The gene encoding Flo11p was originally isolated and characterized independently by two research groups from a
Saccharomyces cerevisiae var.
diastaticus [14][15], which is a yeast able to grow on starch due to the production of a glucoamylase encoded by any one of the members of the
STA genes family
[16]. Two seminal properties of this flocculin were already disclosed in these works, which were the requirement of Flo11p for invasive and pseudohyphal growth under carbon or nitrogen limitation and its involvement in flocculation. Subsequent work revealed other phenotypes, including the formation of velum, which is a biofilm of yeast cells floating on the surface of a wine barrel
[17][18][19], and mats, which correspond to a floral-like biofilm that expands over a wet, semi-solid surface by sliding motility
[20]. Adhesion of yeast cells on solid surfaces such as plastic or polystyrene was reported as a property elicited by Flo11p
[21]. Additionally, Flo11p was shown to be implicated in colony morphologies exhibited by different yeast strains growing on solid media in the presence of various carbon sources
[22]. More recently, we showed that Flo11 molecules could cluster together to form adhesion nanodomains on the cell surface, a property that is dependent on a threshold number of amyloid-β-aggregation prone sequences in the Flo11p (
[23]; discussed below). This wide variety of phenotypes raises the question of whether they can be expressed collectively in a single yeast strain and which domain(s) of Flo11p is/are responsible for these phenotypes. Works from the Hyman
[24] and Verstrepen teams
[25] have in part provided some answers to the first question. They both used the laboratory strain S288c in which
FLO genes are not expressed because of a nonsense mutation in the major transcriptional activator encoded by
FLO8 [26]. While one group integrated individual
FLO genes under the strong
TEF promoter, the other expressed
FLO11 in a high copy plasmid under the galactose inducible
GAL1 promoter. These works led to two major findings regarding differences between Flo11p and the other flocculins. On the one hand, only Flo11p has the ability to trigger strong invasive growth in a semi-solid environment, and thus it is essential in mats formation
[27]. However, this property only occurs if the
FLO8 gene is functional, suggesting that while the absence of
FLO11 prevents the invasion process, the latter requires other factors under the control of
FLO8. On the other hand,
FLO11 is unable to induce a flocculation phenotype, whether
FLO8 is active or not, although agglutination of 10–30 cells can be observed under an optical microscope in a strain that expresses only this gene, indicating that Flo11p promotes cell–cell interactions but not to the extent that it induces aggregation of thousands of cells leading to flocculation. Since this result was at variance to Flo11p-mediated flocculation of the
S. cerevisiae var.
diastaticus strain
[28], it was argued that strain-specific difference in the Flo11p phenotype may result from significant sequence differences in the
FLO11 alleles, rather than quantitative differences in
FLO11 expression
[29]. This assertion was further supported by the fact that laboratory strain S288c is unable to exhibit a flor phenotype or to produce nanodomains on its cell surface even upon dramatic overexpression of
FLO11 [23]. In conclusion, these results highlight the importance of the sequence/structure of the Flo11 protein in the expression of these phenotypes.