Fig. 4.

PGN-binding site of PGRPs. (A) Crystal structure of human PGRP-IαC in complex with MTP, showing interactions at the binding site (22). MTP is drawn in stick representation. Carbon atoms are cyan, nitrogen atoms are dark blue, and oxygen atoms are red. PGRP-IαC is yellow; residues making hydrogen bonds (dashed lines) with MTP are green. In purple are Asn-236 and Phe-237, drawn in ball-and-stick representation, which form van der Waals contacts with the side chain of l-lysine. MurNAc, N-acetylmuramic acid; ALA, l-alanine; IDG, l-isoglutamine; LYS, l-lysine. (B) View of the PGN-binding site of human PGRP-S (35). The orientation is the same as in A. In green are residues of PGRP-S corresponding to MTP-contacting residues in the PGRP-IαC–MTP complex. Gly-68 and Trp-69 (purple) are predicted to contact the side chain of Lys-type PGN or Dap-type PGN. Gly-68 is represented by its carbonyl oxygen. (C) Structure-based sequence alignment of specificity-determining residues of mammalian and insect PGRPs. Residues corresponding to Asn-236 and Phe-237 of human PGRP-IαC in A are highlighted in yellow. Mammals: Bt, Bos taurus; Cd, Camelus dromedarius; Hs, Homo sapiens; Mm, Mus musculus; Rn, Rattus norvegicus; Ss, Sus scrofa. For human and mouse PGRP-Iα and PGRP-Iβ, C and N indicate the C-terminal and N-terminal PGRP domains, respectively. Insects: Ag, Anopheles gambiae; Bm, Bombyx mori; Ce, Calpodes ethlius; Dm, Drosophila melanogaster; Ms, Manduca sexta; Tn, Trichoplusia ni. Sequence alignments were performed by using the program clustalw at ExPASy (www.expasy.ch). The figure was generated by using espript (http://espript.ibcp.fr/ESPript/ESPript).