The Adhesin Hwp1 and the First Daughter Cell Localize to the a/a Portion of the Conjugation Bridge during Candida albicans Mating

The cell wall protein Hwp1 was originally demonstrated to be expressed exclusively in hyphae of Candida albicans and cross-linked to human epithelium by mammalian transglutaminase. Hwp1 is expressed on the walls of hyphae formed by a/α, a/a, and α/α cells. Hence, it is expressed on hyphae independently of mating type. However, Hwp1 is selectively expressed on the wall of conjugation tubes formed by a/a cells, but not α/α cells, in the mating process. This was demonstrated in all possible crosses between four unrelated natural a/a strains and four unrelated α/α strains. In zygotes, Hwp1 is restricted to that portion of the wall of the conjugation bridge contributed by the a/a parent cell. Hwp1 staining further revealed that the first daughter bud that emerges from the conjugation bridge does so from the a/a-contributed portion. Hwp1 expression and localization during the mating process is, therefore, mating type specific, opaque phase specific, and α-pheromone induced. These results indicate that the mating type-specific contributions to the conjugation bridge during the mating process in C. albicans are qualitatively and functionally distinct and that the a/a portion of the bridge, which selectively contains Hwp1, bears the first daughter cell in the mating process.