Recombinant protein expression has been a central tool in basic and applied research for the past 20 years. Despite fairly good methods developed for both prokaryotic and eukaryotic expression of many soluble cytoplasmic proteins, integral transmembrane proteins have presented major challenges for obtaining high yields in every expression system. The low expression levels are mainly related to the topologies of these proteins. GPCRs, in particular, with their seven transmembrane (7TM) domains, have generated disappointingly low expression levels due to the requirements for folding and transport and the cellular toxicity related to heterogenous expression in membranes. Much effort has been dedicated to improving expression, and the efforts have evolved along the three main paths described below.
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