Evidence against the presence of fibres or chemically distinct layers in the cell wall ofSaccharomyces

The gross chemical composition of material extracted from yeast cell walls with various solvents or enzymes was studied. Attempts were made to locate these materials in situ by comparing electron micrographs of negatively stained and sectioned cell walls with those of the residues of the extraction procedures. There are at least two chemically distinct species of carbohydrate polymers which can be extracted with strong alkali: one yielding mainly mannose and some amino acid on hydrolysis and the other yielding mannose, glucose and amino acid. The alkali-insoluble material also yielded glucose, mannose and amino acid on hydrolysis but the glucose/mannose ratio was much higher. It was shown that none of these polymers constituted a physically distinct layer in the yeast cell wall. However, there does seem to be a region at the outer surface with distinctive properties. This is not fibrillar in nature and after extraction with ethylene diamine forms a double-layered structure. Materials which react with KMnO4 to produce an electron-dense material are located throughout the wall but tend to be concentrated in the outer and inner regions. Procedures which remove this material also remove up to 80% of the mannose, 40% of the glucose and 35 % of the protein of the original wall material. It was shown that fibres do not constitute a major fraction of the normal cell wall, except possibly in the region of the bud scars but may be produced fairly readily by certain specific treatments. The classical view of the yeast cell wall with the structural integrity being maintained by a fibrous network of β 1–3, 1–6 linked glucose residues is challenged and evidence to support an alternative view is presented.