Ultrastructure of purple membrane and cell wall of Halobacterium halobium

Abstract

The purple membrane and other membrane systems of Halobacterium halobium were studied by the freeze-replica method, employing rotary shadowing, and by thin sectioning combined with freeze substitution. The trimeric structure of bacteriorhodopsin in the purple membrane was observed directly under the electron microscope. The existence of distinct pits on E face of the purple membrane was confirmed. This suggests that bacteriorhodopsin is a transmembranous protein. A new type of purple membrane arrangement yielding optically triclinic diffraction patterns with center-to-center spacing of 8 nm was distinguished from the usual type, which produces hexagonal diffraction patterns with spacings of 6.2 nm. The relationship between two arrangements was discussed with reference to the formation of purple membrane. Freeze-deep etching revealed detailed structures of the cell wall in three dimensions. The true surfaces of purple membrane and red membrane were also observed.