High-yield production of a human therapeutic protein in tobacco chloroplasts

Transgenic plants have become attractive systems for production of human th erapeutic proteins because of the reduced risk of mammalian viral contamina nts, the ability to do targe scale-up at low cost, and the tow maintenance requirements. Here we report a feasibility study for production of a human therapeutic protein through transplastomic transformation technology, which has the additional advantage of increased biological containment by appare nt elimination of the transmission of transgenes through pollen. We show th at chloroplasts can express a secretory protein, human somatotropin, in a s oluble, biologically active, disulfide-bonded form. High concentrations of recombinant protein accumulation are observed (>7% total soluble protein), more than 300-fold higher than a similar gene expressed using a nuclear tra nsgenic approach. The plastid-expressed somatotropin is nearly devoid of co mplex posttranslational modifications, effectively increasing the amount of usable recombinant protein. We also describe approaches to obtain a somato tropin with a non-methionine N terminus, similar to the native human protei n. The results indicate that chloroplasts are a highly efficient vehicle fo r the potential production of pharmaceutical proteins in plants.