Optimizing Recombinant Protein Expression in Soybean

Abstract

The production of biopharmaceuticals represents the fastest growing segment in the pharmaceutical industry. Currently, the majority of biopharmaceuticals are produced in recombinant microbe expression systems. However, microbes have certain limitations in the classes of proteins that can be economically produced, and in the post-translational processing that can be achieved. To solve this problem, insect and mammalian cell cultures have also been utilized for eukaryotic protein production (Lubiniecki and Lupker, 1994; Kost and Condreay, 1999). However, a significant problem with these systems, in particular cell cultures, is that production costs are prohibitively high for many proteins. Therefore, an increased demand for biopharmaceuticals will require improved and cost effective manufacturing practices as well as practical transportation and delivery methods for a global community. Over the past two decades, there has been substantial research on the expression of heterologous proteins in plants as a means to produce biopharmaceuticals that can meet current and future global needs. Several excellent review articles describe these recent advances (Streatfield 2007; Karg and Kallio 2009; Franconi et al., 2010). While numerous plant systems have been shown to support expression of heterologous proteins, we believe that soybean is perhaps the most practical of these systems. Soybean is often overlooked as an expression system in part due to a technically challenging, lengthy, and costly transformation process. However, there is enormous potential for the use of transgenic soybean as a factory for the economic production of pharmaceutical proteins. The soybean system has distinct advantages which offer a practical alternative to existing expression systems. First, while soybeans are traditionally thought of as high oil seeds, they are also high protein seeds. Soybeans contain nearly 40% protein by dry mass, and represent one of the richest natural sources of protein known. Given this high protein content, it is therefore possible to express in excess of a milligram of transgenic protein in a single soybean seed. There are few, if any, host systems (plant or non-plant) that can produce such levels of foreign protein based on weight. Second, soybean is a relatively easy and inexpensive plant to grow. Therefore, the production of biopharmaceuticals in soybeans is extremely cost-effective. For example, given the high protein content of seeds and a foreign protein expression level of 1%-4% of total soluble protein (TSP), large greenhouses could produce millions of doses of a vaccine for pennies per dose. Also contributing to the low cost of heterologous protein expression in soy are the well-known and established procedures for processing soybeans