The cell envelope of Thermoproteus tenax: three‐dimensional structure of the surface layer and its role in shape maintenance

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 disclination 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.