Structural Insights into the Preferential Binding of PGRP-SAs from Bumblebees and Honeybees to Dap-Type Peptidoglycans Rather than Lys-Type Peptidoglycans.

The peptidoglycan recognition protein SAs (PGRP-SAs) from Bombus ignitus (Bi-PGRP-SA), Apis mellifera (Am-PGRP-SA), and Megachile rotundata PGRP-SA (Mr-PGRP-SA) exhibit an intrinsic ability to preferentially bind to Dap-type peptidoglycan (PGN) from Bacillus subtilis rather than Lys-type PGN from Micrococcus luteus This ability is more analogous to the binding exhibited by PGRP-LCx and PGRP-SD than to that exhibited by PGRP-SA in Drosophila Moreover, Bi-PGRP-SA and Am-PGRP-SA share greater sequence identity with Drosophila PGRP-LCx than with PGRP-SD and retain several conserved contact residues, including His37/His38, His60/His61, Trp66/Trp67, Ala150/Ala151, and Thr151/Thr152 However, the corresponding contact residue Arg85 is not a major anchor residue in bees (e.g., bumblebees, honeybees, and leaf-cutting bees), and an in silico analysis indicated that the residues Thr151/Thr152 and Ser153/Ser154 of Bi-PGRP-SA and Am-PGRP-SA are deduced to be anchor residues. In addition, the nonconserved residues Asp67 in Bi-PGRP-SA and Mr-PGRP-SA and His68 in Am-PGRP-SA are deduced to be involved in the binding to Dap-type PGNs in bumblebees, honeybees, and leaf-cutting bees. We conclude that the structures and specificities of PGRP-SAs in bees are more analogous to those of PGRP-LCx than to those of Drosophila PGRP-SA. This phenomenon might be explained by the fact that the evolutionary clade of Hymenoptera is more ancient than that of Diptera.