Escherichia coli is the most widely used host for producing membrane proteins. Thus far, to study the consequences of membrane protein overexpression in E. coli, we have focussed on prokaryotic membrane proteins as overexpression targets. Their overexpression results in the saturation of the Sec translocon, which is a protein-conducting channel in the cytoplasmic membrane that mediates both protein translocation and insertion. Saturation of the Sec translocon leads to (i) protein misfolding/aggregation in the cytoplasm, (ii) impaired respiration, and (iii) activation of the Arc response, which leads to inefficient ATP production and the formation of acetate. The overexpression yields of eukaryotic membrane proteins in E. coli are usually much lower than those of prokaryotic ones. This may be due to differences between the consequences of the overexpression of prokaryotic and eukaryotic membrane proteins in E. coli. Therefore, we have now also studied in detail how the overexpression of a eukaryotic membrane protein, the human KDEL receptor, affects E. coli. Surprisingly, the consequences of the overexpression of a prokaryotic and a eukaryotic membrane protein are very similar. Strain engineering and likely also protein engineering can be used to remedy the saturation of the Sec translocon upon overexpression of both prokaryotic and eukaryotic membrane proteins in E. coli.
