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. 2008 Oct 22;75(1):1–12. doi: 10.1124/mol.108.051938

Fig. 5.

Fig. 5.

Comparison of β1-adrenergic receptor with rhodopsin, β2-adrenergic receptor, and A2A adenosine receptor structures. The turkey β1-adrenergic receptor was mutated to facilitate its crystallization. The mutated receptor evidenced enhanced thermostability and preferentially existed in an antagonist-binding state. Stretches of amino acid sequence were deleted from the N-terminal region (i.e., the loop connecting helices V and VI and the C-terminal region). A, the β1-adrenergic receptor is colored by helix: helix I (blue), helix II (cyan), helix III (violet), helix IV (red), helix V (orange), helix VI (yellow), helix VII (green), and helix VIII (magenta). Thermostabilizing mutations (R68S, M90V, Y227A, A282L (not resolved in the shown structure), F327A, and F338M) and those that either increased functional expression (C116L) or eliminated a palmitoylation site (C358A) are shown as balls/sticks. B, structure of the β1-adrenergic receptor (colored by helix) is shown with bovine rhodopsin (green ribbon) to illustrate key structural differences. Despite some differences in the transmembrane helices, the main distinction is in the organization of the cytoplasmic (C-) loops. The C-II loop in the β1-adrenergic receptor structure forms a short α-helix, whereas this loop is more extended in rhodopsin. In addition, rhodopsin has a native C-III loop that is absent from the β1-adrenergic receptor structure as a result of deletions needed for crystallization. C, the structure of the β1-adrenergic receptor (colored by helix) is displayed with the human β2-adrenergic receptor (green ribbon) to show the key structural differences. In this case, the C-II loops of both the β1- and β2-adrenergic receptors are similar, but the α-helical conformation of the β1-adrenergic receptor cannot be accommodated within the two crystallized structures of the β2-adrenergic receptor because of lattice contacts with adjacent molecules. D, the structure of the β1-adrenergic receptor (colored by helix) is displayed with the human A2A adenosine receptor (green ribbon) to show key structural differences. The main difference between these two structures lies in the distinct folding of the extracellular loops, with the adenosine receptor adopting a more open surface for ligand binding as a result of this folding.