Enzymically inactive members of the trans-sialidase family from Trypanosoma cruzi display beta-galactose binding activity

trans-sialidase is a unique sialidase in that, instead of hydrolizing siali c acid, it preferentially transfers the monosaccharide to a terminal beta-g alactose in glycoproteins and glycolipids, This enzyme, originally identifi ed in Trypanosoma crud, belongs to a large family of proteins. Some members of the family lack the enzymatic activity. No function has been yet assign ed 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 th at genes encoding inactive members are not a few, but rather, are present i n the same copy number (60-80 per haploid genome) as those encoding active trails-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-sialyl ated 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 enzymatic ally inactive through a mutation ire a single amino acid also retained the galactose binding activity, The binding was competed by lactose and was dep endent on conservation of the protein native conformation, Therefore, at le ast some molecules in the trans-sialidase family that have lost their enzym atic function still retain their Gal-binding properties and might have a fu nction as lectins in the parasite-host interaction.