Mutant glycosidases for labeling sialoglycans with high specificity and affinity.

Affinity labeling of biomacromolecules is vital for bioimaging and functional studies. However, affinity probes recognizing glycans with high specificity remain scarce. Here we report the development of glycan recombinant affinity binders (GRABs) based on mutant bacterial sialidases, which are enzymatically inactive but preserve stringent specificity for sialoglycan substrates. By mutating a key catalytic residue of Streptococcus pneumoniae neuraminidase A (SpNanA) and Ruminococcus gnavus neuraminidase H (RgNanH), we develop GRAB-Sia and GRAB-Sia3 recognizing total sialoglycans and α2,3-sialosides, respectively. The GRABs exhibit strict substrate and linkage specificity, and tetramerization with streptavidin substantially increases their avidity. The GRABs and tetrameric GRABs (tetra-GRABs) are effective tools for probing sialoglycans in immunoblotting, flow cytometry, immunoprecipitation, and fluorescence imaging. Furthermore, multiplex analysis with tetra-GRABs uncovers spatially distinct sialoglycans in the various mouse organs. This work provides a versatile toolkit for labeling and analyzing sialoglycans with high specificity, sensitivity, and convenience.