Structural Insights into the Preferential Binding of PGRP-SAs from Bumblebees and Honeybees to Dap-Type Peptidoglycans Rather than Lys-Type Peptidoglycans [MOLECULAR AND STRUCTURAL IMMUNOLOGY]

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.

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