title: The three complementarity-determining region-like loops in the second extracellular domain of human Fc alpha/mu receptor contribute to its binding of IgA and IgM.
abstract: The Fc alpha/mu receptor (Fcα/μR, CD351) is a receptor that has dual specificity for IgA and IgM. Its second extracellular domain (EC2) has an Ig variable region-like structure and is predicted to be the ligand binding domain. EC2 is homologous to the first Ig-like domain (D1) of polymeric Ig receptor (pIgR) and has three complementarity-determining region (CDR)-like loops. A peptide that includes the CDR1-like loop region has been found to be responsible for IgA and IgM binding. However, whether the CDR2- and CDR3-like loops of EC2 contribute to ligand binding has remained unclear. In this work, we made three chimaeric receptors composed of the hFcα/μR backbone but having the CDR1-, CDR2- and CDR3-like loops of EC2 replaced by their counterpart loops from human pIgR D2, which itself does not bind IgA or IgM. Flow cytometry and confocal microscopy analysis showed that substitution of either the CDR1- or the CDR2-like loop abrogated IgA and IgM binding, indicating that both the CDR1- and the CDR2-like loops were important for ligand binding. In comparison, substitution of CDR3-like loop resulted in significant loss of IgM binding but has only a small negative effect on IgA binding. In addition, site-directed mutagenesis of the three CDR-like loops showed that residues Val29, Arg31, Asn54, Gln55, Glu98, Asn99 and Asn100 were involved in both IgA and IgM binding, and substitution of Glu98 within the CDR3-like loop increased IgA binding but decreased IgM binding.
authors.affiliation: Department of Immunology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences, School of Basic Medicine, Peking Union Medical College, 5 Dong Dan San Tiao, 100005 Beijing, China.
authors.forename: Xi
authors.initials: X
authors.lastname: Yang
authors.forename: Qing
authors.initials: Q
authors.lastname: Zhao
authors.forename: Liping
authors.initials: L
authors.lastname: Zhu
authors.forename: Wei
authors.initials: W
authors.lastname: Zhang
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D015703:Antigens, CD / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D001665:Binding Sites; D045744:Cell Line, Tumor; D022801:Complementarity Determining Regions / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D000939:Epitopes; D006801:Humans; D007070:Immunoglobulin A / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D007075:Immunoglobulin M / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D008024:Ligands; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011961:Receptors, Fc / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D017450:Receptors, Opioid, mu / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D019056:Receptors, Polymeric Immunoglobulin / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D016415:Sequence Alignment
chemical_list: D000596:Amino Acids; D015703:Antigens, CD; D022801:Complementarity Determining Regions; D000939:Epitopes; C036832:Fc(alpha) receptor; D007070:Immunoglobulin A; D007075:Immunoglobulin M; D008024:Ligands; D011961:Receptors, Fc; D017450:Receptors, Opioid, mu; D019056:Receptors, Polymeric Immunoglobulin; D011993:Recombinant Fusion Proteins