Exploration of mucin-protein interactions using liquid glycan array

M13 phage makes it possible to produce DNA-encoded display of any molecules stable in water by prospective DNA-barcoding. For example, chemical conjugation of a glycan to an M13 virion that contains a prospectively introduced DNA barcode in the M13 genome creates a DNA-barcoded glycophage. In this glycophage, there is a 1:1 correspondence between the DNA sequence inside the phage and the structure of the glycan displayed on the surface of the phage. The same construct simultaneously illustrates the concept of DNA-encoded multivalency because there is a one-to-one correspondence between the DNA barcode and the displayed density. In this chapter, we show that a collection of these DNA-barcoded multivalent glycophages, known as Liquid Glycan Array (LiGA), can measure the glycan-recognition profile of mucin-binding proteins. Using DNA-encoded multivalency, we make an intriguing observation that a simple ultra-dense display of D-galactose on M13 "looks like mucin" because it binds to the proteolytically inactive E447D mutant of mucinase StcE (StcE^E447D). The binding is specific because StcE^E447D does not bind to the ultra-high-density display of the enantiomer control, L-galactose, nor to the diastereomer controls (D-glucose and D-mannose).