Single bacteriorhodopsin molecules revealed on both surfaces of freeze-dried and heavy metal-decorated purple membranes.

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Abstract
The flat sheets of the purple membrane from Halobacterium halobium contain only a single protein (bacteriorhodopsin) arranged in a hexagonal lattice. After freeze-drying at -80.degree. C (a method that is superior to air-drying), shadowing with tantalum/tungsten, and image processing, structural details on both surfaces are portrayed in the range of 2 nm. One surface is rough and lattice lines are clearly visible, whereas the other is smooth and the hexagonal order seems to be absent. The optical diffraction patterns indicate a hexagonal lattice for both surfaces. These diffraction patterns are characteristic and easily distinguished. The orientation of the 2 surfaces was identified by Ag decoration: partial condensation of Ag on purple membranes enabled the smooth surface to be identified as the plasmatic and the rough surface as the exoplasmatic surface. After image processing, the exoplasmatic surface shows a triplet structure which exactly fits the projected structure determined by Unwin and Henderson (1975) at molecular resolution; on the plasmatic surface, 4 image details per unit cell are visible. Three of them match the arrangement of bacteriorhodopsin, whereas the 4th must be located over a lipidic array. It is thus possible to show the part of each single bacteriorhodopsin protein that is present in the surfaces of the purple membrane. By shadowing the membranes perpendicularly, it was shown that these components of the surfaces are mainly portrayed by a decoration effect of the tantalum/tungsten condensate.