Phylogeny and properties of a novel lectin family with β-trefoil folding in mussels

A novel lectin, termed "MytiLec", was isolated and characterized from the mussel Mytilus galloprovincialis, an important food and environmental indicator species found in marine coastal areas worldwide. MytiLec binds to the sugar moiety of globotriose (Gb3), an α-galactoside, leading to apoptosis of Gb3-expressing Burkitt's lymphoma cells. The amino acid sequence of MytiLec is unusual, but 3-dimensional structural analysis reveals the presence of β-trefoil fold, a well-known feature in "R-type" lectins, a family of galactose-binding proteins found in many types of organisms. To date, MytiLec has been found only in a few species of the mollusk family Mytilidae and the phylum Brachiopoda, which also express typical R-type lectins. In this minireview, we discuss: (i) possible reasons for the unusual coexistence of two distinct lectin families in the same animal family; (ii) structural models of MytiLec that are useful for design of lectins with improved anti-cancer properties; (iii) construction of "Mitsuba", an artificial lectin based on MytiLec that has similar carbohydrate-binding activity but a more stable monomeric form; (iv) regulation of cell growth by MytiLec and related lectins through binding to glycans.