The D-galactose-binding lectin of the octocoral Sinularia lochmodes: characterization and possible relationship to the symbiotic dinoflagellates

A D-galactose binding lectin (SLL-2) was isolated from Sinularia lochmodes, an octocoral, by a combination of affinity chromatography on acid-treated agarose and FPLC on Superdex 200. SLL-2 agglutinated rabbit and horse eryth rocytes while SLL-1, a minor component, reacted only with rabbit erythrocyt es. SLL-2 is a glycoprotein with a molecular mass of 122 kDa and is compose d of eight identical subunits (15 kDa). The sequence of the amino terminal region of SLL-2 did not show any apparent homology to the sequences of othe r animal and plant lectins. D-Galactose, N-acetyl-D-galactosamine, lactose, and melibiose were moderate inhibitors to the agglutination of rabbit eryt hrocytes. In contrast, horse erythrocytes were much more susceptible to agg lutination by SLL-2, which was inhibited by sugars and glycoproteins such a s D-galactose, N-acetyl-D-galactosamine, lactose, melibiose, and porcine st omach mucin. SLL-2 showed considerable tolerance to heating and kept its ac tivity after heating at 80 degrees C for 60 min. In immuno-histochemical st udies using an anti-SLL-2 antiserum and protein A gold conjugate, SLL-2 was found to be present in high amounts in the nematocysts. SLL-2 was also det ected on the surface of symbiotic dinoflagellate, Symbiodinium sp. cells ir respective whether they were surrounded with or without host cells. These o bservations suggest the presence of lectin-mediated interaction between sym biotic dinoflagellates and S. lochmodes. (C) 2000 Elsevier Science Inc. All rights reserved.