Structure and function of Awa1p, which is responsible for cell surface hydrophobicity of sake yeasts.
Masashi Shimizu, Kiyoshi Ito, Hitoshi Shimoi
Genetic Engineering Division, Nat. Res. Institute of Brewing, 3-7-1, Kagamiyama, Higashihiroshima, 739-0046, Japan
Sake yeasts, which are classified as Saccharomyces cerevisiae, are used in the brewing of sake, a traditional Japanese alcoholic beverage. The surfaces of sake yeast cells are hydrophobic, which causes them to accumulate at bubble surfaces. This results in a thick foam layer on sake mash. We have cloned a gene required for cell surface hydrophobicity and foam formation of a sake yeast, K7. This gene, named AWA1, encodes a GPI-anchored protein that is localized on the cell surface. Although Awa1p is homologous to YOL155C of S. cerevisiae S288C, the N-terminal region of Awa1p is also similar to a part of YJR151C of S288C. To elucidate the relationship between structure and function of Awa1p, we constructed four kinds of deletion mutants of AWA1 and used them to transform a mutant strain of K7 that lacks AWA1 (K701). The mutants lacking the N-terminal region that is homologous to YJR151C and a GPI-anchor signal had less hydrophobic cell surfaces and did not make foam in sake mash. On the other hand, the mutants lacking a serine-rich repeat and a C-terminal repeat had hydrophobic cell surfaces and made foam. K701 transformed with YOL155C of S288C had a hydrophilic cell surface and did not make foam. However, when the part of YJR151C was inserted into YOL155C, the transformed K701 acquired a hydrophobic cell surface and made foam. These results indicate that the YJR151C homologous part is required for cell surface hydrophobicity of sake yeasts and foam formation in sake mash.