Identification of amino acid residues at the active site of endosialidase that dissociate the polysialic acid binding and cleaving activities in Escherichia coli K1 bacteriophages.

Endosialidase (endo-N-acetylneuraminidase) is a tailspike enzyme of bacteriophages specific for human pathogenic Escherichia coli K1, which specifically recognizes and degrades polySia (polysialic acid). polySia is also a polysaccharide of the capsules of other meningitis- and sepsis-causing bacteria, and a post-translational modification of the NCAM (neural cell-adhesion molecule). We have cloned and sequenced three spontaneously mutated endosialidases of the PK1A bacteriophage and one of the PK1E bacteriophage which display lost or residual enzyme activity but retain the binding activity to polySia. Single to triple amino acid substitutions were identified, and back-mutation constructs indicated that single substitutions accounted for only partial reduction of enzymic activity. A homology-based structural model of endosialidase revealed that all substituted amino acid residues localize to the active site of the enzyme. The results reveal the importance of non-catalytic amino acid residues for the enzymatic activity. The results reveal the molecular background for the dissociation of the polySia binding and cleaving activities of endosialidase and for the evolvement of 'host range' mutants of E. coli K1 bacteriophages.

Construction of antibody mimics from a noncatalytic enzyme-detection of polysialic acid.

We have used a conceptually novel way to construct antibody mimics based on the binding of a noncatalytic enzyme to its substrate. Bacteriophage-derived endosialidase cleaves polysialic acid (polySia), an important oncofetal and bacterial antigen, which is poorly immunogenic. We fused to green fluorescent protein (GFP) a catalytically inactive endosialidase known to bind but not degrade polysialic acid. The fusion protein is a convenient single-step reagent in fluorescence microscopy, binding assays and immunoblots. It efficiently and specifically detected polysialic acid in developing brain, neuroblastoma cells and bacteria causing meningitis. Enzyme-substrate interactions represent an unexploited source of molecular recognition events. Some of these could be used in designing well-defined substitute antibodies for the study of target molecules which are difficult to purify, available in low quantities, are unstable or have poor immunogenity.