Abstract
The glycoprotein which accounts for approximately 50% of the protein and all of the nonlipid carbohydrate of the cell envelope of Halobacterium salinarium (Mescher, M. F., Strominger, J. L., and Watson S. W. (1974) J. Bacteriol. 120, 945-954) has been purified and partially characterized. The glycoprotein has an apparent molecular weight of 200,000, is extremely acidic, and has a carbohydrate content of approximately 10 to 12%. The carbohydrate included neutral hexoses, amino sugar, and uronic acid. Information regarding the number, composition, and mode of attachment of the carbohydrate chains was obtained by isolation and examination of the glycopeptides derived from degradation of cell envelope protein with trypsin and pronase. Trypsin digestion resulted in two glycopeptides. One of these was large (approximately 55,000 daltons) and had most of the neutral hexose linked to it. The carbohydrate moieties consisted of di- and trisaccharides of glucosylgalactose and (uronic acid, glucose)-galactose attached via O-glycosidic linkages between galactose and threonine. The other tryptic glycopeptide had a relatively large heterosaccharide attached to it via an alkaline-stable linkage. The heterosaccharide contained 1 glucose, 8 to 9 galactose, 1 mannose, and 10 to 11 glucosamine residues, and approximately 6 residues of an unidentified amino augar. The alkaline stability of the linkage and the amino acid composition of glycopeptides resulting from Pronase digestion of the tryptic glycopeptide showed that the heterosaccharide was attached to an asparagine residue, presumably via an N-glycosylamine bond to the amide group. The intact glycoprotein has a single N-linked heterosaccharide, 22 to 24 O-linked disaccharides, and 12 to 14 O-linked trisaccharides per molecule. N- and O-glycosidic linkages are the most common carbohydrate-protein linkages in mammalian glycoproteins but, to our knowledge, this is the first report of either type of linkage in a prokaryotic cell envelope protein.
Highlights
The glycoprotein has an apparent molecular weight of 200,000, is extremely acidic, and has a carbohydrate content of approximately
The results of the structural studies reported here show the high molecular weight H. salinarium glycoprotein to have oligosaccharide chains attached to the polypeptide via two different types of linkage
The large amino sugar-containing heterosaccharide is attached to the protein via an alkaline-stable linkage, an N-glycosylamine bond to the amide group of asparagine
Summary
The glycoprotein has an apparent molecular weight of 200,000, is extremely acidic, and has a carbohydrate content of approximately. The alkaline stability of the linkage and the amino acid composition of glycopeptides resulting from Pronase digestion of the tryptic glycopeptide showed that the heterosaccharide was attached to an asparagine residue, presumably via an N-glycosylamine bond to the amide group. Complex carbohydrate-containing polymers are an almost universal feature of the cell envelope and cell surface of both prokaryotic and eukaryotic cells In prokaryotic organisms these cell envelope carbohydrates typically occur as long chain polysaccharides composed of repeating units as in the case of peptidoglycans, lipopolysaccharides, and teichoic acids. In some cases these polymers have been shown to be covalently linked to proteins (I-3). In contrast to prokaryotic organisms, the majority of the cell surface carbohydrates of eukaryotic cells are present in glycoproteins and occur as relatively short heterosaccharide chains covalently linked to the polypeptides
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