Peptidoglycan Recognition Proteins Involved in 1,3-β -D-Glucan-dependent Prophenoloxidase Activation System of Insect

The prophenoloxidase (proPO) cascade is a major innate immune response in invertebrates, which is triggered into its active form by elicitors, such as lipopolysaccharide, peptidoglycan, and 1,3-β-D-glucan. A key question of the proPO system is how pattern recognition proteins recognize pathogenic microbes and subsequently activate the system. To investigate the biological function of 1,3-β-D-glucan pattern recognition protein in the proPO cascade system, we isolated eight different 1,3-β-D-glucan-binding proteins from the hemolymph of large beetle (Holotrichia diomphalia) larvae by using 1,3-β-D-glucan immobilized column. Among them, a 20- and 17-kDa protein (referred to as Hd-PGRP-1 and Hd-PGRP-2) show high sequence identity with the short forms of peptidoglycan recognition proteins (PGRPs-S) from human and Drosophila melanogaster. To be able to characterize the biochemical prop. erties of these two proteins, we expressed them in Drosophila S2 cells. Hd-PGRP-1 and Hd-PGRP-2 were found to specifically bind both 1, 3-β-D-glucan and peptidoglycan. By BIAcore analysis, the minimal 1,3-β-D-glucan structure required for binding to Hd-PGRP-1 was found to be laminaritetraose. Hd-PGRP-1 increased serine protease activity upon binding to 1,3-β-D-glucan and subsequently induced the phenoloxidase activity in the presence of both 1,3-β-D-glucan and Ca²⁺, but no phenoloxidase activity was elicited under the same conditions in the presence of peptidoglycan and Ca²⁺. These results demonstrate that Hd-PGRP-1 can serve as a receptor for 1,3-β-D-glucan in the insect proPO activation system.