| Galectins are a family of carbohydrate-binding proteins that are widely distributed in nature having been identified in vertebrates, invertebrates, and protists. Members of this family are defined by two main criteria (a) their binding affinity for β-galactosides and (b) the presence of a conserved sequence motif within their carbohydrate recognition domain (CRD). Recent studies in mammals have demonstrated that galectins participate both directly and indirectly in mediating immune responses such as host-pathogen interactions, inflammation, and autoimmunity. Given their conservation throughout evolution galectins may function as part of an ancient and conserved mechanism of host defense. To gain insight into the origin and evolution of galectin function in the innate immune response we have isolated four proteins using lactose affinity chromatography with relative mobilities of 37, 33, 16, and 14 kDa from the protochordate, Clavelina picta. We show by western and northern analysis that the 16 kDa band (designated CpGal-16) is specifically localized and synthesized in the circulating hemocytes, the cells which carry out many of the immune functions in the tunicate. In addition, CpGal-16 appears to be a novel member of the galectin family in terms of its primary structure and carbohydrate specificity. Analysis of it gene regulation shows that CpGal-16 is up-regulated in response to bacterial challenge, and we have further evidence that suggests this up-regulation may be the result of a protein-carbohydrate interaction. These results suggest that galectins have an evolutionarily conserved role in the innate immune response. |
