Studies on the Proteinaceous Structure Present on the Surface of the Saccharomyces cerevisiae Spore Wall

The surface of the Saccharomyces cerevisiae spore wall exhibits a ridged appearance. The outermost layer of the spore wall is believed to be a dityrosine layer, which is primarily composed of a crosslinked dipeptide bisformyl dityrosine. The dityrosine layer is impervious to protease digestion; indeed, most of bisformyl dityrosine molecules remain in the spore after protease treatment. However, we find that the ridged structure is removed by protease treatment. Thus, a ridged structure is distinct from the dityrosine layer. By proteomic analysis of the spore wall-bound proteins, we found that hydrophilin proteins, including Sip18, its paralog Gre1, and Hsp12, are present in the spore wall. Mutant spores with defective hydrophilin genes exhibit functional and morphological defects in their spore wall, indicating that hydrophilin proteins are required for the proper organization of the ridged and proteinaceous structure. Previously, we found that RNA fragments were attached to the spore wall in a manner dependent on spore wall-bound proteins. Thus, the ridged structure also accommodates RNA fragments. Spore wall-bound RNA molecules function to protect spores from environmental stresses.