Fig. 6.

A model summarizing the molecular events in the initiation of the Toll and prophenoloxidase pathways. Although a few PGRP-SA (R) molecules bind to intact PG, it is not able to activate the immune responses (A). PG of Gram-positive bacteria is digested partially (B) or completely (B′) by lysozyme. Whereas the partially digested PG recruits more PGRP-SA molecules binding to the bacterial surface (B), the fully digested PG cannot recruit PGRP-SA on the bacterial surface leading to lysis of the bacterial cell (B′). The clustered PGRP-SA molecules recruit GNBP1 and a modular serine protease (SP) containing low-density lipoprotein receptor A repeat domains (LDL), resulting in the activation of the modular serine protease (C). Then the activated SP triggers the proteolytic cascade leading to activation of the Toll and prophenoloxidase (proPO) pathways that produce antimicrobial peptide (AMP) and melanin around the invading bacteria.