Secreted arginases from phylogenetically far-related lichen species act as cross-recognition factors for two different algal cells

Purified arginases secreted from Evernia prunastri and Xanthoria parietina thalli hydrolyze arginine in a Mn 2+ -dependent reaction. Ca 2+ cannot replace Mn 2+ , but its addition to reaction mixtures in the presence of Mn 2+ significantly inhibited arginase activity. Arginases from both lichen species also show lectin function, binding to the cell wall of both homologous and heterologous algae. Such binding is enhanced by both Ca 2+ and Mn 2+ and results in cytoagglutination, which is counteracted by α-D-galactose. A putative ligand for these lectins consists of a glycosylated urease, the polysaccharide moiety of which is uniquely composed of α-D-galactose. Binding of lectins inhibits its enzymatic activity, which is recovered after desorption of the lectin with α-D-galactose. Urease is also eluted from arginase-agarose columns by using α-D-galactose as eluent. Data demonstrate ligand-dependent retention of the fungal lectin on the algal cell surface and this is consistent with a model of recognition of compatible algae, through which algal cells would form a lichen with a lectin-secreting fungus only when these cells contain the specific ligand for the lectin in their cell walls. This is, lectin binding is used as a mechanism for ensuring specificity in the association.