S-Layer Glycoproteins from Moderately and Extremely Halophilic Archaeobacteria
The first procaryotic glycoprotein was discovered in the S-layer of halobacteria of the genus Halobacterium by Mescher and Strominger (1976). Saturated sodium chloride solutions are the natural habitat of these rod-shaped and flagellated archaeobacteria. The original finding was that the halobacterial S-layer contains a single glycoprotein with an M. of 200 kDa and a carbohydrate content of about 10% by weight. Since procaryotes lack all the organelles engaged in eucaryotic glycoprotein biosynthesis, this discovery has stimulated further work on the structure and biosynthesis of this glycoprotein (for a review, see Sumper 1987; Lechner and Wieland, 1989). The halobacterial S-layer is very tightly joined to the plasma membrane as these cells lack a rigid sacculus as well as an outer membrane. S-layer proteins represent the outermost component of the cell envelope and, as a consequence, these proteins are in immediate contact with the environment. Therefore they are considered ideal model systems to study the adaptation of protein structures to unfavourable conditions. The extreme habitat of halobacteria imposes particularly serious problems with respect to the stabilization of the protein structure. Since other members of the halobacterial family only tolerate moderately halophilic conditions (e.g., Haloferax volcanii requires 2.3M NaC1 for optimum growth), a comparison of the corresponding S-layer protein structures should give hints on how adaptation to high salt conditions works.
KeywordsGlycosylation Site High Salt Condition Negative Charge Density Sulfate Residue Saturated Sodium Chloride Solution
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