Purification and antibacterial characterization of a novel isoform of the Manila clam lectin (MCL-4) from the plasma of the Manila clam, Ruditapes philippinarum

In many bivalve molluscs, lectins are present in the hemolymph and are thought to be important for internal host defense mechanisms. For this study, we purified a novel isoform of the Manila clam lectin (designated MCL-4) from the plasma of the Manila clam, Ruditapes philippinarum, using affinity chromatography and gel filtration. Native PAGE results showed that the MCL-4 consisted of 70 kDa protein. MCL-4 was found to be composed of 58-kDa and 43-kDa bands when examined using SDS-PAGE under reducing and non-reducing conditions. The native MCL-4 was revealed as a 147 kDa molecular mass protein by gel filtration. The purified MCL-4 agglutinates calcium-dependently in the erythrocytes of sheep and rabbit, but not in cells of the three species of marine bacteria tested. However, the phagocytic ability of the R. philippinarum hemocytes for the MCL-4-opsonized Vibrio tubiashii cells was significantly greater than that for the BSS-treated bacterial cells. Addition of purified MCL-4 markedly suppressed Alteromonas haloplanktis growth. These results suggest that MCL-4, because of its opsonizing and bacteriostatic properties, might contribute to the host defense mechanisms against invading microorganisms in R. philippinarum.