The Molecular History of Eukaryotic Life  Seven Transmembrane Segment Receptors


David Nelson  Dec. 14, 2000

	Seven transmembrane segment receptors are eukaryotic proteins.  Animals are 
especially fond of these receptors.  The genome of the nematode worm is now done 
and it contains an estimated 1049 G-protein coupled receptors(1). These may be chemosensory
receptors.  Almost all of these are orphan receptors without known ligands.  In fact, 
only one ligand has been linked to its chemoreceptor in C. elegans and that is diacetyl. 
This ligand binds to the odr-10 gene product that normally mediates attraction to 
diacetyl.  However, if the gene is expressed in another neuron that usually mediates 
repulsion from a substance then the worm is repelled by diacetyl(2).  This shows that the 
receptor only triggers a pathway and the response is dependent on which neuron the gene 
is expressed in.  The worms genome has 19,099 genes total, so 1049 odor receptors is 
about 5.5% of the worms genome dedicated to chemosensory receptors.  Current estimates 
suggest that 300 of these genes are pseudogenes and about 200 are not expressed in 
neurons, so they are not likely to be chemoreceptors, but that still leaves 500 
serpentine receptor genes that are probable functioning chemoreceptors.  This is the 
largest family of genes in the worms genome.(additional references 3-5)

	Yeast have only 3 GPCRs: STE2(6), STE3(7) and GPR1(8).  These are receptors for 
pheromone (STE2 and STE3) and a receptor that is induced during starvation. The ligand is 
probably glucose which causes a cAMP spike in starved yeast cells.  The trimeric G protein 
alpha subunits for these receptors are named GPA1 (for STE2 and STE3) and GPA2 (for GPR1).  
There only appear to be two G protein alpha subunits in yeast, and two in the fission yeast 
S. pombe.  Based on these results, the fungi have not exploited these useful receptors very 
much.  They seem to be used for mating purposes and for sensing the concentration of 
glucose in the environment. 

	Dictyostelium branches on the eukaryotic lineage within the crown group of 
eukaryotes before animals and fungi, but sometime after plants.  The signal 
transduction mechanisms present in Dictyostelium should be present in animals and fungi, 
but not necessarily in plants There are genome projects being done now that include 
Dictyostelium and Giardia, one of the most distant eukaryotic organisms known.  These 
projects should aid greatly in understanding the earliest events in eukaryotic evolution, 
including development of signal transduction pathways.  But the genomes are not done yet, 
so the story is fragmentary so far.  

	 Dictyostelium also uses GPCRs for cell to cell communication in the developmental 
program of aggregation and sporulation.  Searches for seven transmembrane receptors and G-
proteins in Dictyostelium show that there are four cAMP receptors called cAR1 to cAR4.  
These are able to sense cAMP secreted by the Dictyostelium cells as they aggregate to form 
a stalk.  These four receptors are expressed at different times during the developmental 
program of the organism.  However, one report (9) refers to deletion of the single G-
protein beta subunit.  There appears to be only one beta subunit that is shared by all four 
receptors.  The situation is not the same for the alpha subunits, since there is sequence 
data for at least eight distinct alpha subunits.  It is not clear if there are eight 
different seven transmembrane receptors for all these alpha subunits, or if the four cAR 
receptors use more than one, possibly at different times.  

	One interesting finding concerning these serpentine receptors in Dictyostelium is 
that some responses are not dependent on the beta subunit.  These seem to be G-protein 
independent responses that are linked to the receptor.  One of these is calcium influx, 
though the amount of calcium influx is less than with the intact G-proteins.  Here we may 
be seeing a remnant of early evolution of the serpentine receptor/G-protein couple.  At 
one point in the past there had to be a receptor without G-proteins like the 
bacteriorhodopsin light driven pump discussed below.  If the cAMP receptors can carry out 
some of the same functions, but at a reduced efficiency, then this might be a vestige of 
the original receptor function without G-proteins.  A related issue is activation of 
receptors without ligand bound.  The Nov. 98 TIBS has an article on Agonist Independent 
Regulation of Consitutively Active G-protein Coupled Receptors.  Constitutive signaling 
without ligand bound has been documented now for several GPCRs.

	The seven transmembrane receptors have a remarkable similarity to 
bacteriorhodopsin and halorhodopsin.  These are a light driven proton pump and a light 
driven chloride pump from the archaebacteria Halobacterium salinarium (previously named 
halobium).  The seven transmembrane segments enclose a retinal attached to a lysine 
residue of the last helix by a schiffs base. This retinal undergoes an all-trans to 13-
cis isomerization when it absorbs a photon.  The conformational change is transmitted to 
the protein and it is used to pump protons or chloride and form a gradient.  In 
vertebrate eyes, the protein rhodopsin is also a seven transmembrane segment protein with 
retinal attached to a lysine also in the last helix.  In this protein the conformational 
change is 11-cis to all trans and it activates a heterotrimeric G protein that activates 
a cGMP phosphodiesterase which reduces cGMP concentrations.  The result is closing of 
sodium ion channels and electrical stimulation of a nerve. 

	The common ancestry between bacteriorhodopsin (BR) and eukaryotic receptors 
might be considered unconvincing if the retinal attachment and seven transmembrane 
segments were the only evidence in its favor.  However, the relationship is supported based 
on more detailed evidence.  BR has two close relatives in Halobacterium salinarium that 
are light driven sensory receptors SRI and SRII.  These also contain retinal, but they 
are tightly coupled to two transducer proteins HtrI for SRI and HtrII for SRII.  If the 
sensory rhodopsins are expressed without the transducers they function as proton pumps.  
The tight association between the sensory rhodopsins and the transducer prevents proton 
pumping. Instead, the conformational change caused by light is used to stimulate the 
transducers. These proteins are histidine kinases that control a phosphorylation cascade 
that regulates the flagellar motor as in E. coli chemotaxis.  Different wavelengths of 
light can either attract or repel the bacteria.  In this way, the transducer proteins are 
similar to two component regulators described below.  

	The most convincing evidence for common ancestry of bacterial and eukaryotic 
rhodopsins is in the mechanism of signal transduction.  Both bacterial and vertebrate 
rhodopsins form a protonated schiffs base on the G helix where retinal is attached.  Both 
form a salt bridge between this protonated schiffs base and a conserved carboxyl group on 
helix C.  Absorbance of a photon breaks the salt bridge and shifts the proton to the 
carboxyl group.  In BR the net result is opening of a half channel facing the cytoplasm 
and transport of 2 H+ across the membrane. In sensory rhodopsin, the signal is 
transmitted to the Htr transducer protein and on into the cell by phosphorylation.  In 
eukaryotes, the trimeric G proteins separate into alpha and (beta, gamma) subunits.  

	There is no sequence similarity between bacteriorhodopsin and vertebrate rhodopsin.  
However, the overall structure and the placement of retinal are two pieces of evidence that 
suggest a common ancestor.  If this is correct, then the archaebacterial ancestor of 
eukaryotes may have contributed a seven transmembrane precursor of modern seven 
transmembrane receptor proteins.  J.L. Spudich, an expert in these proteins does not 
believe that archaeal and visual rhodopsins are descended from a common ancestor.  He 
argues that the 22 amino acids that make up the binding pocket for retinal are not 
conserved in the vertebrate rhodopsins and the retinal structure is significantly 
different.  High resolution crystal structures are available now for bacteriorhodopsin 
trapped in various stages of the transport cycle.(10); halorhodopsin (11) and bovine 
rhodopsin (12)  The interpretation of the structures suggests that the seven 
helices are arranged in different ways.  The consensus seems to be that the vertebrate 
rhodopsins (and therefore all the GPCRs) are not related to the archaeal rhodopsins.

	Therefore, the origin of seven transmembrane segment receptors cannot be traced back to 
light sensitive receptors in archaebacteria.  The similarity between these proteins seems 
to be a spectacular case of convergent evolution.  A new development this year is the 
discovery of a fungal protein called nop-1 from Neurospora crassa that has been 
overexpressed in Pichia pastoris.  The protein binds retinal as a schiff base and probably 
acts as a rhodopsin like molecule in N. crassa (13)  There are other related proteins in N. 
crassa that do not have the lysine residue that makes the schiff base, so these may have 
lost the ability to use retinal as a stimulus, but this is expected in the evolution of the 
seven transmembrane receptors that do not contain retinal and are driven by ligand binding. 
(for reviews search for JL Spudich).At the current time the origin of the seven 
transmembrane receptors is not clear.

	A 1998 report claims to have found the first seven transmembrane receptor in plants
(14).  These receptors are not common in plants. The completion of the Arabidopsis genome (Dec 
14 Nature 2000) confirms the presence of GPCRs.  The automated analysis of the genome identified 
27 GPCR domains.  There are only one each of G alpha (GPA1) and G beta (AGB1) genes in the 
Arabidopsis genome.  These proteins are then found in the four main branches at the top of the 
eukaryote tree, animals, fungi, amoebozoa and plants.  Beyond that no sequence data is available 
for GPCRs in the lower eukaryote groups.

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References


    

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  9. EMBO J 1998 Sep 1;17(17):5076-84

  10. Science 286, Oct. 8, 255-260 and 252 commentary 1999

  11. Science 288 1390-1396 2000

  12. Science 289, 733-734, 739-745 2000

  13. PNAS 96, 8034-8039 1999

  14. Plakidou-Dymock S, Dymock D, Hooley R. A higher plant seven-transmembrane receptor that 
influences sensitivity to cytokinins. Curr Biol 1998 Mar 12;8(6):315-24