Currently, a high-resolution structure of an entire GPCR is available only for bovine rhodop-sin (Palczewski et al. 2000), because of the difficulties inherent in producing, purifying, and crystallizing other GPCRs. As shown in Fig. 1.3, the mostly a-helical TMDs are arranged in a closely packed bundle forming the transmembrane receptor core. The N terminus of the poly-peptide is located in the extracellular space, whereas the C terminus shows an intracellular localization. The seven transmembrane helices are connected by six alternating intracellular (i1-i3) and extracellular (e1-e3) loops. As predicted by nuclear magnetic resonance (NMR), in site-directed spin-labelling and mutagenesis studies most TMDs show a cytosolic a-helical extension (Altenbach et al. 1996; Farahbakhsh et al. 1995; Schulz et al. 2000b; Yeagle et al. 1997). The current rhodopsin model still lacks detailed structural information of some parts of the connecting loops and the C terminus.
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