Crystallization of bacteriorhodopsin from bicelle formulations at room temperature

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Abstract
We showed previously that high‐quality crystals of bacteriorhodopsin (bR) from Halobacterium salinarum can be obtained from bicelle‐forming DMPC/CHAPSO mixtures at 37°C. As many membrane proteins are not sufficiently stable for crystallization at this high temperature, we tested whether the bicelle method could be applied at a lower temperature. Here we show that bR can be crystallized at room temperature using two different bicelle‐forming compositions: DMPC/CHAPSO and DTPC/CHAPSO. The DTPC/CHAPSO crystals grown at room temperature are essentially identical to the previous, twinned crystals: space group P21 with unit cell dimensions of a = 44.7 Å, b = 108.7 Å, c = 55.8 Å, β = 113.6°. The room‐temperature DMPC/CHAPSO crystals are untwinned, however, and belong to space group C2221 with the following unit cell dimensions: a = 44.7 Å, b = 102.5 Å, c = 128.2 Å. The bR protein packs into almost identical layers in the two crystal forms, but the layers stack differently. The new untwinned crystal form yielded clear density for a previously unresolved CHAPSO molecule inserted between protein subunits within the layers. The ability to grow crystals at room temperature significantly expands the applicability of bicelle crystallization.