Unique Chicken Tandem-Repeat-Type Galectin: Implications of Alternative Splicing and a Distinct Expression Profile Compared to Those of the Three Proto-Type Proteins

Animal galectins (lectins with specificity for beta-galactosides of glycan chains) are potent effectors in diverse aspects of cell sociology. Gene divergence has led to different groups and a marked interspecies variability in the number of members per group. Since the suitability of a model for studying functionality in the galectin network will be distinguished by a rather simple degree of complexity, we have focused on chicken galectins (CGs). Starting from partial expression sequence tag information, we here report on cloning of full-length cDNA for the first avian tandem-repeat-type galectin. It is termed CG-8 on the basis of its sequence similarity to galectin-8 from mammals. Systematic sequence searches revealed its unique character among CGs. Detection of two mature mRNA species points to production of isoforms. Alternative splicing affecting exon V generates the two proteins with linkers of either 9 (CG-8I) or 28 amino acids (CG-8II). Both proteins form monomers with a shape comparable to that of the proto-type proteins CG-1A/B in solution, act as cross-linkers in hemagglutination, and bind cells with a strict dependence on galactose. Western blotting revealed the presence of either CG-8II or the mixture in organ extracts. No evidence of a truncated form was obtained. Preparation of a specific antibody also enabled immunohistochemical localization. Prominent sites of its presence were defense cells in the I. propria mucosae, in addition to immune cells in distinct organs such as alveolar macrophages and thymocytes. Overall, we extend the network of CGS to a tandem-repeat-type protein and provide a detailed characterization from gene and protein structures to expression.