It is generally believed that the external residues on the trans membrane portions of the heptahelical G-protein coupled recep tors do not have any functional role other than to provide a hydrophobic environment to anchor the receptor in the membrane.
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Gouldson, P. R., Bywater, R. P., & Reynolds, C. A. (1997). Correlated mutations amongst the external residues of G-protein coupled receptors. Biochemical Society Transactions, 25(3), [529S]. https://doi.org/10.1042/bst025529s