A prerequisite for structural genomics and related projects is to standardize the process of gene overexpression and protein solubility screening to enable automation for higher throughput. We have tested a methodology to rapidly subclone a large number of human genes and screen these for expression and protein solubility in Escherichia coli. The methodology, which can be partly automated, was used to compare the effect of six different N-terminal fusion proteins and an N-terminal 6*His tag. As a realistic test set we selected 32 potentially interesting human proteins with unknown structures and sizes suitable for NMR studies. The genes were transferred from cDNA to expression vectors using subcloning by recombination. The subcloning yield was 100% for 27 (of 32) genes for which a PCR fragment of correct size could be obtained. Of these, 26 genes (96%) could be overexpressed at detectable levels and 23 (85%) are detected in the soluble fraction with at least one fusion tag. We find large differences in the effects of fusion protein or tag on expression and solubility. In short, four of seven fusions perform very well, and much better than the 6*His tag, but individual differences motivate the inclusion of several fusions in expression and solubility screening. We also conclude that our methodology and expression vectors can be used for screening of genes for structural studies, and that it should be possible to obtain a large fraction of all NMR-sized and nonmembrane human proteins as soluble fusion proteins in E. coli.

We have studied the effect of solubilising N-terminal fusion proteins on the yield of target protein after removal of the fusion partner and subsequent purification using immobilised metal ion affinity chromatography. We compared the yield of 45 human proteins produced from four different expression vectors: three having an N-terminal solubilising fusion protein (the GB1-domain, thioredoxin, or glutathione S-transferase) followed by a protease cleavage site and a His tag, and one vector having only an N-terminal His tag. We have previously observed a positive effect on solubility for proteins produced as fusion proteins compared to proteins produced with only a His tag in Escherichia coli. We find this effect to be less pronounced when we compare the yields of purified target protein after removal of the solubilising fusion although large target-dependent variations are seen. On average, the GB1+His fusion gives significantly higher final yields of protein than the thioredoxin+His fusion or the His tag, whereas GST+His gives lower yields. We also note a strong correlation between solubility and target protein size, and a correlation between solubility and the presence of peptide fragments that are predicted to be natively disordered.