Large-scale protein expression for proteome research.

Access to pure and soluble recombinant proteins is essential for numerous applications in proteome research, such as the production of antibodies, structural characterization of proteins, and protein microarrays. Through the German cDNA Consortium we have access to more than 1500 ORFs encoding uncharacterized proteins. Preparing a large number of recombinant proteins calls for the careful refinement and re-evaluation of protein purification tools. The expression and purification strategy should result in mg quantities of protein that can be employed in microarray-based assays. In addition, the experimental set-up should be robust enough to allow both automated protein expression screening and the production of the proteins on a mg scale. These requirements are best fulfilled by a bacterial expression system such as Escherichia coli. To develop an efficient expression strategy, 75 different ORFs were transferred into suitable expression vectors using the Gateway cloning system. Four different fusion tags (E. coli transcription-termination anti-termination factor (NusA), hexahistidine tag (6xHis), maltose binding protein (MBP) and GST) were analyzed for their effect on yield of induced fusion protein and its solubility, as determined at two different induction temperatures. Affinity-purified fusion proteins were confirmed by MALDI-TOF MS.

Structure and assembly of the RNA binding domain of bluetongue virus non-structural protein 2.

Bluetongue virus non-structural protein 2 belongs to a class of highly conserved proteins found in orbiviruses of the Reoviridae family. Non-structural protein 2 forms large multimeric complexes and localizes to cytoplasmic inclusions in infected cells. It is able to bind single-stranded RNA non-specifically, and it has been suggested that the protein is involved in the selection and condensation of the Bluetongue virus RNA segments prior to genome encapsidation. We have determined the x-ray structure of the N-terminal domain (sufficient for the RNA binding ability of non-structural protein 2) to 2.4 A resolution using anomalous scattering methods. Crystals of this apparently insoluble domain were obtained by in situ proteolysis of a soluble construct. The asymmetric unit shows two monomers related by non-crystallographic symmetry, with each monomer folded as a beta sandwich with a unique topology. The crystal structure reveals extensive monomer-monomer interactions, which explain the ability of the protein to self-assemble into large homomultimeric complexes. Of the entire surface area of the monomer, one-third is used to create the interfaces of the curved multimeric assembly observed in the x-ray structure. The structure reported here shows how the N-terminal domain would be able to bind single-stranded RNA non-specifically protecting the bound regions in a heterogeneous multimeric but not polymeric complex.