Protein self‐association of N‐terminal domain of β‐1,3‐glucan recognition protein upon binding to β‐1,3‐glucan stimulates the prophenoloxidase activation in Manduca sexta (1007.4)

Insect β‐glucan recognition protein (βGRP), a pathogen recognition receptor for innate immune responses, detects β‐1,3‐glucan on fungal surfaces via its N‐terminal carbohydrate binding domain (N‐βGRP) and triggers serine protease cascades leading to the activation of prophenoloxidase (proPO) or Toll pathway. Here we use biochemical and biophysical techniques to probe the molecular interaction of N‐βGRP from Manduca sexta with laminarin, soluble β‐1,3‐glucan. Intriguingly, the carbohydrate binding induces the formation of two types of protein self‐association of N‐βGRPs depending on the molar ratio of carbohydrate to protein (C/P). Insoluble aggregation occurs at the lower molar ratio (C/P ~1), and a soluble oligomer, likely containing at least six N‐βGRP molecules, forms at the higher molar ratio (C/P >5). Further, the PO activation is significantly enhanced in the naïve plasma supplemented with the insoluble aggregate of N‐βGRP. The soluble complex formed in plasma with laminarin also stimulates the PO activation, but at a lower level. Our results indicate that the self‐association of βGRPs on binding to β‐1,3‐glucan is an essential step to stimulate the immune responses against fungal infection. We also discuss the possible involvement of other hemolymph proteins in the self‐association complex of βGRP for downstream signaling. Grant Funding Source : Supported by NIH grant GM‐41247