The Molecular History of Eukaryotic Life  Evolution of Signal Transduction


David Nelson  Dec. 13, 2000

	Signal transduction involves a cell responding to a stimulus.  Both proteins and small 
molecule second messengers are key players.  This section will deal with the evolution of 
these pathways.  Where did they arise, and how far back on the tree of life do they exist.
Some of these mechanisms predate eukaryotic life, however we will discuss their occurrence 
in bacteria and archaea even though that is outside the scope of these pages.

	There are a variety of ways to transmit a signal across a cell membrane and into the 
interior of the cell, or even into the nucleus.  Many of these pathways use the same 
components, so these may be viewed as modules of the signal transduction process. 
A list of some of these components is given below.

    

  1. Seven transmembrane segment receptors (also called serpentine receptors) coupled to 
heterotrimeric G proteins. G-protein coupled receptors (GPCRs)


  2. Single transmembrane segment receptors (receptor tyrosine kinases, receptor guanylyl 
cyclases, histidine kinases and two component regulators)


  3. Nuclear Receptors


  4. Ion channels, either voltage gated, ligand gated or mechanosensitive


  5. Calcium/ calmodulin


  6. Phospholipases/ sphinomylinase and products diacylglycerol, phosphoinisitides and 
ceramide


  7. Nitric oxide synthase and soluble guanylate cyclases


  8. Cyclic nucleotides cAMP, cGMP, adenylate cyclase and phosphodiesterases


  9. Protein kinase cascades and associated phosphatases


	We will look at the evolutionary history of these systems.  On the tree of life, 
animals and fungi branch more recently than other eukaryotes.  The order of branching of 
the other crown group eukaryotes has been subject to revision in recent years.  For 
example, Dictyostelium discoideum (a cellular slime mold) had been considered a fairly deep 
branching species often depicted as diverging before the crown group based on rRNA trees.  
More recent assessment of the position of Dictyostelium places it just before the animal-
fungi clade and after plants(1,2).  This has implications for interpretation of the 
molecular features seen in Dictyostelium and other Amoebozoa.  The earlier placement of 
Dictyostelium would suggest that conserved features found in this species and animals or 
fungi would predate the origin of plants so that plants might be expected to have these 
features also.  This is no longer the case.  For example, the seven transmembrane segment 
cAMP receptors cAR1 to cAR4 and associated G-proteins may have arisen after Dictyostelium 
diverged from plants.  Plants do not necessarily have to have this class of receptors.  In 
fact, Plants do seem to have G-protein coupled receptors(3-5), but thay are relatively rare 
and not much exploited in plants, at least so far as we know.

	Dictyostelium discoidium has been well studied because it assembles from single cells 
to form a slug that develops into a fruiting body that makes spores.  Of course, this 
involves signal transduction, so we will look at some signal transduction pathways in 
Dictyostelium as a representative primitive eukaryote. We will also consider yeast, since 
the whole genome is sequenced and it has been thoroughly studied as a model organism.  In 
1998, C. elegans the nematode worm had its genome sequence completed.  In March 2000 the 
Drosophila genome was placed in Genbank.  These genomes will offer insights into signal 
transduction in animals, but it may take a few years to analyze all the gene data. 

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References

    

  1. Baldauf SL, Roger AJ, Wenk-Siefert I, Doolittle WF.
A kingdom-level phylogeny of eukaryotes based on combined protein data.
Science. 2000 Nov 3;290(5493):972-7.


  2. Baldauf SL, Doolittle WF. Origin and evolution of the slime molds.
Proc Natl Acad Sci U S A. 1997 Oct 28;94(22):12007-12.


  3. 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


  4. Josefsson LG.
Evidence for kinship between diverse G-protein coupled receptors.
Gene. 1999 Nov 1;239(2):333-40.


  5. Josefsson LG, Rask L. Cloning of a putative G-protein-coupled receptor from Arabidopsis 
thaliana.  Eur J Biochem. 1997 Oct 15;249(2):415-20.