Glycosylation of synthetic peptides breaks helices. Phosphorylation results in distorted structure

Two proposed glycosylation sites are located within T cell epitopes of rabies virus glycoprotein, namely VVEDEGCTNLSGF (VF13; amino acids 29-41) and GKAYTIFNKTLM (GM12; amino acids 312-323). To explore the effects on peptide conformation due to post-translational modifications, we synthesized glycosylated and phosphorylated versions of the two peptides and compared their structures with the native peptide using CD and FT-IR spectroscopy. After the modifications, i.e., glycosylation on Asn with one or two N-acetyl-glucosamine or glucose residues or phosphorylation on Ser, the low to medium degree of helicity of the unmodified peptides disappears as indicated by CD measurements in water-trifluoroethanol mixtures. Incorporation of one sugar moiety into either peptide resulted with a high probability in a type I (III) beta-turn formation with almost identical spectra for the different peptides. Elongation of the carbohydrate in GM12 only slightly enhanced this effect. In contrast, phosphorylation of VF13 caused distorted conformation of the peptide backbone. This novel and direct demonstration of a change in secondary structure by glycosylation (or phosphorylation) might be an important element in determining peptide antigen structure and function.