KDEL sequence

The foregoing paragraphs demonstrate that a variety of routes are involved in assembly of the proteins of the ER membranes a similar situation probably holds for other membranes (eg, the mitochondrial membranes and the plasma membrane). Precise targeting sequences have been identified in some instances (eg, KDEL sequences). 

Several molecular strategies have been successful in increasing foreign protein production in cultured plant cells. These include using promoters for inducible expression [10, 12, 29], optimizing codon usage [30] and adding the KDEL sequence to ensure protein retention in the endoplasmic reticulum [31]. Application of different... 

Proteins produced in plant cells can remain within the cell or are secreted into the apoplast via the bulk transport (secretory) pathway. In whole plants, because levels of protein accumulated intracellularly, e. g. using the KDEL sequence to ensure retention in the endoplasmic reticulum, are often higher than when the product is secreted [58], foreign proteins are generally not directed for secretion. However, as protein purification from plant biomass is potentially much more difficult and expensive than protein recovery from culture medium, protein secretion is considered an advantage in tissue culture systems. For economic harvesting from the medium, the protein should be stable once secreted and should accumulate to high levels in the extracellular environment. 

Similar results were recently obtained with a human monoclonal antibody, with KDEL sequences fused to the C-termini of both heavy chains, expressed in tobacco [33]. As observed for the invertase-HDEL fusion, about 90% of the N-linked glycans on this antibody were of the high-mannose type, with 6-9 mannose residues, while a fraction contained the immunogenic P(l,2)-xylose glyco-epitope (Fig. 15.6). However, this antibody was not a(l,3)-fucosylated, a glycan modification occurring in the trans Golgi [34]. 

Some reports indicate that ER-targeting, using a C-terminal KDEL sequence, is necessary to achieve high protein levels [36-39]. It has been suggested that proteins... 

The bacterial RNase, barnase, linked to Pseudomonas aeruginosa exotoxin A or to a non-toxic deletion mutant of the toxin lacking the enzymatic domain, was found to be toxic to cells to a greater extent than either component alone (Prior et al., 1991 1992). A C-terminal KDEL sequence was required for toxicity. Cells resistant to the intracellular action of the toxin were also sensitive to the fusion protein. This... 

Schouten A, Roosien ), van Engelen FA, et al. The C-terminal KDEL sequence increases the expression level of a single-chain antibody designed to be targeted to both the cytosol and the secretory pathway in transgenic tobacco. Plant Mol. Biol., 1996 30(4) 781-793. 

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