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All eukaryotic sialyltransferases have in common the presence in their catalytic domain of several conserved peptide regions (sialylmotifs L, S, and VS). Functional analysis of sialylmotifs L and S previously demonstrated their involvement in the binding of donor and acceptor substrates. The region comprised between the sialylmotifs S and VS contains a stretch of four highly conserved residues, with the following consensus sequence (H/y)Y(Y/F/W/h)(E/D/q/g). (Capital letters and lowercase letters indicate a strong or low occurrence of the amino acid, respectively.) The functional importance of these residues and of the conserved residues of motif VS (HX₄E) was assessed using as a template the human ST3Gal I. Mutational analysis showed that residues His²⁹⁹ and Tyr³⁰⁰ of the new motif, and His³¹⁶ of the VS motif, are essential for activity since their substitution by alanine yielded inactive enzymes. Our results …