Self-association of an insect β-1,3-glucan recognition protein upon binding laminarin stimulates prophenoloxidase activation as an innate immune response

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 for the activation of prophenoloxidase (pro-PO) or Toll pathways. Using biophysical and biochemical methods, we characterized the interaction of the N-terminal domain from Manduca sexta βGRP2 (N-βGRP2) with laminarin, a soluble form of β-1,3-glucan. We found that carbohydrate binding by N-βGRP2 induces the formation of two types of protein-carbohydrate complexes, depending on the molar ratio of carbohydrate to protein ([C]/[P]). Precipitation, analytical ultracentrifugation, and chemical cross-linking experiments have shown that an insoluble aggregate forms when the molar ratio of carbohydrate to protein is low ([C]/[P] ∼ 1). In contrast, a soluble complex, containing at least five N-βGRP2 molecules forms at a higher molar ratio of carbohydrate/protein ([C]/[P] >5). A hypothesis that this complex is assembled partly due to protein-protein interactions was supported by chemical cross-linking experiments combined with LC-MS/MS spectrometry analysis, which permitted identification of a specific intermolecular cross-link site between N-βGRP molecules in the soluble complex. The pro-PO activation in naive plasma was strongly stimulated by addition of the insoluble aggregates of N-βGRP2. The soluble complex with laminarin formed in the plasma also stimulated pro-PO activation, but at a lower level. Taken together, these results provide experimental evidence for novel mechanisms in which associations of βGRP with microbial polysaccharide promotes assembly of βGRP oligomers, which may form a platform needed to trigger the pro-PO pathway activation cascade.