This document summarizes key developments in structural studies of G protein-coupled receptors (GPCRs). It notes that the first membrane protein structure characterized was bacteriorhodopsin in 1975, which contains seven alpha helices and transports protons across membranes. The amino acid sequence of bacteriorhodopsin was determined in 1978-1979. In 1983, the amino acid sequence of bovine rhodopsin, the first GPCR, was determined. Also in 1983, the cDNA and gene for bovine rhodopsin were cloned. In 1986, the beta-2 adrenoceptor was cloned, representing the first non-sensory GPCR. Finally, in 1988 an "orphan" GPCR was identified that was later

1. Structural studies of GPCRs
Tony Harmar
University of Edinburgh

2. Structure of the first membrane
protein
The first membrane protein that was structurally characterised, by RichardHenderson
(left) and Nigel Unwin (right) in 1975, was bacteriorhodopsin, a light-harvesting
membrane protein from the archaean Halobacterium halobium that acts as a lightdriven proton pump and is the only protein constituent of the purple membrane, a
two-dimensional crystal lattice naturally present as part of the plasma membrane of
the bacterium.

3. Bacteriorhodopsin -the first 7TM protein
Using electron diffraction, Henderson
& Unwin showed that the protein
contains seven alpha-helices that
enclose
an
all-trans retinal
chromophore
that undergoes an
isomerisation process upon light
absorption that results in the
translocation of a proton from the
cytoplasmic side to the extracellular
side
of
the
membrane.They
commented, almost prophetically“The
purple membrane thus seems to provide
a simple example of an 'intrinsic'
membrane protein, a class of structure
to which many molecular pumps and
channels must belong. We would not be
surprised if the simple arrangement of
helices found here also occurs in some of
these other intrinsic membrane proteins”

4. Amino acid sequence of bacteriorhodopsin
The amino acid sequence of bacteriorhodopsin was first published, almost
simultaneously, by the groups of Yuri Ovchinnikov in 1978 and Nobel Laureate Har
Gobind Khorana in1979. Each study represented a tour de force of protein
chemistry.

5. The first depiction of the 7TM topology of bacteriorhodopsin, from Ovchinnikov.

6. Amino acid sequence of the first GPCR
1983: Complete amino acid sequence of bovine rhodopsin
determined by the laboratories of Ovchinnikov (Russia) and
Hargrave (USA.

7. First cDNA and gene sequences
1983:cloning of cDNA and gene encoding bovine rhodopsin by Jeremy Nathans (left)
and David Hogness (right). Using a “citation classic” technique for homology
screening devised by Hogness, they later identified three related visual pigment

8. Amino acid sequence of the first GPCR
1983: Complete amino acid sequence of bovine rhodopsin
determined by the laboratories of Ovchinnikov (Russia) and
Hargrave (USA).

9. 1986:Cloning of β2 adrenoceptor – the first nonsensory GPCR

10. 1986:Cloning of β2 adrenoceptor – the first nonsensory GPCR

11. Cloning the β 2 adrenoceptor
•
Receptor from hamster lung solubilised in detergent and purified by
affinity chromatography on alprenolol-sepharose
•
Progress of purification monitored by binding of [ 125 I]-cyanopindolol
•
Attempts to obtain amino acid sequence of the intact protein failed
•
Purified protein was subjected to chemical cleavage with cyanogen
bromide (CNBr), which cleaves proteins after every methionine
residue
•
Cyanogen bromide fragments were purified by HPLC and
sequenced

12. Cloning the β 2 adrenoceptor

13. Cloning the β 2 adrenoceptor

14. 1988:the first "orphan" GPCR
Nature 335: 358-360 (1988)
G-21 was a genomic clone with homology to the β2AR: at
first its endogenous ligand was unknown, i.e. it encoded
an “orphan” GPCR
Nature 335: 358-360 (1988)

15. 1988:5-HT1A receptor “deorphanised”
When expressed in cell lines and studied in a radioligand
binding assay, G-21 exhibited the pharmacology of the 5HT1A receptor