| Recent studies have revealed that glycoproteins on cell membranes are usually involved in the process of cell recognition. This study provides evidence to demonstrate the role of cell surface glycoproteins of zooxanthellae in establishing a symbiotic relationship between algae and a sea anemone host. When freshly isolated zooxanthellae from the sea anemone, Aiptasia pulchella, were incubated with trypsin, α-amylase, N-glycosidase F, or O-glycosidase, the algae could not infect bleached animals as efficiently as did the control algae. If the glycoproteins on the surface of the zooxanthellae were masked with lectins, the infection rate also declined significantly. Further, the glycoproteins on the cell walls of zooxanthellae were isolated and characterized with SOS-polyacrylamide gel electrophoresis and Western blotting. There are 10 protein bands in total revealed by Coomassie blue and silver staining, of which 5 are identified as glycoproteins. Two types of terminal sugar residues, mannose-mannose and galactose-β (1-4)-N-acetylglucosamine, were characterized among them. The most abundant glycoprotein with a molecular weight of 64 kDa carries a terminal sugar mannose. These data indicate that the glycoproteins on cell walls of zooxanthellae are a crucial factor in the successful establishment of the symbiotic relationship between zooxanthellae and A. pulchella. |