As indicated in the introduction, no single expression system is ideal for all GPCRs. Table 8.1 summarizes the different expression systems and lists the advantages, disadvantages, and examples of GPCRs expressed. Generally, the cell-free expression system has provided only poor expression of GPCRs; the only examples to date are unpublished studies of human p2 AR. However, the simplicity and speed of cellfree translation and the flexibility that allows evaluation of expression from PCR products quickly and directly makes this an attractive approach. The system requires further development and the scale-up costs continue to be of concern. E. coli expression definitely has advantages in that it is simple and rapid to use and scale-up costs are reasonable. The possibility of isotope labeling for NMR and x-ray crystallography studies has some value. GPCRs have been difficult to express at
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