Cell-free systems: Functional modules for synthetic and chemical biology

The main goal of cell-free protein synthesis is to produce correctly folded and functional proteins in reasonable amounts for further downstream applications. Especially for eukaryotic proteins, functionality is often directly linked to the presence of posttranslational modifications. Thus, it is of highest interest to develop novel cell-free expression systems that enable the synthesis of posttranslationally modified proteins. Here we present recent advances for the synthesis of glycoproteins, proteins containing disulfide bridges, membrane proteins, and fluorescently labeled proteins. The basis for the expression of these difficult-to-express target proteins is a translationally active cell extract which can be prepared from eukaryotic cell lines such as Spodoptera frugiperda 21 (Sf21) and Chinese hamster ovary (CHO) cells. Due to a very mild lysate preparation procedure, microsomal vesicles derived from the endoplasmic reticulum (ER) can be maintained in the eukaryotic lysate. These vesicles are translocationally active and serve as functional modules facilitating protein translocation and enrichment as well as posttranslational modification of de novo synthesized proteins. In particular, for the synthesis of membrane proteins microsomal vesicles are the essential prerequisite for the insertion of the desired protein into a biologically active membrane scaffold providing a natural environment. We anticipate that the use of such translationally active eukaryotic cell lysates containing translocationally active vesicles may solve a large number of problems still persistent when expressing eukaryotic proteins in vitro.