Structural Mechanism of Insect Cuticular Protein Binding to Chitin Revealed by Solid-State NMR

The exoskeleton of an insect is a fascinating example of how nature employs organic substances to craft high-performance materials, characterized by their hardness, tensile strength, and lightweight. Dissecting the atomic-level arrangement of these multifunctional organics, primarily proteins and chitin, within insect cuticles will aid in deciphering this enigma. Despite its importance, revealing the mechanism of interaction between cuticular proteins and chitin polysaccharides in heterogeneous systems remains a challenge. Nuclear magnetic resonance (NMR) spectroscopy, renowned for its ability to provide atomic-resolution insights, is adept at obtaining distance information and local structure without the need for long-range order. Here, cuticular proteins and chitin polysaccharides, which are prominent components of the insect cuticle, were studied using combining solution and solid-state NMR methods. Our results indicate that the larval cuticle proteins of Ostrinia furnacalis (OfLCP30-C) are intrinsically disordered in aqueous solution but undergo a conformational transition from unfolded to folded upon binding to chitin polysaccharides. High-resolution 1H-detected solid-state NMR spectra enable us to obtain the atomic-resolution structure of OfLCP30-C in its chitin-binding state. Aromatic amino acids located on the same side of the planar-shaped structure act as adhesive patches, adhering to the chitin surface and exerting a critical influence during the chitin-protein binding process. Our results provide a feasible basis for studying the ubiquitous interactions between cuticular proteins and polysaccharides.