Lipopolysaccharide and β-1,3-glucan-binding protein (LGBP) bind to seaweed polysaccharides and activate the prophenoloxidase system in white shrimp Litopenaeus vannamei.

Lipopolysaccharide and β-1,3-glucan-binding protein (LGBP), important pattern recognition proteins (PRPs), recognize lipopolysaccharide (LPS) and β-1,3-glucan (βG), known as pathogen-associated molecular patterns (PAMPs), and subsequently trigger innate immunity. Several seaweed polysaccharides and seaweed extracts increase immune parameters and resistance to pathogens. Here, we constructed the expression vector pET28b-LvLGBP and transferred it into Escherichia coli BL21 (DE3) for protein expression and to produce the recombinant protein LGBP (rLvLGBP) in white shrimp Litopenaeus vannamei. We examined the binding of rLvLGBP with seaweed-derived polysaccharides including alginate, carrageenan, fucoidan, laminarin, Gracilaria tenuistipitata extract (GTE), and Sargassum duplicatum extract (SDE), and examined the phenoloxidase activity of shrimp haemocytes incubated with a mixture of rLvLGBP and each polysaccharide. We also examined the binding of rLvLGBP with LPS and βG, and the phenoloxidase activity of shrimp haemocytes incubated with a mixture of rLvLGBP and LPS (rLvLGBP-LPS) or a mixture of rLvLGBP and βG (rLvLGBP-βG). An ELISA binding assay indicated that rLvLGBP binds to LPS, βG, alginate, carrageenan, fucoidan, laminarin, GTE, and SDE with dissociation constants of 0.1138-0.1770 μM. Furthermore, our results also indicated that the phenoloxidase activity of shrimp haemocytes incubated with a mixture of rLvLGBP and LPS, βG, alginate, carrageenan, fucoidan, laminarin, GTE, and SDE significantly increased by 328%, 172%, 200%, 213%, 197%, 194%, 191%, and 197%, respectively compared to controls (cacodylate buffer). We conclude that LvLGBP functions as a PRP, recognizes and binds to LPS, βG, alginate, carrageenan, fucoidan, laminarin, GTE, and SDE, and subsequently leads to activating innate immunity in shrimp.