Background
Agglutination, a type of aggregation specific to mating in Sacchromyces cerevisiae, occurs with two surface proteins: alpha-agglutinin and a-agglutinin. Alpha-agglutinin is produced only by alpha-cells and a-agglutinin is produced only by a-cells. During mating, each cell releases pheromones and accepts pheromones secreted by other cells. The alpha-cells respond to a-cell pheromones while the a-cells respond to alpha pheromones. When each cell has received enough of the correct molecule, it will begin to make its agglutinin. In close proximity, the alpha-agglutinin and the a-agglutinin will aggregate, causing cell-cell adhesion. The two yeast cells will come close enough and eventually fuse into one diploid cell.
We took a closer look into the protein, a-agglutinin, and found that it was composed of two subunits: Aga1p and Aga2p. The Aga1 and Aga2 genes lie under a promoter that switches on when the cell has received pheromones from a mate. When the proteins are expressed, Aga1p is attached to the cell surface by a GPI anchor and is also connected to Aga2p through disulfide bonds.
For our project, we decided to utilize the a-agglutinin half of the aggregation display system to form artificial biofilms. By placing sequences of strong adhesive proteins behind the GPI anchor-Aga1-Aga2 gene and overexpressing them, we were able to create yeast cells that could adhere to different surfaces and even other yeast cells. A couple genes we used to attach to the end of the Aga1p-Aga2p complex were mice cadherins, proteins that mussels used to stick to rocks, and proteins used by other yeast species to initiate biofilm formation.
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