Biochemical characterization of evasion from peptidoglycan recognition by Staphylococcus aureus D-alanylated wall teichoic acid in insect innate immunity.

Abstract We recently reported that d -alanylation of Staphylococcus aureus wall teichoic acid (WTA) mitigates an induction of the Toll-mediated humoral response in Drosophila by interfering with peptidoglycan (PG) recognition by PG recognition protein-SA (PGRP-SA). Here, we investigated the mode of this interference by using an in vitro cell free system from larvae of the coleoptran insect Tenebrio molitor . WTA modification on PG had a potent inhibitory effect on PGRP-SA-mediated Toll proteolytic cascade activation, and the d -alanylation of WTA enhanced its inhibitory effect. Purified d -alanylated WTA released from PG lost its inhibitory action on both Toll cascade activation and PGRP-SA binding to insoluble PG. The inhibition of PGRP-SA binding to PG by d -alanylated WTA took place not only on polymeric PG but also on WTA-attached disaccharide units of monomeric PG. These results suggest that d -alanylation-mediated evasion requires the covalent bonding of d -alanylated WTA to PG, but not net-like cross-linking structure of PG.