The thylakoid membrane of Rhodopseudomonas viridis contains extensive, regular arrays of photoreceptor complexes arranged on a hexagonal lattice with a repeat distance of ˜130 Å. Single membrane sheets were obtained by mild treatment of the thylakoid fraction with the detergent Triton X‐100. Heavy metal shadowing and electron microscopy of isolated thylakoids indicated a strong asymmetry of the membrane, showing a smooth plasmic and a rough exoplasmic side. Fourier processing of rotary‐shadowed specimens showed the different surface relief on both sides of the membrane. Structural units on both sides were roughly circular and showed 6‐fold symmetry at a resolution close to 20 Å. The structural unit was characterised by a central core that seemed to extend through the membrane, protruding on the exoplasmic side. The core was surrounded by a ring showing 12 subunits on the plasmic side. Rotary‐shadowed as well as negatively‐stained membranes indicated a handedness of the structure. Treatment of thylakoid vesicles with higher detergent concentrations yielded a fraction of particles showing the same features as Fourier maps of the structural units. The isolated particles therefore appeared to represent structurally intact units of photosynthesis.

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A three‐dimensional reconstruction from electron micrographs of negatively stained cell envelopes of Halobacterium volcanii has revealed the structure of the surface glycoprotein to a resolution of 2 nm. The glycoprotein is arranged on a p6 lattice with a lattice constant of 16.8 nm. It forms 4.5 nm high, dome‐shaped, morphological complexes with a narrow pore at the apex opening into a ‘funnel’ towards the cell membrane. The polarity of the structure was derived from freeze‐etching experiments and ‘edge’ views. Six radial protrusions emanate from each morphological complex and join around the 3‐fold axis to provide lateral connectivity. Using the primary structure of the surface glycoprotein of the closely related species Halobacterium halobium (Lechner and Sumper, 1987) and the cell envelope profile from a previous X‐ray analysis of the same species (Blaurock et al., 1976) we have integrated our reconstruction into a model of halobacterial cell envelope.

The sulphur-dependent archaebacterium Thermoproteus tenax has a cylindrical cell shape variable in length, but constant in diameter. Its whole surface is covered by a regular protein layer (S-layer). The lattice has p6 symmetry and a lattice constant of 32.8 nm. The three-dimensional reconstruction from a tilt series of isolated and negatively stained S-layer shows a complex mass distribution of the protein: a prominent, pillar-shaped protrusion is located at the 6-fold crystallographic axis with radiating arms connecting neighbouring hexamers in the vicinity of the 3-fold axis. The base vectors of the S-layer lattice have a preferred orientation with respect to the longitudinal axis of the cell. The layer can be seen as a helical structure consisting of a right-handed, two-stranded helix, with the individual chains running parallel. Supposing that new S-layer protein is inserted at lattice faults (wedge disclinations) near the poles, growing of the layer would then proceed by moving a discination at the end of the helix. The constant shape of the cell, as well as the particular structure of the layer, strongly suggest that this S-layer has a shape-maintaining function.

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