Lectins in Marine Invertebrates

(Update Issue: May.9, 2007)
 Although a number of lectins of various molecular weights have been found in marine invertebrates, very limited information concerning their structures has thus far been obtained. Most of the lectins with known amino acid sequences belong to the C-type lectin family. Unlike vertebrate C-type lectins, many of which contain several distinct functional domains, most of the invertebrate C-type lectins consist of only single C-type carbohydrate recognition domains (CRDs) (1,2), except for horseshoe crab (Tachypleus tridentatus) factor C, a serine protease zymogen in the hemolymph coagulation system. This protein comprises several domains, including a serine protease domain and a C-type CRD whose carbohydrate-binding capacity is unclear (3). This suggests that there may still be other C-type lectins with multiple domains in marine invertebrates. C-type CRDs in marine invertebrates show rather low homology with each other; sequence identities are generally below 40% even between closely related species. Each CRD has one carbohydrate-binding site, many of which have specificity toward galactose or N-acetylgalactosamine. In contrast to C-type lectins, some other types of lectins with novel structures have also been found. For example, a lipopolysaccharide-binding lectin from horseshoe crab consists of six tandem repeats of 33-38 amino acid residues. A novel lectin with no homology with other proteins was also found in sea urchin (Anthocidaris crassispina) eggs.
Figure Legend
Binding of lectins with opsonin activity to foreign substances such as bacteria promote phagocytosis (A). After binding to the carbohydrate chains on erythrocyte surface, hemolytic lectins induce membrane damage (e.g., by forming pores), leading to hemolysis (B).
One of the most probable roles of marine invertebrate lectins is to act as humoral factors in the defense mechanism, as do immunoglobulins in vertebrates. This is suggested from some observations such as the activation of phagocytes by the binding of lectin to foreign cells (opsonin activity) or the enhancement of lectin production in body fluids after injection of foreign substances. On the other hand, direct hemolytic activity has recently been found for a sialic acid-specific lectin from horseshoe crab and a galactose-specific lectin from the sea cucumber Cucumaria echinata. After binding to the specific carbohydrate chains on the erythrocyte surface, these lectins damage the cell membrane, leading to cell lysis.

These lectins may play an important role against bacterial infections or natural enemies. A lectin with biological activities such as mitogenic and chemotactic activities has been found in the venom of the pedicellariae (spines) of the sea urchin Toxopneustes pileolus, suggesting its involvement in toxic action. Other functions of marine invertebrate lectins have also been suggested for some C-type lectins: calcium carbonate crystallization (the acorn barnacle, Megabalanus rosa), morphogenesis (the tunicate, Polyandrocarpa misakiensis), and vitelline coat lysis (the blue mussel, Mytilus edulis sperm).These examples suggest that there may be various lectins with different physiological roles in marine invertebrates.
Hatakeyama Tomomitsu (Nagasaki University, Faculty of Engineering)
References(1) Hatakeyama, T, Ohuchi, K, Kuroki, M, Yamasaki, N: Amino acid sequence of a C-type lectin CEL-IV from the marine invertebrate Cucumaria echinata. Biosci. Biotech. Biochem. 59, 1314-1317, 1995
(2) Takagi, T, Nakamura, A, Deguchi, R, Kyozuka, K: Isolation, characterization, and primary structure of three major proteins obtained from Mytilus edulis sperm. J. Biochem. 116, 598-605, 1994
(3) Muta, T, Miyata T, Misumi, Y, Tokunaga, F, Nakamura, T, Toh, Y, Ikehara, Y, Iwanaga, S: Limulus factor C. J. Biol. Chem. 266, 6554-6561, 1991
Mar.15, 1998

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