trans-sialidase is a unique sialidase in that, instead of hydrolizing sialic acid, it preferentially transfers the monosaccharide to a terminal beta-galactose in glycoproteins and glycolipids. This enzyme, originally identified in Trypanosoma cruzi, belongs to a large family of proteins. Some members of the family lack the enzymatic activity. No function has been yet assigned to them. In this work, the gene copy number and the possible function of inactive members of the trans -sialidase family was studied. It is shown that genes encoding inactive members are not a few, but rather, are present in the same copy number (60-80 per haploid genome) as those encoding active trans -sialidases. Recombinant inactive proteins were purified and assayed for sialic acid and galactose binding activity in agglutination tests. The enzymatically inactive trans -sialidases were found to agglutinate de-sialylated erythrocytes but not untreated red blood cells. Assays made with mouse and rabbit red blood cells suggest that inactive trans -sialidases bind to beta, rather than alpha, terminal galactoses, the same specificity required by active trans -sialidases. A recombinant molecule that was made enzymatically inactive through a mutation in a single amino acid also retained the galactose binding activity. The binding was competed by lactose and was dependent on conservation of the protein native conformation. Therefore, at least some molecules in the trans -sialidase family that have lost their enzymatic function still retain their Gal-binding properties and might have a function as lectins in the parasite-host interaction.
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