Albizia lebbeck Seed Coat Proteins Bind to Chitin and Act as a Defense against Cowpea Weevil Callosobruchus maculatus

The seed coat is an external tissue that participates in defense against insects. In some nonhost seeds, including Albizia lebbeck, the insect Callosobruchus maculatus dies during seed coat penetration. We investigated the toxicity of A. lebbeck seed coat proteins to C. maculatus. A chitin-binding protein fraction was isolated from seed coat, and mass spectrometry showed similarity to a C1 cysteine protease. By ELM program an N-glycosylation interaction motif was identified in this protein, and by molecular docking the potential to interact with N-acetylglucosamine (NAG) was shown. The chitin-binding protein fraction was toxic to C. maculatus and was present in larval midgut and feces but not able to hydrolyze larval gut proteins. It did not interfere, though, with the intestinal cell permeability. These results indicate that the toxicity mechanism of this seed coat fraction may be related to its binding to chitin, present in the larvae gut, disturbing nutrient absorption.