The crystal structure of Nictaba reveals its carbohydrate-binding properties and a new lectin dimerization mode

Abstract

Nictaba is a (GlcNAc)n-binding, stress-inducible lectin fromNicotiana tabacum, that serves as the archetypical lectin for the family of Nictaba-related lectins. Nictaba and Nictaba-related lectins play pivotal roles in plant defense mechanisms and stress response pathways. Despite extensive research into the variety of biological activities and physiological role(s) of the lectin, the three-dimensional structure of Nictaba remained largely unknown. Here, we report crystal structures for Nictaba in the apo form and bound to chitotriose. The structures reveal a jelly-roll fold for the Nictaba protomer and the assembly thereof into a novel dimerization interface among lectins. The chitotriose binding mode centers around the central GlcNAc residue providing insights into the determinants of specificity of Nictaba towards carbohydrate structures. Indeed, by integrating such structural insights with inputs from glycan arrays, molecular docking, and molecular dynamics simulations, we propose that Nictaba employs a single carbohydrate-recognition domain within each of the two subunits in the dimer to display pronounced specificity towards GlcNAc-containing carbohydrates. Furthermore, we identified amino acid residues involved in the extended binding site capable of accommodating structurally diverse high-mannose and complexN-glycans, highlighting the lectin potential to recognizeN-glycan structures. Glycan array andin silicoanalyses revealed interactions centered around the conserved Man3GlcNAc2 core, explaining the broad recognition ofN-glycan structures. Collectively, the integrated structural and biochemical insights presented here fill a hitherto substantial void into the atlas of lectin structure-function relationships and pave the way for future developments in plant stress biology and lectin-based applications.