Characterization of a novel Manduca sexta beta-1, 3-glucan recognition protein (βGRP3) with multiple functions

Recognition of pathogens by insect pattern recognition receptors is critical to mount effective immune responses. In this study, we reported a new member (beta GRP3) of the beta-1, 3-glucan recognition protein (beta GRP) family from the tobacco hornworm Manduca sexta. Unlike other members of the M. sexta beta GRP family proteins, which contain an N-terminal small glucan binding domain and a C-terminal large glucanase-like domain, beta GRP3 is 40-45 residues shorter at the N-terminus and lacks the small glucan binding domain. The glucanase-like domain of beta GRP3 is most similar to that of M. sexta microbe binding protein (MBP) with 78% identity. beta GRP3 transcript was mainly expressed in the fat body, and both its mRNA and protein levels were not induced by microorganisms in larvae. Recombinant beta GRP3 purified from Drosophila S2 cells could bind to several Gram-negative and Gram-positive bacteria and yeast, as well as to laminarin (beta-1, 3-glucan), mannan, lipopolysaccharide (LPS), lipoteichoic acid (LTA), and meso-diaminopimelic acid (DAP)-type peptidoglycan (PG), but did not bind to Lysine-type PG. Binding of beta GRP3 to laminarin could be competed well by free laminarin, mannan, LPS and LTA, but almost not competed by free PGs. Recombinant beta GRP3 could agglutinate Bacillus cereus and Escherichia coli in a calcium-dependent manner and showed antibacterial (bacteriostatic) activity against B. cereus, novel functions that have not been reported for the beta GRP family proteins before. M. sexta beta GRP3 may serve as an immune surveillance receptor with multiple functions. (C) 2014 Elsevier Ltd. All rights reserved.