Localization of the Putative Sialic Acid-binding Site on the Immunoglobulin Superfamily Cell-surface Molecule CD22

P Anton Van Der Merwe,Paul R Crocker,Mary Vinson,A Neil Barclay,Roland Schauer,S⊘Rge Kelm

doi:10.1074/jbc.271.16.9273

Abstract

B-lymphocyte antigen CD22 is a member of the recently described sialoadhesin family of immunoglobulin-like cell-surface glycoproteins that bind glycoconjugates terminating in sialic acid. One prominent ligand for CD22 is the highly glycosylated leukocyte surface protein CD45. Using surface plasmon resonance spectroscopy, we characterized the interaction of recombinant mouse CD22 with native CD45 purified from rat thymus (CD45-thy). By in situ desialylation and resialylation of immobilized CD45-thy, we show that mouse CD22 binds to the sialoglycoconjugate NeuGc alpha 2-6Gal beta 1-4GlcNAc carried on CD45-thy N-glycans. Previous studies have shown that the sialic acid-binding site lies within the two membrane-distal domains of CD22 (domains 1 and 2), which are V-set and C2-set immunoglobulin superfamily domains, respectively. To further localize the binding site, we have made 42 single amino acid substitutions throughout both domains. All 12 mutations that abrogated binding to CD45-thy without disrupting antibody binding were of residues within the GFCC'C" beta-sheet of domain 1. These residues are predicted to form a contiguous binding site centered around an arginine residue in the F strand that is conserved in all members of the sialoadhesin family. Our results provide further evidence that immunoglobulin superfamily cell adhesion molecules use the GFCC'C" beta-sheet of membrane-distal V-set domains to bind structurally diverse ligands, suggesting that this surface is favored for cell-cell recognition.

Highlights

B-lymphocyte antigen CD22 is a member of the recently described sialoadhesin family of immunoglobulin-like cell-surface glycoproteins that bind glycoconjugates terminating in sialic acid
Our results provide further evidence that immunoglobulin superfamily cell adhesion molecules use the GFCC؅C؆ ␤-sheet of membrane-distal V-set domains to bind structurally diverse ligands, suggesting that this surface is favored for cell-cell recognition
The binding of anti-CD22 antibody-coated beads to B-cells decreases the activation threshold of the B-cell antigen receptor (BCR), presumably by removing CD22 from the vicinity of the BCR [12]. These findings suggest that physiological interactions between CD22 and natural cell-surface ligands may function to modulate signaling through the BCR [12]

Summary

Introduction

B-lymphocyte antigen CD22 is a member of the recently described sialoadhesin family of immunoglobulin-like cell-surface glycoproteins that bind glycoconjugates terminating in sialic acid. Our results provide further evidence that immunoglobulin superfamily cell adhesion molecules use the GFCC؅C؆ ␤-sheet of membrane-distal V-set domains to bind structurally diverse ligands, suggesting that this surface is favored for cell-cell recognition.

Results

Conclusion