J Biol Chem, Vol. 274, Issue 44, 31445-31455, October 29, 1999
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
Pascal
Crottet
,
Manuel C.
Peitsch¶,
Catherine
Servis
, and
Blaise
Corthésy
**
From the
Institut Suisse de Recherches,
Expérimentales sur le Cancer, CH-1066 Epalinges, ¶ Glaxo
Welcome Experimental Research, 16 chemin des Aulx, 1228 Plan-les-Ouates,
Institut de Biochimie, Université de
Lausanne, CH-1066 Epalinges, and ** Division d'immunologie et
d'allergie, Centre Hospitalier Universitaire Vaudois,
CH-1011 Lausanne, Switzerland
Recombinant secretory immunoglobulin A containing
a bacterial epitope in domain I of the secretory component (SC) moiety
can serve as a mucosal delivery vehicle triggering both mucosal and systemic responses (Corthésy, B., Kaufmann, M., Phalipon, A., Peitsch, M., Neutra, M. R., and Kraehenbuhl, J.-P. (1996) J. Biol. Chem. 271, 33670-33677). To load recombinant secretory IgA
with multiple B and T epitopes and extend its biological functions, we
selected, 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 mutants 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 substitution of the FLAG into four of
the mutants induced the formation of disulfide-linked homodimers. Three
of the dimers and two of the monomers from SC mutants could be
affinity-purified using an antibody to the FLAG, mapping them as
candidates for insertion. FLAG-induced dimerization also occurred with
the polymeric immunoglobulin receptor (pIgR) and might reflect the
so-far nondemonstrated capacity of the receptor to oligomerize. By
co-expressing in COS-7 cells and epithelial Caco-2 cells two pIgR
constructs tagged at the carboxyl terminus with hexahistidine or FLAG,
we provide the strongest evidence reported to date that the pIgR
dimerizes noncovalently in the plasma membrane in the absence of
polymeric IgA ligand. The implication of this finding is discussed in
terms of IgA transport and specific antibody response at mucosal surfaces.
Copyright © 1999 by The American Society for Biochemistry and Molecular Biology, Inc.