DOMAIN-STRUCTURE OF THE ACETOGENIUM-KIVUI SURFACE-LAYER REVEALED BY ELECTRON CRYSTALLOGRAPHY AND SEQUENCE-ANALYSIS

The three-dimensional structure of the Acetogenium kivui surface layer (S-layer) has been determined to a resolution of 1.7 nm by electron c rystallographic techniques. Two independent reconstructions were made from layers negatively stained with uranyl acetate and Na-phosphotungs tate. The S-layer has p6 symmetry with a center-to-center spacing of a pproximately 19 nm. Within the layer, six monomers combine to form a r ing-shaped core surrounded by a fenestrated rim and six spokes that po int towards the axis of threefold symmetry and provide lateral connect ivity to other hexamers in the layer. The structure of the A. kivui S- layer protein is very similar to that of the Bacillus brevis middle wa ll protein, with which it shares an N-terminal domain of homology. Thi s domain is found in several other extracellular proteins, including t he S-layer proteins from Bacillus sphaericus and Thermus thermophilus, Omp alpha from Thermotoga maritima, an alkaline cellulase from Bacill us strain KSM-635, and xylanases from Clostridium thermocellum and The rmoanaerobacter saccharolyticum, and may serve to anchor these protein s to the peptidoglycan. To our knowledge, this is the first example of a domain conserved in several S-layer proteins.