SELECTIVE F O R M A T I O N OF BACTERIO-OPSIN TRIMERS BY CHEMICAL CROSSLINKING OF PURPLE MEMBRANE Hans-Georg DELLWEG and Manfred SUMPER

The purple membrane ofHalobacterium halobium contains a single protein known as bacteriorhodopsin, which is composed of 1 mol retinal]mol bacterio-opsin [ 1 ]. The organization of the protein within each purple membrane sheet is that of an exact 2-dimensional crystal. X-ray and electron diffraction data have revealed that the bacteriorhodopsin in purple membrane is arranged as trimeric units in a hexagonal lattice [2]. The average distance between bacteriorhodopsin molecules within a trimeric unit is significantly shorter than the spacing between bacteriorhodopsin molecules from adjacent trimers. One could predict that crosslinking of purple membrane with bifunctional reagents would lead to the selective formation of crosslinked trimers. We demonstrate here that this is indeed the case. The biosynthetic precursor of purple membrane is the brown membrane, which also contains bacterioopsin [3 ]. When this membrane is treated with crosslinking reagents, polymeric protein molecules are formed exclusively. Crosslinked purple membrane exhibits the same photointermediates as the native membrane, although the cycle of events is slowed down.