Solution structure of the silkworm βGRP/GNBP3 N-terminal domain reveals the mechanism for β-1,3-glucan-specific recognition

The β-1,3-glucan recognition protein (βGRP)/Gram-negative bacteria-binding protein 3 (GNBP3) is a crucial pattern-recognition receptor that specifically binds β-1,3-glucan, a component of fungal cell walls. It evokes innate immunity against fungi through activation of the prophenoloxidase (proPO) cascade and Toll pathway in invertebrates. The βGRP consists of an N-terminal β-1,3-glucan-recognition domain and a C-terminal glucanase-like domain, with the former reported to be responsible for the proPO cascade activation. This report shows the solution structure of the N-terminal β-1,3-glucan recognition domain of silkworm βGRP. Although the N-terminal domain of βGRP has a β-sandwich fold, often seen in carbohydrate-binding modules, both NMR titration experiments and mutational analysis showed that βGRP has a binding mechanism which is distinct from those observed in previously reported carbohydarate-binding domains. Our results suggest that βGRP is a β-1,3-glucan-recognition protein that specifically recognizes a triple-helical structure of β-1,3-glucan.