Galectins from the nematode Caenorhabditis elegans and the genome project

The finding of Caenorhabditis elegans galectin (32 kDa) demonstrated, for the first time, the presence of the ''tandem-repeat type'' of galectin, which consists of two homologous domains (ca. 16 kDa). Its N- and C-terminal half domains show relatively low sequence similarity to each other (ca. 30% identity), though most (but not all) of the amino acids involved in the carbohydrate binding are conserved. The nematode 32-kDa galectin shows strong hemagglutinating activity, but its saccharide specificity is rather complex. The individual half domains have considerably distinct features in the binding to asialofetuin-agarose. Though the endogenous ligand for them is not known, these observations imply that the 32-kDa nematode galectin functions as a possible ''heterobifunctional crosslinker''. Since this galectin is localized most abundantly in the adult cuticle, it possibly plays a role in the formation of tight and insoluble epidermal layers. A recently isolated novel nematode galectin (16-kDa) forms a non-covalent dimer, and exhibits significant hemagglutinating activity, which is inhibitable by lactose. The current progress in the C. elegans genome project has revealed the presence of a number of galectin-related genes, and at least four other tandem-repeat-type galectins (40-75% identical to the 32-kDa galectin) have been proved to be expressed. Two closely related genes encoding CRDs (carbohydrate-recognition domains) having a somewhat longer C-terminal tail have also been predicted. Because the complete genome sequence of C. elegans (1 x 108 bp) will be obtained in the near future, galectin research utilizing this model animal will hopefully provide us with new concepts about both the biological and evolutionary significance of multivalent galectin-carbohydrate interactions.