Fig. 2

Proposed scheme of the evolution of MRs and GPCRs. Only three helices of MRs and GPCRs are depicted. Helix 3 (helix C of MRs) is colored blue, helix 6 (helix F of MRs) is colored brown in the “closed” conformation and orange in the “open” conformation, helix 7 (helix G of MRs) is colored green. The scheme illustrates the proposed order of appearance of functions in evolution as a series of gains and losses. The losses of retinal could lead to numerous bacterial 7TM receptors [18, 19] (not shown on the scheme) and to the Na⁺-binding precursor of most GPCRs. In the course of further evolution, many GPCRs seemingly lost the ability to bind Na⁺ (not shown on the scheme). The reacquisition of the retinal by a class A GPCR yielded a visual rhodopsin. See the text for further details