Secreted arginases from phylogenetically farrelated 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 Mn2+ -dependent reaction. Ca2+ cannot replace Mn2+, but its addition to reaction mixtures in the presence of Mn2+ 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 Ca2+ and Mn2+ and results in cytoagglutination, which is counteracted by alpha-D-galactose. A putative ligand for these lectins consists of a glycosylated urease, the polysaccharide moiety of which is uniquely composed of alpha-D-galactose. Binding of lectins inhibits its enzymatic activity, which is recovered after desorption of the lectin with alpha-D-galactose. Urease is also eluted from arginase-agarose columns by using alpha-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.