The N-terminal domain of the Flo1 flocculation protein from Saccharomyces cerevisiae shows homotypic binding properties

An example of controlled cell adhesion is the phenomenon of flocculation of Saccharomyces cerevisiae cells. Often, carbohydrates are involved in this process of biomolecular recognition, acting as the receptor molecule for lectins. However, the role of direct carbohydrate-carbohydrate interactions in the mechanism of yeast flocculation has not been explored yet. The Flo1 flocculins, mediating the flocculation event, have a heavily glycosylated N-terminal domain (Goossens et al., 2011). This domain is able to bind mannose and longer carbohydrate chains in a Ca2+- dependent way. In this study, the role played by the glycosylated N-Flo1p and the glycans themselves in the flocculation of S. cerevisiae was studied using surface plasmon resonance and single-molecule force spectroscopy. Firstly, it was demonstrated that N-Flo1p shows a homotypic adhesion phenotype by interacting with glycans on other N-Flo1p in the presence of calcium. Secondly, it was shown that glycans aggregate in the presence of calcium. Combining those two major results, the flocculation mechanism was refined and extended at the molecular level by suggesting that the mechanism of flocculation is based on two types of interactions: lectin-carbohydrate interactions, extended with carbohydrate-carbohydrate interactions.