Covalent homodimers of murine secretory component induced by epitope substitution unravel the capacity of the polymeric Ig receptor to dimerize noncovalently in the absence of IgA ligand

Recombinant secretory immunoglobulin A containing a bacterial epitope in do main I of the secretory component (SC) moiety can serve as a mucosal delive ry vehicle triggering both mucosal and systemic responses (Corthesy, B., Ka ufmann, M., Phalipon, A., Peitsch, M., Neutra, M. R., and Kraehenbuhl, J.-P . (1996) J. Biol. Chem. 271, 33670-33677), To load recombinant secretory Ig A with multiple B and T epitopes and extend its biological functions, we se lected, based on molecular modeling, five surface-exposed sites in domains II and III of murine SC, Loops predicted to be exposed at the surface of SC domains were replaced with the DYKDDDDK octapeptide (FLAG), Another two mu tants were obtained with the FLAG inserted in between domains II and III or at the carboxyl terminus of SC. As shown by mass spectrometry, internal su bstitution of the FLAG into four of the mutants induced the formation of di sulfide-linked homodimers, Three of the dimers and two of the monomers from SC mutants could be affinity-purified using an antibody to the FLAG, mappi ng them as candidates for insertion. FLAG-induced dimerization also occurre d with the polymeric immunoglobulin receptor (pIgR) and might reflect the s o-far nondemonstrated capacity of the receptor to oligomerize, By co-expres sing in COS-7 cells and epithelial Caco-2 cells two pIgR constructs tagged at the carboxyl terminus with hexahistidine or FLAG, we provide the stronge st evidence reported to date that the pIgR dimerizes noncovalently in the p lasma membrane in the absence of polymeric IgA ligand, The implication of t his finding is discussed in terms of IgA transport and specific antibody re sponse at mucosal surfaces.