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

Paired immunoglobulin-like type 2 receptor alpha (PILR alpha) and beta (PILR beta) belong to the PILR family and are related to innate immune regulation in various species. Despite their high sequence identity, PILR alpha and PILR beta are shown to have variant sialic acid (SA) binding avidities. To explore the molecular basis of this interaction, we solved the crystal structures of PILR alpha and PILR beta at resolutions of 1.6 and 2.2 angstrom, 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 PILR alpha form the SA binding site equivalent to those in siglecs but are arranged in a unique linear mode. PILR beta differs from PILR alpha in one of these three residues (L108), explaining its inability to engage gB. Mutation of L108 to tryptophan in PILR beta restored the gB-binding capacity. We further solved the structure of this PILR beta 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.