GALECTIN-8 - A NEW RAT LECTIN, RELATED TO GALECTIN-4

A protein of 35 kDa which has the characteristic properties of galectins (S-type lectins) was cloned from rat liver cDNA expression library. Since names for galectins 1-7 were already assigned, this new protein was named galectin-8. Three lines of evidence demonstrate that galectin-8 is indeed a novel galectin: (i) its deduced amino acid sequence contains two domains with conserved motifs that are implicated in the carbohydrate binding of galectins, (ii) in vitro translation products of galectin-8 cDNA or bacterially expressed recombinant galectin-8 are biologically active and possess sugar binding and hemagglutination activity, and (iii) a protein of the expected size (34 kDa) that binds to lactosyl-Sepharose and reacts with galectin-8-specific antibodies is present in rat liver and comprises similar to 0.025% of the total Triton X-100-soluble hepatic proteins. Overall, galectin-8 is structurally related (34% identity) to galectin-4, a soluble rat galectin with two carbohydrate-binding domains in the same polypeptide chain, joined by a link peptide. Nonetheless, several important features distinguish these two galectins: (i) Northern blot analysis revealed that, unlike galectin-4 that is confined to the intestine and stomach, galectin-8 is expressed in liver, kidney, cardiac muscle, lung, and brain; (ii) unlike galectin-4, but similar to galectins-1 and -2, galectin-8 contains 4 Cys residues; (iii) the link peptide of galectin-8 is unique and bears no similarity to any known protein; (iv) the N-terminal carbohydrate-binding region of galectin-8 contains a unique WG-E-I motif instead of the consensus WG-E-R/K motif implicated as playing an essential role in sugar-binding of all galectins. Together with galectin-4, galectin-8 therefore represents a subfamily of galectins consisting of a tandem repeat of structurally different carbohydrate recognition domains within a single polypeptide chain.