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.

译文

:Fcα/μ受体(Fcα/μR,CD351)是对IgA和IgM具有双重特异性的受体。它的第二个细胞外结构域(EC2)具有Ig可变区样结构,并被预测为配体结合结构域。 EC2与聚合Ig受体(pIgR)的第一个Ig样结构域(D1)同源,并具有三个互补决定区(CDR)样环。已经发现包括CDR1样环区的肽负责IgA和IgM结合。但是,尚不清楚EC2的CDR2和CDR3样环是否有助于配体结合。在这项工作中,我们制备了三个由hFcα/μR骨架组成的嵌合受体,但EC2的CDR1,CDR2和CDR3样环被来自人类pIgR D2的对应环所取代,后者本身不结合IgA或IgM。流式细胞术和共聚焦显微镜分析表明,CDR1-或CDR2-样环的取代消除了IgA和IgM的结合,表明CDR1-和CDR2-样环对配体结合都很重要。相比之下,CDR3样环的取代导致IgM结合的显着丧失,但对IgA结合的负面影响很小。此外,三个CDR样环的定点诱变表明残基Val29,Arg31,Asn54,Gln55,Glu98,Asn99和Asn100参与IgA和IgM的结合,并且在CDR3样环内的Glu98取代增加了。 IgA结合,但IgM结合降低。

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