Specific lipid-protein interactions in a novel honeycomb lattice structureof bacteriorhodopsin

In the purple membrane of Halobacterium salinarium, bacteriorhodopsin trime rs are arranged in a hexagonal lattice. When purple membrane sheets are inc ubated at high temperature with neutral detergent, membrane vesicularizatio n takes place, yielding inside-out vesicles with a diameter of 50 nm, The v esicular structure becomes unstable at low temperature, where successive fu sion of the vesicles yields a crystal which is composed of stacked planar m embranes. X-ray crystallographic analysis reveals that the bacteriorhodopsi n trimers are arranged in a honeycomb lattice in each membrane layer and th at neighbouring membranes orient in opposite directions. The native structu re of the trimeric unit is preserved in the honeycomb lattice, irrespective of alterations in the in-plane orientation of the trimer. One phospholipid tightly bound to a crevice between monomers in the trimeric unit is sugges ted to act as a glue in the formation of the trimer.