LCP-FRAP Assay for Pre-Screening Membrane Proteins for in Meso Crystallization

Fluorescence recovery after photobleaching was used to study the diffusion of two integral membrane proteins, bacteriorhodopsin and beta2-adrenergic receptor, in lipidic cubic phase (LCP). We found that the diffusion properties within the LCP matrix strongly depend on the protein construct and applied screening conditions. Common precipitants often induce restriction on diffusion of proteins in LCP and thereby impede their chances for crystallization. A high protein mobile fraction and a fast diffusion rate correlate very well with known crystallization conditions. Using this knowledge, one can now pre-screen precipitant conditions with microgram quantities of material to rule out conditions that are not conducive to diffusion, nucleation, and crystal growth. The results of this assay will narrow membrane protein crystallization space by identifying suitable protein constructs, stabilizing compounds and precipitant conditions amenable to in meso crystallization. Crystallization pre-screening will significantly increase the chances of obtaining initial crystal hits, expediting efforts in generating high-resolution structures of challenging membrane protein targets.