Assembly, secretion, and vacuolar delivery of a hybrid immunoglobulin in plants.
Secretory immunoglobulin (Ig) A is a decameric Ig composed of four alpha-heavy chains, four light chains, a joining (J) chain, and a secretory component (SC). The heavy and light chains form two tetrameric Ig molecules that are joined by the J chain and associate with the SC. Expression of a secretory monoclonal antibody in tobacco (Nicotiana tabacum) has been described: this molecule (secretory IgA/G [SIgA/G]) was modified by having a hybrid heavy chain sequence consisting of IgG gamma-chain domains linked to constant region domains of an IgA alpha-chain. In tobacco, about 70% of the protein assembles to its final, decameric structure. We show here that SIgA/G assembly and secretion are slow, with only approximately 10% of the newly synthesized molecules being secreted after 24 h and the bulk probably remaining in the endoplasmic reticulum. In addition, a proportion of SIgA/G is delivered to the vacuole as at least partially assembled molecules by a process that is blocked by the membrane traffic inhibitor brefeldin A. Neither the SC nor the J chain are responsible for vacuolar delivery, because IgA/G tetramers have the same fate. The parent IgG tetrameric molecule, containing wild-type gamma-heavy chains, is instead secreted rapidly and efficiently. This strongly suggests that intracellular retention and vacuolar delivery of IgA/G is due to the alpha-domains present in the hybrid alpha/gamma-heavy chains and indicates that the plant secretory system may partially deliver to the vacuole recombinant proteins expected to be secreted.
Frigerio
L
L
Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom.
Vine
N D
ND
Pedrazzini
E
E
Hein
M B
MB
Wang
F
F
Ma
J K
JK
Vitale
A
A
D020126:Brefeldin A / Q000494:pharmacology; D007071:Immunoglobulin A, Secretory / Q000235:genetics* / Q000378:metabolism; D007074:Immunoglobulin G / Q000235:genetics* / Q000378:metabolism; D007150:Immunohistochemistry; D018613:Microscopy, Confocal; D016253:Microscopy, Immunoelectron; D018515:Plant Leaves / Q000378:metabolism; D010947:Plants, Toxic; D011233:Precipitin Tests; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D011993:Recombinant Fusion Proteins / Q000235:genetics* / Q000378:metabolism; D014026:Nicotiana / Q000235:genetics* / Q000378:metabolism / Q000648:ultrastructure; D014617:Vacuoles / Q000378:metabolism* / Q000648:ultrastructure
D007071:Immunoglobulin A, Secretory; D007074:Immunoglobulin G; D011500:Protein Synthesis Inhibitors; D011993:Recombinant Fusion Proteins; D020126:Brefeldin A