Molecular Cloning, Carbohydrate Specificity and the Crystal Structure of Two Sclerotium rolfsii Lectin Variants

Abstract

SRL is a cell wall associated developmental-stage specific lectin secreted by Sclerotium rolfsii, a soil-born pathogenic fungus. SRL displays specificity for TF antigen (Gal13GalNAc--Ser//Thr) expressed in all cancer types and has tumour suppressing effects in vivo. Considering the immense potential of SRL in cancer research, we have generated two variant gene constructs of SRL and expressed in E. coli to refine the sugar specificity and solubility by altering the surface charge. SSR1 and SSR2 are two different recombinant variants of SRL, both of which recognize TF antigen but only SSR1 binds to Tn antigen (GalNAc-Ser/Thr). The glycan array analysis of the variants demonstrated that SSR1 recognizes TF antigen and their derivative with high affinity similar to SRL but showed highest affinity towards the sialylated Tn antigen, unlike SRL. The carbohydrate binding property of SSR2 remains unaltered compared to SRL. The crystal structures of the two variants were determined in free form and in complex with N-acetylglucosamine at 1.7 angstrom and 1.6 angstrom resolution, respectively. Structural analysis highlighted the structural basis of the fine carbohydrate specificity of the two SRL variants and results are in agreement with glycan array analysis.