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.