J chain is covalently bound to both monomer subunits in human secretory IgA.

A Garcia-Pardo,M.E Lamm,A.G Plaut,B Frangione

doi:10.1016/s0021-9258(19)68467-x

Abstract

Previous work has established that the secretory component (SC) in human secretory IgA is covalently linked to only one of the two IgA monomer subunits, but it has not been clear whether the J chain is covalently linked to one or to both of these subunits. In view of the asymmetry in the disulfide bonding between SC and the IgA subunits, an arrangement which follows disulfide interchange, several models for the disulfide linkage of J chain and the bonds between IgA subunits were envisaged and investigated. When sIgA was gel filtered through Sephadex G-200 in acetic acid, a single major symmetrical peak eluted at the front. This material contained SC, alpha and L chains, and all of the J chain. The greater resolution afforded by polyacrylamide gel electrophoresis in detergent confirmed that human sIgA contains no major noncovalently linked components in the 150,000-200,000 molecular weight range. In another series of experiments the Fc monomer, which is not covalently attached to SC, isolated after treatment of sIgA with IgA protease and cyanogen bromide, was investigated to learn whether alpha chain COOH-terminal octapeptides could be released by reduction. The results were negative. The available data thus favor a model in which J chain is disulfide-bonded to both IgA monomer subunits in sIgA.

Highlights

Amide gel electrophoresisin detergent confirmed that chain is disulfide-bonded to only one subunit has been subhuman sIgA contains no major noncovalently linked sequently proposed (Mestecky, 1976).In view of the fact that components in the160,000-200,000molecularweight the union of SC to J chain-containing dimeric IgA in the range
The SC. (Fd2.Jfragment was cleaved with cyanogen bromide and gel filtered on Sephadex G 150 in 1 M acetic acid (Fig. 1,which is taken from Garcia-Pardo et al, 1979).Peak ZZZ contains the Fc monomer not linked to SC, and Peak ZVcontains the J chain linked to COOH-terminal octapeptides of two a chains
If the two subunits are not covalently linked (Fig. 7, Model I), there are two possibilities.In one, J chain is linked to one monomer and SC to the other, as illustrated

Summary

Introduction

Final assembly of the sIgA’ molecule takes place in the mucosal epithelium where SC is synthesized (Poger and Lamm, 1974; Brandtzaeg, 1974; Brown et al, 1976). no covalent interaction between J chain and SC exists (Mestecky et al,, 1974a),the J chain is believed to play a role in the binding of IgA to SCby imposing the optimal conformation (Brandtzaeg, 1976) This is consistent with experiments showing that dimeric IgA, which contains J chain, but not monomeric IgA, which lacks J chain, binds to SC

Objectives

Methods

Conclusion