Three-Dimensional Structure of the Crystalline Protein Envelope Layer of the Hyperthermophilic Archaebacterium Pyrobaculum islandicum

Pyrobaculum islandicum strain GE03 is a hyperthermophilic sulfur-reducing archaebacterium growing optimally at 100°C. A major component of the cell envelope is a hexagonally arranged two-dimensional (2-D) crystalline protein layer which appears to maintain the strictly invariant diameter of the cell. Protrusions from the layer span an interspace of uniform width between the body of the layer and the cytoplasmic membrane, and may insert into the membrane. The structure of the layer was examined by means of three-dimensional (3-D) reconstructions from tilted electron microscopic projections of negatively stained, detergent-extracted crystalline layers in both possible orientations with respect to the carbon film. A highly interconnected network of mass is revealed in which each putative protein monomer makes contacts at the sixfold, threefold, and twofold crystallographic axes. The convergence of six monomers gives rise to the interspace-spanning protrusion, only the base of which appears in the reconstructions. The remarkable similarity of this structure to that of the shape-determining layer of Thermoproteus tenax leads us to propose that the two organisms are closely related. In freeze-etched cells, a fibrous coat resembling a eubacterial capsule is observed to cover the smooth outer face of the crystalline protein layer. Thus, in this organism, the protein array does not constitute the outermost surface of the cell