To fuse or not to fuse: what is your purpose?

Abstract

Since the dawn of time, or at least the dawn of recombinant DNA technology (which for many of today's scientists is the same thing), investigators have been cloning and expressing heterologous proteins in a variety of different cells for a variety of different reasons. These range from cell biological studies looking at protein-protein interactions, post-translational modifications, and regulation, to laboratory-scale production in support of biochemical, biophysical, and structural studies, to large scale production of potential biotherapeutics. In parallel, fusion-tag technology has grown-up to facilitate microscale purification (pull-downs), protein visualization (epitope tags), enhanced expression and solubility (protein partners, e.g., GST, MBP, TRX, and SUMO), and generic purification (e.g., His-tags, streptag, and FLAG™-tag). Frequently, these latter two goals are combined in a single fusion partner. In this review, we examine the most commonly used fusion methodologies from the perspective of the ultimate use of the tagged protein. That is, what are the most commonly used fusion partners for pull-downs, for structural studies, for production of active proteins, or for large-scale purification? What are the advantages and limitations of each? This review is not meant to be exhaustive and the approach undoubtedly reflects the experiences and interests of the authors. For the sake of brevity, we have largely ignored epitope tags although they receive wide use in cell biology for immunopreciptation.

Keywords: GST; His6; MBP; SUMO; TRX; fusion tag; protein expression.

Figure 1

Structures of two proteins expressed as SUMO-fusions in E. coli. Panel A shows the structure of a peptidyl-prolylcis-trans isomerase from Burkholderiapsuedomallei bound to 8-deethyl-8-[but-3-enyl]-ascomycin (solid surface). The protein was crystallized and the structure determined to 1.9Å resolution with SUMO still attached (labeled). (PDB ID 3UQB, Fox III, Abendroth, Staker, and Stewart, Seattle Structural Genomics Center for Infectious Disease, deposited Nov. 2011). Panel B shows the structure of the human Ub E3 ligase Parkin. SUMO was enzymatically removed prior to crystallization. The structure was determined at 2.0Å resolution (PDB ID 4I1H, Riley et al.42).