The peptidoglycan recognition protein PGRP-S is an innate immunity molecule that specifically interacts with microbial peptidoglycans and other pathogen-associated molecular patterns. We report here two structures of the unique tetrameric camel PGRP-S (CPGRP-S) complexed with (i) muramyl dipeptide (MDP) at 2.5 Å resolution and (ii) GlcNAc and -maltose at 1.7Å resolution. The binding studies carried out using surface plasmon resonance indicated that CPGRP-S binds to MDP with a dissociation constant of 10 ؊7 M, whereas the binding affinities Peptidoglycan recognition proteins (PGRPs) 2 of the innate immune system provide the first line of defense to hosts against infecting microbes. These proteins specifically recognize pathogen-associated molecular patterns (PAMPs) such as peptidoglycan (PGN), LPS, lipoteichoic acid (LTA), mycolic acid, and etc. It is well known that PGN is present on the cell walls of both Gram-negative and -positive bacteria. PGN is a polymer of alternating GlcNAc and N-acetylmuramic acid (MurNAc) in a (134) linkage, cross-linked by short peptide stems composed of alternating L-and D-amino acids. The glycan moiety of PGN is conserved among almost all of the bacteria, but the peptide part displays a considerable diversity (1). According to the amino acid residue at number three position of the peptide stem, PGNs are divided into two major categories, (i) lysine type (Lys-type) and (ii) meso-diaminopimelic acid type. The peptidoglycans or their fragments are recognized by the host through innate immunity proteins, particularly PGRPs (2).PGRPs are highly conserved molecules (3-7), which bind to PGN and other PAMPs as well as to their fragments and derivatives with varying affinities (8). There are four types of mammalian PGRPs designated as short PGRP (PGRP-S), PGRP-L, PGRP-I␣, and PGRP-I representing short (molecular mass, 20 -25 kDa), long (molecular mass, up to 90 kDa), and intermediate (molecular mass, 40 -45 kDa), respectively. PGRP-S contains ϳ200 amino acid residues and represents a single PGRP domain, wheraes PGRP-L and PGRP-I also contain a similar C-terminal PGRP domain. PGRP-S shows direct bacterial recognition. The potency of its recognition determines its strength in controlling bacterial infections. Initially, PGRP-S was detected in bone marrow (6), whereas subsequently, it was also observed as a soluble protein in the granules of polymorphonuclear leukocytes (9). This observation implied that PGRP-S might have a role as an antibacterial agent (6). Recently, its presence has also been shown in milk of certain species of mammals (10) and in the intestinal M cells (11). As far as mammary gland secretions are concerned, so far, PGRP-S has been detected in the mammary secretions of porcine and camel only (10). As far as structural studies are concerned, the crystal structures of truncated human PGRP-S (HPGRP-S) (amino acid residues from 9 to 191) and a full-length (amino acid residues from 1 to 171) camel PGRP-S (CPGRP-S) have been reported (12,13) showing truncated HPGRP-S as a monom...