Organization and interactions of cell envelope proteins of the extreme thermoacidophile Sulfolobus acidocaldarius

To address basic questions concerning proteins of the Sulfolobus acidocaldarius cell envelope, cell ghosts (empty cells consisting of cytoplasmic membrane complexed with the glycoprotein S-layer) were isolated and analyzed by standard biochemical methods. The constituent proteins included the two dissimilar subunits also found in the baglike structures (sacculi) of S-layer (purified from whole cells by detergent extraction), plus approximately 30 additional protein species. When S-layer-dissociating, conditions were applied to intact cell ghosts, the large sacculus subunit was released, whereas the small subunit remained bound to the cytoplasmic membrane. These results provided evidence that the small subunit may anchor the S-layer to the S. acidocaldarius cell membrane. The S. acidocaldarius S-layer was highly resistant to proteolysis in its native state but not when dissociated into subunits. Virtually all other membrane-associated proteins were readily digested by pronase treatment of cell ghosts. Pronase treatment of whole cells, in contrast, removed only a defined subset of the membrane-associated proteins, suggesting that this subset represents a few proteins normally exposed to the external medium. Several putative membrane glycoproteins were detected in cell ghosts that were not demonstrably associated with the S-layer.