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.
Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom.
L
L
Frigerio
N D
ND
Vine
E
E
Pedrazzini
M B
MB
Hein
F
F
Wang
J K
JK
Ma
A
A
Vitale
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