Single-molecule pair studies of the interactions of the alpha-GalNAc (Tn-antigen) form of porcine submaxillary mucin with soybean agglutinin.

Mucins form a group of heavily O-glycosylated biologically important glycoproteins that are involved in a variety of biological functions, including modulating immune response, inflammation, and adhesion. Mucins are also involved in cancer and metastasis and often express diagnostic cancer antigens. Recently, a modified porcine submaxillary mucin (Tn-PSM) containing GalNAcalpha1-O-Ser/Thr residues was shown to bind to soybean agglutinin (SBA) with approximately 10(6)-fold enhanced affinity relative to GalNAcalpha1-O-Ser, the pancarcinoma carbohydrate antigen. In this study, dynamic force spectroscopy is used to investigate molecular pairs of SBA and Tn-PSM. A number of force jumps that demonstrate unbinding or rebinding events were observed up to a distance equal to 2.0 microm, consistent with the length of the mucin chain. The unbinding force increased from 103 to 402 pN with increasing force loading rate. The position of the activation barrier in the energy landscape of the interaction was 0.1 nm. The lifetime of the SBA-TnPSM complex in the absence of applied force was determined to be in the range 1.3-1.9 s. Kinetic parameters describing the rate of dissociation of other sugar lectin interactions are in the range 3.3 x 10(-3)-2.5 x 10(-3) s. The long lifetime of the SBA-TnPSM complex is compatible with a binding model in which lectin molecules "bind and jump" from alpha-GalNAc residue to alpha-GalNAc residue along the polypeptide chain of Tn-PSM before dissociating. These findings have important implications for the molecular recognition properties of mucins.