Some human recombinant proteins are produced in mammalian cell culture systems and other in the milk of transgenic animals. In late April, a California biotech company, Planet Biotechnology, Inc., announced that they had successfully completed the first human clinical trial of a recombinant therapeutic protein produced in transgenic plants. Human clinical trials involving two other plant-synthesized pharmaceutical proteins are also underway. Thus, they are very safe sources of recombinant protein. Plant biotechnologists at the Fralin Biotechnology Center are using transgenic tobacco to produce a variety of complex human proteins.
Tobacco is the easiest plant to genetically engineer.
Carole Cramer and Elizabeth Grabau and researchers in their laboratories at the Fralin Center have recently introduced genes for two human therapeutic proteins into tobacco. Assembly of this protein is extremely complicated, requiring more than a dozen additional processing steps after the initial peptide backbone is made. Thus, it serves to test the limits of plants for bioproduction of human proteins. Preliminary results suggest that tobacco can perform many of the steps required to make this complex protein, although the amounts produced are quite low. However, plants cannot gamma-carboxylate the protein, a modification that allows Protein C to associate with the correct site in blood vessels during clot formation.