Wortfish wrote:Rumraket wrote:Wortfish wrote:But the nematode's phototaxis depends on a robust signal transduction, amplification and interpretation mechanism that is required for it to respond to the presence of light.
All of this already exists for GPCR receptors of all kinds. What matters are the mutations to the receptor protein that make it sensitive enough to light to effectuate the usual conformational change that activates the G-protein (transducins in the case of opsins), effectively making the receptor protein into an opsin.
No, it does not.
Now you're just in denial. Read this:
http://genomewiki.ucsc.edu/index.php/Opsin_evolution:_orgins_of_opsinsOpsins are definitely not the 'original' GPCR because these were already widely deployed at much earlier divergence nodes -- yeast, protozoa, choanflagellates, trichoplax have GPCR but lack opsins. Nor are opsins the prototype for the 'rhodopsin class' R of the GRAFS classification of GPCR which again was established far earlier. Indeed, even the Ralpha subgroup with of rhodopsin class GPCR was well-established prior to the first metazoan opsin.
Opsins are thus latecomers, not pioneers, to a rapidly expanding paralogous gene clade within already full-featured GPCR. Judging by their closest extant blastp relatives among tens of thousands of GPCR at GenBank, opsins specifically arose as a gene duplication within the peptide receptor subgroup PEP. Indeed, certain of these proteins list opsins among their top ten best back-blast matches (ie have better matches than to almost all non-opsin GPCR). Note here that blast scores can be misleading because the 'floor' of percent identity is about 25% just due to universal conserved residues plus accidental matches.
Note an 'intermediate' GPCR does not exist: either lysine is present at K296 or it isn't. Reconstructing ancestral states from the best contemporary set of GPCR proteins lacking K296 cannot produce a lysine there by any rational methodology. The 20 encoded amino acids can be clustered into subgroups (eg by polarity or bulk) but ultimately form a unorderable discrete set not furnishing continuum transitional states.
Most likely the parental gene had several introns and the original opsins inherited this pattern (ie the duplication was segmental rather than retroprocessional as in some cnidarian opsins). The history of introns within opsins is already complex and becomes quite problematic within the enveloping GPCR gene family. Opsins (with the exception of a fragmentary sea urchin melanopsin) lack the ubiquitous phase 21 intron breaking the DRY motif arginine.
Intracellular targeting of early opsins was likely to cytoplasmic or endoplasmic reticulum membranes as isolated monomers, with limited microvillar or especially ciliary specialization (to motile larva) also plausible. These opsins were the first eyes to the world but only in the sense of indicating the intensity (and later directionality) of sunlight striking the cell utilizing already refined GPCR second messenger signal transduction.
Opsin creation does not imply saltatory evolution because the basics had been established far earlier -- the 7-transmembrane helical structure with fixed topology, the TM1-TM2 salt bridge N55-D83 that could serve as initial counter-ion, the DRY ionic lock, the GWS.Y..E.....C..DW........SY region of EC2, the NPxxY terminal helix, the conformational shift upon binding of ligand that could trigger signaling, the Galpha protein binding site needed for the signaling cascade, and an arrestin-type mechanism signaling termination. The earliest opsins contained and continued all of these features from the get-go, adapting them over the course of time to various photoreceptive functions.
A few mutations to a protein does not generate a phototransductive system.
Apparently they do. It is possible that a single mutation in the ancestral GPC receptor caused an amino acid change that significantly altered the photosensitivity of the molecule, since apparently the main mechanism of activation of the opsin receptor is dependent on protonation of a single lysine residue at position 296 in the protein.
You are also assuming the pre-existence of transducins without which any changes to the GPCR receptor would be useless.
I'm sorry this is just what the evidence shows, it's not an assumption. Transducin is just the name for the G-protein activated by the Opsin protein receptor (what in the above quote is known as the 7-transmembrane helical structure common to all GPCRs). Other non-opsin GPCRs have G-proteins too (which is why they are called
G-
Protein
Coupled
Receptors), clearly homologous to transducin, and they activate their own transducin-like G-proteins in the same way by conformational change when some extracellular stimuli is detected. So yes, "transducin" G-proteins definitely pre-existed the origin of opsins.
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