Prospects for membrane protein crystals in NMX.

Adding hydrogen atoms and protonation states to structures of membrane proteins requires successful implementation of neutron macromolecular crystallography (NMX). This information would significantly increase our fundamental understanding of the transport processes membrane proteins undertake. To grow the large crystals needed for NMX studies requires significant amounts of stable protein, but once that challenge is overcome there is no intrinsic property of membrane proteins preventing the growth of large crystals per se. The calcium-transporting P-type ATPase (SERCA) has been thoroughly characterized biochemically and structurally over decades. We have extended our crystallization efforts to assess the feasibility of growing SERCA crystals for NMX-exploring microdialysis and capillary counterdiffusion crystallization techniques as alternatives to the traditional vapor diffusion crystallization experiment. Both methods possess crystallization dynamics favorable for maximizing crystal size and we used them to facilitate the growth of large crystals, validating these approaches for membrane protein crystallization for NMX.

Membrane-protein crystals for neutron diffraction.

Neutron macromolecular crystallography (NMX) has the potential to provide the experimental input to address unresolved aspects of transport mechanisms and protonation in membrane proteins. However, despite this clear scientific motivation, the practical challenges of obtaining crystals that are large enough to make NMX feasible have so far been prohibitive. Here, the potential impact on feasibility of a more powerful neutron source is reviewed and a strategy for obtaining larger crystals is formulated, exemplified by the calcium-transporting ATPase SERCA1. The challenges encountered at the various steps in the process from crystal nucleation and growth to crystal mounting are explored, and it is demonstrated that NMX-compatible membrane-protein crystals can indeed be obtained.