Opsonic activity of cell adhesion proteins and beta-1,3-glucan binding proteins from two crustaceans

A beta-1,3-glucan binding protein (beta GBP) from the shore crab Carcinus maenas was purified from plasma by precipitation of the protein at low ionic strength. The protein had a molecular mass of 110 kDa, and was shown to affinity precipitate with laminarin, a soluble beta-1,3-glucan, and to cross-react with an antiserum directed toward beta GBP from the crayfish Pacifastacus leniusculus. Also, a protein from the haemocytes of C. maenas with a molecular mass of 80 kDa was found to mediate cell attachment and cause degranulation of crab cells, similar to the 76 kDa protein present in the haemocytes of P. leniusculus. Antibodies against the crayfish 76-kDa protein reacted with the crab 80-kDa protein present in the granular cells. No 80-kDa protein could be found in the hyaline cells. Using a method with FITC-conjugated yeast particles in a phagocytosis assay, both the beta GBP and the 80-kDa protein from C. maenas were shown to have opsonic activity as had beta GBP and 76-kDa protein from P. leniusculus, resulting in higher levels of phagocytosis by the crab hyaline cells. Treatment of the yeast particles with beta GBP previously reacted with laminarin (beta GBP-L) only resulted in a minor increase of phagocytosis. Moreover, if the phagocytic cells were preincubated with beta GBP-L or with the 80-kDa protein, the enhancement of the phagocytic activity by beta GBP or the 80-kDa protein were abolished, indicating that a saturable number of one kind of cell surface receptor seem to be involved in phagocytosis.