressure is a convenient thermodynamic parameter to probe the dynamics of proteins as it is intimately related to volume which is essential for protein function. To be biologically active, a protein fluctuates between different substates. Pressure perturbation can promote some hidden substates by modifying the population between them. High pressure macromolecular crystallography (HPMX) is a perfect tool to capture and to characterize such substates at a molecular level providing new insights on protein dynamics. The present chapter describes the use of the diamond anvil cell to perform HPMX experiments. It also provides tips on sample preparation and optimal data collection as well as on efficient analysis of the resulting high-pressure structures.

Pushed to extremes: distinct effects of high temperature vs. pressure on the structure of an atypical phosphatase

Protein function hinges on small shifts of three-dimensional structure. Elevating temperature or pressure may provide experimentally accessible insights into such shifts, but the effects of these distinct perturbations on protein structures have not been compared in atomic detail. To quantitatively explore these two axes, we report the first pair of structures at physiological temperature vs. high pressure for the same protein, STEP (PTPN5). We show that these perturbations have distinct and surprising effects on protein volume, patterns of ordered solvent, and local backbone and side-chain conformations. This includes novel interactions between key catalytic loops only at physiological temperature, and a distinct conformational ensemble for another active-site loop only at high pressure. Strikingly, in torsional space, physiological temperature shifts STEP toward previously reported active-like states, while high pressure shifts it toward a previously uncharted region. Together, our work argues that temperature and pressure are complementary, powerful, fundamental macromolecular perturbations.

Comparative study of the effects of high hydrostatic pressure per se and high argon pressure on urate oxidase ligand stabilization

The stability of the tetrameric enzyme urate oxidase in complex with excess of 8-azaxanthine was investigated either under high hydrostatic pressure per se or under a high pressure of argon. The active site is located at the interface of two subunits, and the catalytic activity is directly related to the integrity of the tetramer. This study demonstrates that applying pressure to a protein–ligand complex drives the thermodynamic equilibrium towards ligand saturation of the complex, revealing a new binding site. A transient dimeric intermediate that occurs during the pressure-induced dissociation process was characterized under argon pressure and excited substates of the enzyme that occur during the catalytic cycle can be trapped by pressure. Comparison of the different structures under pressure infers an allosteric role of the internal hydrophobic cavity in which argon is bound, since this cavity provides the necessary flexibility for the active site to function.

Equilibria between conformational states of the Ras oncogene protein revealed by high pressure crystallography

In this work, we experimentally investigate the allosteric transitions between conformational states on the Ras oncogene protein using high pressure crystallography. Ras protein is a small GTPase involved in central regulatory processes occurring in multiple conformational states. Ras acts as a molecular switch between active GTP-bound, and inactive GDP-bound states, controlling essential signal transduction pathways. An allosteric network of interactions between the effector binding regions and the membrane interacting regions is involved in Ras cycling. The conformational states which coexist simultaneously in solution possess higher Gibbs free energy than the ground state. Equilibria between these states can be shifted by applying pressure favouring conformations with lower partial molar volume, and has been previously analyzed by high-pressure NMR spectroscopy. High-pressure macromolecular crystallography (HPMX) is a powerful tool perfectly complementary to high-pressure NMR, allowing characterization at the molecular level with a high resolution the different allosteric states involved in the Ras cycling. We observe a transition above 300 MPa in the crystal leading to more stable conformers. Thus, we compare the crystallographic structures of Ras(wt)·Mg²⁺·GppNHp and Ras(D33K)·Mg²⁺·GppNHp at various high hydrostatic pressures. This gives insight into per-residue descriptions of the structural plasticity involved in allosteric equilibria between conformers. We have mapped out at atomic resolution the different segments of Ras protein which remain in the ground-state conformation or undergo structural changes, adopting excited-energy conformations corresponding to transient intermediate states. Such in crystallo phase transitions induced by pressure open the possibility to finely explore the structural determinants related to switching between Ras allosteric sub-states without any mutations nor exogenous partners.

The equilibria between structural states induced by pressure within the crystal structure of Ras are illustrated with different colors corresponding to different Ras substates.