Structure of the cell envelope of Halobacterium halobium

Creators: Blaurock, Allen E.; Stoeckenius, Walther; Oesterhelt, Dieter; Scherphof, Gerrit L.

Abstract

The structure of the isolated cell envelope of Halobacterium halobium is studied by X-ray diffraction, electron microscopy, and biochemical analysis. The envelope consists of the cell membrane and two 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 A-thick layer of protein. It is a third 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 Å-wide space which has an electron density very close to that of the suspending medium, and which can be etched after freeze-fracture. The space is tentatively identified as the periplasmic space. At NaCl concentrations below 2.0 M, both protein layers of the envelope disintegrate. Gel filtration and analytical ultracentrifugation of the soluble components from the two protein layers reveal two major bands of protein with apparent mol wt of ~16,000 and 21,000. At the same time, the cell membrane stays essentially intact as long as the Mg++ concentration is kept at ≥ 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 core of the lipid bilayer.

Additional Information

© 1976 The Rockefeller University Press. Received for publication 3 September 1974, and in revised form 26 May 1976. We thank Miss Jill Hill for preparing cells and envelopes; Mr. Z. Gabor for prints of the X-ray patterns; and Mr. Istvan Marot for help with the freeze-fracturing and freeze-etching experiments. This work was supported by National Heart and Lung Institute program project grant HL 06285. Note Added in Proof: In a recent paper on the "Purification and characterization of a prokaryotic glycoprotein from the cell envelope of Halobacterium salinarium" (J. Biol. Chem. 1976. 251:2005-2014), Mescher and Strominger describe a glycoprotein of apparent mol wt of 200,000. This protein accounts for about half of the protein of the cell envelope. We note that EM of the envelope of the halobacteria (21, 43, 45) shows particles in a hexagonal array in what we now call the outer protein layer. The rows of particles are about 130 Å apart, center to center. This dimension defines a hexagonal unit cell of area 19,500 Å^2 assuming that the outer protein layer is 40 Å thick, the volume of the unit cell is 780,000 Å^3. Assuming, as the above authors suspect, that the glycoprotein is in the outer protein layer and taking a protein density of 1.33 g/cm^3 (9), the unit cell can contain just over three molecules. A cluster of three molecules therefore will account for the particle size seen by EM. A threefold rotation axis at the center of each cluster will account for the hexagonal packing as well. Any excess unit-cell volume, above that actually occupied by the three molecules, can reasonably be assigned to one or more pores through the outer protein layer. Such pores will be needed in order for metabolites to pass through the wall. (Note added June 30, 1976.)

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