PILRα and PILRβ have a siglec fold and provide the basis of binding to sialic acid

Abstract

Paired immunoglobulin-like type 2 receptor α (PILRα) and β (PILRβ) belong to the PILR family and are related to innate immune regulation in various species. Despite their high sequence identity, PILRα and PILRβ are shown to have variant sialic acid (SA) binding avidities. To explore the molecular basis of this interaction, we solved the crystal structures of PILRα and PILRβ at resolutions of 1.6 Å and 2.2 Å, respectively. Both molecules adopt a typical siglec fold but use a hydrophobic bond to substitute the siglecspecific disulfide linkage for protein stabilization. We further used HSV-1 glycoprotein B (gB) as a representative molecule to study the PILR-SA interaction. Deploying site-directed mutagenesis, we demonstrated that three residues (Y2, R95, and W108) presented on the surface of PILRα form the SA binding site equivalent to those in siglecs but are arranged in a unique linear mode. PILRβ differs from PILRα in one of these three residues (L108), explaining its inability to engage gB. Mutation of L108 to tryptophan in PILRβ restored the gB-binding capacity. We further solved the structure of this PILRβ mutant complexed with SA, which reveals the atomic details mediating PILR/SA recognition. In comparison with the free PILR structures, amino acid Y2 oriented variantly in the complex structure, thereby disrupting the linear arrangement of PILR residues Y2, R95, and W108. In conclusion, our study provides significant implications for the PILR-SA interaction and paves the way for understanding PILR-related ligand binding.

O-glycans direct selectin ligands to lipid rafts on leukocytes

“…Raft association could involve interactions of glycan determinants on PSGL-1, CD43, and CD44 with a raftresident lectin. Candidates are siglecs and the structurally related paired Ig-like type 2 receptors (PILRs), which bind terminal sialic acids in particular contexts (41,42). Siglec-E, the siglec CD33, and PILRα are expressed on murine myeloid cells.…”

Paired Immunoglobulin-like Type 2 Receptor Alpha G78R variant alters ligand binding and confers protection to Alzheimer’s disease

“…To understand the conformational changes that might occur in the PILRA sialic acidbinding pocket during receptor-ligand interactions in the presence of G78 (AD risk) or R78 (AD-protective) variants, we evaluated available experimental crystal structures ( Fig. 3, A to C) [39,41]. Structures of G78 (AD risk) PILRA reveal a monomeric extracellular domain with a single V-set Ig-like β-sandwich fold that binds O-glycan ligands (Fig.…”

“…Notably, in the structure of R78 (AD protective) PILRA crystallized in the absence of any ligand [41], the long side-chain of R78 is observed to hydrogen bond with Q140 directly (Fig. 3A).…”

Lectin Receptors Expressed on Myeloid Cells

“…Recognition of sialic acid depends on a conserved structural template involving both hydrogen bonding networks, ionic and hydrophobic interactions, together with variable inter-strand loops that make contact with additional glycan residues and confer extended specificity to siglecs (5). A siglec-like Ig fold was recently seen in the regulatory myeloid receptors, PILR-α and -β, which mediate high affinity binding to mucin-like sialylated ligands via both protein-protein and protein-sialic acid interactions (6,7). Siglecs are expressed broadly across the haemopoietic and immune systems, except for MAG which is restricted to the myelin-forming cells of the nervous system, oligodendrocytes and Schwann cells ( Figure 1).…”