STRUCTURE OF CELL-ENVELOPE OF HALOBACTERIUM-HALOBIUM

The structure of the isolated cell envelope of H. halobium is studied by X-ray diffraction, EM and biochemical analysis. The envelope consists of the cell membrane and 2 layers of protein outside. The outer layer of protein shows a regular arrangement of the protein or glycoprotein particles and is therefore identified as the cell wall. Just outside the cell membrane is a 20 .ANG.-thick layer of protein. It is a 3rd structure in the envelope, the function of which may be distinct from that of the cell membrane and the cell wall. This inner layer of protein is separated from the outer protein layer by a 65 .ANG.-wide space which had an electron density very close to that of the suspending medium, and which is etched after freeze-fraction-fracture. The space is tentatively identified as the periplasmic space. At NaCl concentrations < 2.0 M, both protein layers of the envelope disintegrate. Gel filtration and analytical ultracentrifugation of the soluble components from the 2 protein layers reveal 2 major bands of protein with apparent MW of .apprx. 16,000 and 21,000. Concurrently, the cell membrane stays essentially intact when the Mg++ concentration is kept at .gtoreq. 20 mM. The cell membrane breaks into small fragments when treated with 0.1 M NaCl and EDTA, or with distilled water, and some soluble proteins, including flavins and cytochromes, are released. The cell membrane apparently has an asymmetric bilayer structure, with substantial amounts of protein penetrating the hydrophobic core of the lipid bilayer.