Human galectin-1 recognition of poly-N-acetyllactosamine and chimeric polysaccharides

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Abstract
Human galectin-1 is a dimeric carbohydrate binding protein (Gal-1) (subunit 14.6 kDa) widely expressed by many cells but whose carbohydrate binding specificity is not well understood. Because of conflicting evidence regarding the ability of human Gal-1 to recognize N-acetyllactosamine (LN, Galβ4GlcNAc) and poly-N-acetyllactosamine sequences (PL, [-3Galβ4GlcNAcβ1-]n), we synthesized a number of neoglycoproteins containing galactose, N-acetylgalactosamine, fucose, LN, PL, and chimeric polysaccharides conjugated to bovine serum albumin (BSA). All neoglycoproteins were characterized by MALDI-TOF. Binding was determined in ELISA-type assays with immobilized neoglycoproteins and apparent binding affinities were estimated. For comparison, we also tested the binding of these neoglycoconjugates to Ricinus communis agglutinin I, (RCA-I, a galactose-binding lectin) and Lycopersicon esculentum agglutinin (LEA, or tomato lectin), a PL-binding lectin. Gal-1 bound to immobilized Galβ4GlcNAcβ3Galβ4Glc-BSA with an apparent Kd of ∼23 µM but bound better to BSA conjugates with long PL and chimeric polysaccharide sequences (Kd's ranging from 11.9 ± 2.9 µM to 20.9 ± 5.1 µM). By contrast, Gal-1 did not bind glycans lacking a terminal, nonreducing unmodified LN disaccharide and also bound very poorly to lactosyl-BSA (Galβ4Glc-BSA). By contrast, RCA bound well to all glycans containing terminal, nonreducing Galβ1-R, including lactosyl-BSA, and bound independently of the modification of the terminal, nonreducing LN or the presence of PL. LEA bound with increasing affinity to unmodified PL in proportion to chain length. Thus Gal-1 binds terminal β4Gal residues, and its binding affinity is enhanced significantly by the presence of this determinant on long-chain PL or chimeric polysaccharides.

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