Understanding the adhesion mechanism of a mucin binding domain from Lactobacillus fermentum and its role in enteropathogen exclusion.

Lactobacillus species possesses surface exposed Mucin Binding Protein (MucBP) which plays a role in adhesion to gastrointestinal mucin. MucBP contains one or more mucin binding domain (MBD), the functionality of which has yet not been characterized thoroughly. Here, we have characterized a 93-amino acid MBD (MBD) of MucBP (LAF_0673) from Lactobacillus fermentum. Multiple sequence alignment of L. fermentum MBD exhibited ∼60% sequence homology with MBDs from other Lactobacillus species. Further, we cloned, expressed and purified MBD from Escherichia coli as N-terminal histidine-tagged protein (6X His-MBD). The purified MBD was able to bind to mucin and showed strong affinity towards the terminally expressed mucin glycans viz. N-acetylgalactosamine (GalNAc), N-acetylglucosamine (GlcNAc), Galactose (Gal), and Sialic acid (N-acetylneuraminic acid; Neu5Ac). In silico experiments further confirmed the interaction between homology modeled MBD to mucin glycans through hydrogen-bonding with its surface amino acid residues Ser, Pro, Ile, Tyr and Ala. We also have demonstrated that MBD was able to inhibit the adhesion of enteric pathogens, including E. coli, Salmonella Paratyphi A, Shigella sonnei and Proteus vulgaris to mucin. Our results suggested that L. fermentum MBD is a functionally sufficient unit to act as an adhesin and to protect from invading enteric pathogens.