The cell wall polymer of the extremely halophilic archaeon Natronococcus occultus

Partial acid hydrolysis of the cell wall polymer produced soluble fragments that could be separated by HPLC. A gamma-glutamyl dipeptide was isolated. In the intact cell wall polymer, the glutamate residues form a poly-(gamma-glutamine) chain with a length of about 60 monomers, which corresponds to a relative molecular mass of approximately 7700 Da. Two other soluble dimeric fragments, composed of glutamate and either glucosamine or galactosamine in a molar ratio of 1:1, were purified from the hydrolysate, suggesting the presence of two different oligosaccharides linked to the poly-(gamma-glutamine) chain of the intact polymer. The analysis of additional fragments, which were composed of an amino sugar and galacturonic acid or glucose indicated that one oligosaccharide consisted of a glucosamine pentamer in an alpha-1,3 linkage at the reducing end and an oligomer with at least five beta-1,4-linked galacturonic acid residues at the non-reducing end. The second oligosaccharide was comprised of a galactosamine dimer in a beta-1,3 linkage at the reducing end and a maltose unit at the non-reducing end. Both oligosaccharides were linked to the alpha-amide group of the glutamine residues of the poly-(gamma-glutamine) chain. The whole cell wall polymer, which represents a novel type of natural glycoconjugate, has a relative molecular mass of 54 kDa.