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. Author manuscript; available in PMC: 2006 Dec 28.
Published in final edited form as: Biochemistry. 2004 Dec 21;43(50):15643–15656. doi: 10.1021/bi047907k

Figure 1.

Figure 1

Evolving molecular concept of a G protein-coupled receptor. The molecular understanding of GPCRs during different periods in the history of the field is represented for GPCRs acting via AC and for the visual pigment rhodopsin. In the AC branch, agonists (yellow) were initially thought to bind to some part of the cell or tissue (A). The receptor was next thought to be a part of AC (violet) itself (B). It was then discovered that the receptor (blue) was a distinct molecular entity (C). The receptor has been conceptualized as a monomeric protein until relatively recently. Our current understanding is that the receptor exists as an oligomer (D). In the rhodopsin branch, the initial detection of rhodopsin (red) came from its characteristic reddish-purple color in the rod outer segments (E). The color was then determined to derive from a chromophore conjugated to a protein. Early biophysical methods suggested that rhodopsin was monomeric (F). More recent studies using atomic force microscopy point to an oligomeric arrangement of rhodopsin (G).