title: Three-dimensional structure of the regular tetragonal surface layer of Azotobacter vinelandii.
abstract: Fragments of the Azotobacter vinelandii tetragonal surface (S) layer, free of outer membrane material, were obtained by treating whole cells with 100 microM EDTA. The three-dimensional structure of the S layer was reconstructed from tilted-view electron micrographs of the S-layer fragments, after computer-assisted image processing by correlation averaging. At a resolution of 1.7 nm, the S layer exhibited funnel-shaped subunits situated at one fourfold-symmetry axis and interconnected at the other fourfold-symmetry axis to form prominent cruciform linking structures. These data, in conjunction with a relief reconstruction of the surface of freeze-etched whole cells, indicated that the apex of the funnel-shaped subunit was associated with the outer membrane, while the funnel "opening" faced the environment; the cruciform linking structures were formed at the outermost surface of the S layer. Electron microscopy and image enhancement were used to compare the structure of the outer membrane-associated S layer with that of fragments of the S layer dislodged from the outer membrane. This analysis revealed an increase in the lattice constant of the S layer from 12.5 to 13.6 nm and an alteration in the position of the cruciform linking structures in the z direction. These conformational changes resulted in a reduction in the thickness of the S layer (minimum estimate, 5 nm) and an apparent increase in the size of the gaps between the subunits. In terms of the porosity of the S layer, this gave the appearance of a transition from a closed to a more open structure.
authors: Department of Microbiology, University of Alberta, Edmonton, Canada.
authors: W H
authors: WH
authors: Bingle
authors: H
authors: H
authors: Engelhardt
authors: W J
authors: WJ
authors: Page
authors: W
authors: W
authors: Baumeister
mesh_terms: D001395:Azotobacter / Q000648:ultrastructure*; D002462:Cell Membrane / Q000648:ultrastructure; D003198:Computer Simulation; D005583:Fourier Analysis; D008854:Microscopy, Electron; D008961:Models, Structural