Exploring the in meso crystallization mechanism by characterizing the lipid mesophase microenvironment during the growth of single transmembrane α-helical peptide crystals

The proposed mechanism for in meso crystallisation of transmembrane proteins suggests that a protein or peptide is initially uniformly dispersed in the lipid self-assembly cubic phase but that crystals grow from a local lamellar phase, which acts as a conduit between the crystal and the bulk cubic phase. However, there is very limited experimental evidence for this theory. We have developed protocols to investigate the lipid mesophase microenvironment during crystal growth using standard procedures readily available in crystallography laboratories. This technique was used to characterize the microenvironment during crystal growth of the DAP12-TM peptide using synchrotron Small Angle X-ray Scattering with a micro-sized X-ray beam. Crystal growth was found to occur from the Gyroid cubic mesophase. For one in four crystals a highly-oriented local lamellar phase was observed, providing supporting evidence for the proposed mechanism for in meso crystallisation. A new observation of this stud...