Understanding the structural specificity of Tn antigen for its receptor: an NMR solution study

The present work aims at understanding the structural basis of the biological recognition of Tn antigen (GalNAc-α-O-l-Ser), a specific epitope expressed by tumor cells, and the role of its amino acidic moiety in the interaction with its receptor (the isolectin B4 extracted from Vicia villosa). An NMR structural characterization of the α and β anomers, based on J couplings and molecular modeling revealed a structure in very good agreement with data reported in literature for variants of the same molecules. In order to demonstrate the involvement of the amino acid in the ligand–receptor recognition, also GalNAc-α-O-d-Ser was studied; the change in the stereochemistry is in fact expected to impact on the interaction only in case the serine is part of the epitope. Relaxation properties in the presence of the receptor clearly indicated a selective recognition of the natural l form, probably due to the formation of a water-mediated hydrogen bond with Asn 129 of the protein.

The comparison of the solution structures of isoforms of the Tn antigen reveals interesting features that may be at the basis of its selective recognition by the lectin receptor. The interaction with the Isolectin B4 from Vicia villosa demonstrates that the molecular recognition is not limited to the sugar moiety but involves also the directly linked amino acid.Figure optionsDownload as PowerPoint slideHighlights
► Tn antigen (GalNAc-α-O-l-Ser) and its β and d-Ser analogs have been studied by NMR.
► Their solution structures have been determined by means of J couplings and modeling.
► The interaction with B4 lectin from Vicia villosa has been tested.
► Hypothesis on recognition mechanisms is proposed on the basis of X-ray structure.