Structures of Paracrystalline Protein Layers from the Hyperthermophilic Archaeobacterium Pyrobaculum

Abstract

Members of the recently discovered archaeobacterial genus Pyrobaculum are sulfur-reducing hyperthermophiles which grow optimally at 100°C (Huber et al., 1987). All known strains were isolated from superheated, sulfurous fresh water springs or from the superheated outflow of a geothermal power plant. The cells are rod-shaped and vary in length up to 8µm, but their diameter is strikingly uniform. Two strains have been characterized in detail and were found to represent different species, Pyrobaculum islandicum (strain GEO3) and Pyrobaculum organotrophum (strain H10). P. islandicum is a facultative heterotroph capable of chemolithoautotrophic growth on sulfur, H₂, and CO₂, while P. organotrophum is strictly organotrophic. This Chapter reviews the results of investigations into the structure of the cell envelopes of these interesting archaeobacteria (Baumeister et al., 1989, 1990; Phipps et al., 1990, 1991). An element of the cell envelope of most archaeobacteria is a 2D paracrystalline array of protein or glycoprotein subunits, or S-layer, which is often directly adjacent to the cytoplasmic membrane (König, 1988). The array subunit is typically a complex, multi-domain protein and often possesses a putative membrane anchor domain (Baumeister et al., 1989). The unusual structures of Pyrobaculum S-layers are described, and the possible functional roles they fulfill are discussed.