Russell-like bodies sequestering recombinant proteins

Most molecular farming applications have in common the need for high yields of recombinant protein which can be achieved by increasing expression and ensuring the stable accumulation of functional proteins in the most appropriate subcellular locations. Targeting strategies can be used to enhance the accumulation of a protein, and to control posttranslational modifications such as glycosylation. The success of such strategies is variable and appears to depend on the plant species and tissue, but also on the protein under study. In some cases, targeting strategies have to be adapted to specialized tissues to achieve the desired effect, e.g. the deposition in native storage organelles such as protein bodies or protein storage vacuoles of seeds. In other cases, aberrant localization can be attributed to inherent properties of the protein. For example, the unintended accumulation in ER-derived compartments may arise from covalent interactions with endogenous ER-resident proteins. Thus the partial localization of a recombinant antibody in the periphery of prolamin storage organelles was observed, most likely due an interaction between the heavy chain of the antibody and gamma-zein, an endogenous storage protein of maize. Even in the absence of endogenous ER-derived storage organelles, recombinant proteins are occasionally sequestered in ER-derived bodies. In Arabidopsis seeds we have identified an insoluble fraction of recombinant murine interleukin-10 and localized this fraction within ER-derived protein accretions. Electron tomography demonstrates that they are detached from the ER and appear very similar to Russell bodies, which occur in connection with human ER storage diseases. Another example was observed in antibody- producing tobacco leaves, where an insoluble proportion of the recombinant protein was detected in ER- derived vesicular structures. We speculate that the content of these ER-bodies corresponds to a transport- incompetent fraction of the recombinant protein, and that, similar to mammalian plasma cells, plant cells are able to form Russell bodies as a self-protection mechanism to avoid blockage of the secretory pathway.