Chemical composition of the yeast ascospore wall. The second outer layer consists of chitosan.

In a preceding paper (Briza, P., Winkler, G., Kalchhauser, H., and Breitenbach, M. (1986) J. Biol. Chem. 261, 4288-4294), we reported the presence of dityrosine in the outer layers of yeast ascospore walls. Both outer layers seen in electron micrographs of yeast ascospore walls are sporulation-specific. Here we show that the second of these two outer layers consists of chitosan. In intact spores, it is shielded from staining with primulin by the outermost layer. However, in purified spore walls, the second layer is brightly stained by primulin, and hydrolysates of such preparations contain about 10% glucosamine relative to spore wall dry weight. The spore wall material staining with primulin is resistant to chitinase, but readily degraded by treatment with HNO2. Acetylation prior to HNO2 treatment completely prevents its degradation. A partial acid hydrolysate of spore walls contains predominantly soluble poly-beta-(1,4)-glucosamine as determined by 13C NMR spectroscopy. By these criteria, the glucosamine polymer of yeast ascospore walls is chitosan. As spore walls treated with alkali lack the inner layers but contain chitosan and as chitosan is not exposed at the surface of the spore, we conclude that it is localized in the second outer layer of the spore wall.