Role of n-linked glycan in the unfolding pathway of Erythrina corallodendron lectin

Erythrina corallodendron lectin (ECorL) exhibits an exquisitely structured oligosaccharide chain. Interestingly, the bacterially expressed, nonglycosylated counterpart, rECorL, possesses an essentially identical carbohydrate specificity and agglutinating activity as the glycosylated lectin, thus suggesting that the overall structure of the two are identical. This paper reports the unfolding behavior of E. corallodendron lectin in its glycosylated (EcorL) and nonglycosylated (rECorL) forms. ECorL shows a two-state unfolding pattern during isothermal melts and differential scanning calorimetry (DSC). The T-g of ECorL as obtained from isothermal melts is 74 degreesC at pH 7.4. The T-p obtained from DSC studies is between 74.8 to 68.1 degreesC in the pH range of 5.26-7.77. The recombinant lectin (rECorL), which is devoid of carbohydrate, shows, in contrast to the glycosylated protein, a non-two-state unfolding profile as measured by both probes mentioned, but the number of intermediates during unfolding could not be ascertained. Simulated annealing on ECorL, with the sugars removed, reveals that the protein Calpha backbones overlap, indicating that the overall structure, including the mode of dimerization, of rECorL is insignificantly altered as compared to ECorL. The alterations in the folding behavior of rECorL as compared to that observed in ECorL may be due to the fact that, unlike most other glycoproteins, one of the glycans in ECorL is unusually structured and forms many hydrogen bonds with the protein. It therefore appears that while the covalently linked sugar does not contribute appreciably to the final folded structure of ECorL, it does alter its folding process in a significant manner.