SummaryAntibody 2G12 is one of a small number of human immunoglobulin G (IgG) monoclonal antibodies exhibiting potent and broad human immunodeficiency virus-1 (HIV-1)-neutralizing activity in vitro , and the ability to prevent HIV-1 infection in animal models.It could be used to treat or prevent HIV-1 infection in humans, although to be effective it would need to be produced on a very large scale. We have therefore expressed this antibody in maize, which could facilitate inexpensive, large-scale production. The antibody was expressed in the endosperm, together with the fluorescent marker protein Discosoma red fluorescent protein (DsRed), which helps to identify antibody-expressing lines and trace transgenic offspring when bred into elite maize germplasm. To achieve accumulation in storage organelles derived from the endomembrane system, a KDEL signal was added to both antibody chains. Immunofluorescence and electron microscopy confirmed the accumulation of the antibody in zein bodies that bud from the endoplasmic reticulum. In agreement with this localization, N -glycans attached to the heavy chain were mostly devoid of Golgi-specific modifications, such as fucose and xylose. Surprisingly, most of the glycans were trimmed extensively, indicating that a significant endoglycanase activity was present in maize endosperm. The specific antigen-binding function of the purified antibody was verified by surface plasmon resonance analysis, and in vitro cell assays demonstrated that the HIV-neutralizing properties of the maize-produced antibody were equivalent to or better than those of its Chinese hamster ovary cell-derived counterpart.

Protein transport within cereal endosperm cells is complicated by the abundance of endoplasmic reticulum (ER)-derived and vacuolar protein bodies. For wheat storage proteins, two major transport routes run from the ER to the vacuole, one bypassing and one passing through the Golgi. Proteins traveling along each route converge at the vacuole and form aggregates. To determine the impact of this trafficking system on the fate of recombinant proteins expressed in wheat endosperm, we used confocal and electron microscopy to investigate the fate of three recombinant proteins containing different targeting information. KDEL-tagged recombinant human serum albumin, which is retrieved to the ER lumen in leaf cells, was deposited in prolamin aggregates within the vacuole of endosperm cells, most likely following the bulk of endogenous glutenins. Recombinant fungal phytase, a glycoprotein designed for secretion, was delivered to the same compartment, with no trace of the molecule in the apoplast. Glycan analysis revealed that this protein had passed through the Golgi. The localization of human serum albumin and phytase was compared to that of recombinant legumin, which contains structural targeting information directing it to the vacuole. Uniquely, legumin accumulated in the globulin inclusion bodies at the periphery of the prolamin bodies, suggesting a different mode of transport and/or aggregation. Our results demonstrate that recombinant proteins are deposited in an unexpected pattern within wheat endosperm cells, probably because of the unique storage properties of this tissue. Our data also confirm that recombinant proteins are invaluable tools for the analysis of protein trafficking in cereals.

Barley (Hordeum vulgare) and wheat (Triticum aestivum) possess significant phytase activity in the mature grains. Maize (Zea mays) and rice (Oryza sativa) possess little or virtually no preformed phytase activity in the mature grain and depend fully on de novo synthesis during germination. Here, it is demonstrated that wheat, barley, maize, and rice all possess purple acid phosphatase (PAP) genes that, expressed in Pichia pastoris, give fully functional phytases (PAPhys) with very similar enzyme kinetics. Preformed wheat PAPhy was localized to the protein crystalloid of the aleurone vacuole. Phylogenetic analyses indicated that PAPhys possess four conserved domains unique to the PAPhys. In barley and wheat, the PAPhy genes can be grouped as PAPhy_a or PAPhy_b isogenes (barley, HvPAPhy_a, HvPAPhy_b1, and HvPAPhy_b2; wheat, TaPAPhy_a1, TaPAPhy_a2, TaPAPhy_b1, and TaPAPhy_b2). In rice and maize, only the b type (OsPAPhy_b and ZmPAPhy_b, respectively) were identified. HvPAPhy_a and HvPAPhy_b1/b2 share 86% and TaPAPhya1/a2 and TaPAPhyb1/b2 share up to 90% (TaPAPhy_a2 and TaPAPhy_b2) identical amino acid sequences. despite of this, PAPhy_a and PAPhy_b isogenes are differentially expressed during grain development and germination. In wheat, it was demonstrated that a and b isogene expression is driven by different promoters (approximately 31% identity). TaPAPhy_a/b promoter reporter gene expression in transgenic grains and peptide mapping of TaPAPhy purified from wheat bran and germinating grains confirmed that the PAPhy_a isogene set present in wheat/barley but not in rice/maize is the origin of high phytase activity in mature grains.