Assembly, secretion, and vacuolar delivery of a hybrid immunoglobulin in plants

Secretory immunoglobulin (Ig) A is a decameric Ig composed of four a-heavy chains, four light chains, a joining CT) chain, and a secretory component ( SC). The heavy and light chains form two tetrameric Ig molecules that are j oined by the I chain and associate with the SC. Expression of a secretory m onoclonal antibody in tobacco (Nicotiana tabacum) has been described: this molecule (secretory IgA/G [SIgA/G]) was modified by having a hybrid heavy c hain sequence consisting of IgG gamma-chain domains linked to constant regi on domains of an IgA alpha-chain. In tobacco, about 70% of the protein asse mbles to its final, decameric structure. We show here that SIgA/G assembly and secretion are slow, with only approximately 10% of the newly synthesize d molecules being secreted after 24 h and the bulk probably remaining in th e endoplasmic reticulum. In addition, a proportion of SIgA/G is delivered t o the vacuole as at least partially assembled molecules by a process that i s blocked by the membrane traffic inhibitor brefeldin A. Neither the SC nor the I chain are responsible for vacuolar delivery, because IgA/G tetramers have the same fate. The parent IgG tetrameric molecule, containing wild-ty pe gamma-heavy chains, is instead secreted rapidly and efficiently. This st rongly suggests that intracellular retention and vacuolar delivery of IgA/G is due to the rr-domains present in the hybrid alpha/gamma-heavy chains an d indicates that the plant secretory system may partially deliver to the va cuole recombinant proteins expected to be secreted.