Room to move: crystallizing membrane proteins in swollen lipidic mesophases

The cubic phase or in meso crystallization method is responsible for almost 40 solved integral membrane protein structures. Most of these are small and compact proteins. A model for how crystals form by the in meso method has been proposed that invokes a transition between mesophases. In light of this model, we speculated that a more hydrated and open mesophase, of reduced interfacial curvature, would support facile crystallization of bigger and bulkier proteins. The proposal was explored here by performing crystallization in the presence of additives that swell the cubic phase. The additive concentration inducing swelling, as quantified by small-angle X-ray diffraction, coincided with a "crystallization window" in which two, very different transmembranal proteins produced crystals. That the swollen mesophase can grow structure-grade crystals was proven with one of these, the light-harvesting II complex. In most regards, the structural details of the corresponding complex resembled those of crystals grown by the conventional vapour diffusion method, with some important differences. In particular, packing density in the in meso-grown crystals was dramatically higher, more akin to that seen with water-soluble proteins, which accounts for their enhanced diffracting power. The layered and close in-plane packing observed has been rationalized in a model for nucleation and crystal growth by the in meso method that involves swollen mesophases. These results present a rational case for including mesophase-swelling additives in screens for in meso crystallogenesis. Their use will contribute to broadening the range of membrane proteins that yield to structure determination.