Stability of Recombinant Proteins in Plants

In almost all living organisms, proteolytic enzymes are involved in a variety of cellular functions not only associated with the control of specific endogenous metabolic reactions, but also with the degradation of abnormal or exogenous (“foreign”) proteins (1 ). Despite the fundamental importance of proteases in cells, studies on these enzymes, for those involved in gene-expression technology, are devised to develop a means of avoiding or minimizing degradation of the recombinant proteins to be produced. Some proteins are rapidly degraded either during or shortly after their synthesis, and others are lost during their extraction from cells or tissues. Although general strategies have been proposed to minimize extraction-related hydrolytic processes in microbial, animal, and plant systems (2 ,3 ), in vivo proteolysis still represents one of the most significant barriers to recombinant gene expression in any organism (4 ). Some exo- and endoproteases from Escherichia coli (5 ) and yeast (6 ), notably, represent harmful molecules for recombinant (“abnormal”) proteins expressed in these systems, and strategies have been developed to counteract potential or actual hydrolytic processes (7 ,8 ). Concurrently, the posttranslational ubiquitination of foreign proteins recognized as abnormal in yeast and other eukaryotic cells may lead to their rapid degradation by the multicatalytic complex proteasome via the ubiquitin-mediated proteolysis pathway (9 ), rendering necessary the study of ubiquitin conjugates when planning to express recombinant proteins in the cytoplasm of yeast or animal cells (10 ).