In plants, two major strategies have been
developed to control the glycosylation of recombinant proteins: subcellular targeting to prevent
the addition of undesirable sugar residues (40, 48, 106) and glycoengineering to prevent the
addition of plant glycans and even replace them with human-like counterparts (5).
Targeting is arguably the most straightforward approach because it can be controlled at the
level of the transgene construct by adding particular protein tags (see sidebar Protein Targeting
in Molecular Pharming along with Figure 1). Targeting recombinant proteins to the endoplasmic
reticulum using the tetrapeptide tag KDEL and similar derivatives prevents the addition of
complex-type glycan structures by avoiding the Golgi apparatus, thus ensuring that all proteins are
expressed with high-mannose glycans common to all eukaryotes. Another key example of glycan
control through targeting is directing proteins to the vacuole, a strategy that Protalix Biotherapeutics
uses for the production of taliglucerase alfa. The protein is manufactured in carrot cells and is
targeted to the vacuole because the resultingmannose-tipped paucimannosidicN-glycans promote
interactions with macrophages. When the enzyme is produced in mammalian cells, the glycans
have terminal sialic acid residues that prevent uptake by macrophages, so they need to be trimmed
enzymatically in vitro, which adds to the processing costs.