Incorporation of the dopamine D2L receptor and bacteriorhodopsin within bicontinuous cubic lipid phases. 1. Relevance to in meso crystallization of integral membrane proteins in monoolein systems

The dopamine D2L receptor and bacteriorhodopsin (bR), which are integral membrane proteins, have been incorporated within bicontinuous cubic mesophases formed by the lipid monoolein, which is the standard lipid used for in meso crystallization experiments. In both cases the incorporated membrane protein was found to promote a structural transition within the underlying cubic phase. The structural effects observed were found to depend strongly on the size and shape of the membrane protein. For incorporation of bR the structural transition observed, from a Q(II)(G) to a Q(II)(D) cubic architecture, is consistent with the effect of the hydrophobic domain of bR on the curvature of the polar-apolar interface and results in a thicker bilayer, alleviating hydrophobic mismatch between the protein and the lipid bilayer. The significantly increased hydrophilic domain size associated with the D2L receptor results in the reverse transition, from a Q(II)(D) to a Q(II)(G) phase, with a concomitant increase in water channel diameter. The observed structural effects have ramifications in designing in meso crystallization trials where the underlying cubic structure must be retained.