Strategies for Optimizing Recombinant Protein Synthesis in Plant Cells: Classical Approaches and New Directions

At present, pharmacologically significant proteins are synthesized in different expression systems, from bacterial to mammalian and insect cell cultures. The plant expression systems (especially suspension cell culture) combine the simplicity and low cost of bacterial systems with the ability to perform eukaryotic-type posttranslational protein modifications. A low (compared with bacterial systems) yield of the target recombinant protein is one of the shortcomings of the plant expression systems. In this review, methods, developed over the past two decades, to increase the level of recombinant gene expression and methods to prevent silencing, caused by a random insertion of the target gene into heterochromatin region, are considered. The emergence of CRISPR/Cas technologies led to the creation of a new approach to increase the gene expression level, directional insertion of “pharmaceutical” protein genes in specific, knowingly transcriptionally active genome regions. The plant cell housekeeping gene loci, actively expressed throughout the interphase, are these regions. The organization of some housekeeping genes, most promising for transferring recombinant protein genes in their loci, is considered in detail.