The peptidoglycan recognition protein PGRP-SC1a is essential for Toll signaling and phagocytosis of Staphylococcus aureus in Drosophila

From a forward genetic screen for phagocytosis mutants in Drosophila melanogaster, we identified a mutation that affects peptidoglycan recognition protein (PGRP) SC1a and impairs the ability to phagocytose the bacteria Staphylococcus aureus, but not Escherichia coli and Bacillus subtilis. Because of the differences in peptidoglycan peptide linkages in these bacteria, our data suggest that PGRP-SC1a is necessary for recognition of the Lys-type peptidoglycan typical of most Gram(+) bacteria. PGRP-SC1a mutants also fail to activate the Toll/NF-kappaB signaling pathway and are compromised for survival after S. aureus infection. This mutant phenotype is the first found for an N-acetylmuramoyl-l-alanine amidase PGRP that cleaves peptidoglycan at the lactylamide bond between the glycan backbone and the crosslinking stem peptides. By generating transgenic rescue flies that express either wild-type or a noncatalytic cysteine-serine mutant PGRP-SC1a, we find that PGRP-SC1a amidase activity is not necessary for Toll signaling, but is essential for uptake of S. aureus into the host phagocytes and for survival after S. aureus infection. Furthermore, we find that the PGRP-SC1a amidase activity can be substituted by exogenous addition of free peptidoglycan, suggesting that the presence of peptidoglycan cleavage products is more important than the generation of cleaved peptidoglycan on the bacterial surface for PGRP-SC1a mediated phagocytosis.