Functional equivalence of proteins among animals
Homologous proteins from primitive species can functionally substitute for their modern counterpart
One of the well known and widely accepted fact about evolution is theory of common descent stated by Charles Darwin, which states that that all organisms on earth are related and connected to each other like branches of a giant tree calledâ€œTREE OF LIFEâ€.All living things are related to one another (to different extents) through common decent ,,meaning all life forms have developed from other species and share a single common ancestor. Previously we have seen many evidences for evolution in action ( you can read it here) and today we take a look into some amazing examples showing “functional equivalence of proteins” among animals of distant lineages.
Theory of common descent states all life forms on earth have evolved from common ancestor which lived some 4 billion years ago .This means that all life forms must share many vital aspects of life and which is becoming more and more evident with the genome sequences of many organisms being worked out. Sequence results coming out from primitive animals like Ctenophores, Sponges , Trichoplax and Nematostella reveal that they also share many important features including signalling proteins and Transcription factors similar to ones which operate in designing complex body plans in higher vertebrates.
Despite over a billion years of independent evolution in different lineages , many metazoan proteins display functional equivalence in vivo when substituted for one another. A look at these homologs(orthologs and paralogs) present in different animals at sequence level also show marked conservation with some remarkable match in functional motifs.
First experiment of substitution of protein homologs dates back to early 90′s ,when group of william McGinnis performed the landmark experiments by putting the mouse Antennapedia-like genes in developing Drosophila cells under control of a heat shock promoter, can induce homeotic transformations that are nearly identical to those caused by ectopic expression of Antp. Later they did similar kind of experiments with Human Deformed protein and found that Human Hox-4.2 and Drosophila deformed encode similar regulatory specificities in Drosophila embryos and larvae.
The most famous among substituional experiments came in 1995 ,when Walter Gehring’s lab showed that Mouse Pax-6 protein can induce eye formation in Drosophila ,just like Drosophila homolog of Pax-6 (eyeless) protein.The ectopic expression of mouse Pax-6 cDNA under the control of GAL4 induces the formation of ectopic eyes in imaginal discs ( undfferentiated cells).
So far the examples which we saw involved substitutions from vertebrates genes into arthropods but remarkably, proteins from basal/primitive animals in tree of life like Sponges and Cnidarians also work perfectly well like their respective homologs in Drosophila.
When a Cnidarian Achaete-Scute ( CN ACSC) homolog is expressed using UAS- GAL4 system ,it induces formation of sensory organs in Drosophila just as the Drosophila homologs do.Additionally cnidarian ACSC protein forms heterodimers with endogenous drosophila binding partner “daughterless”,and these dimers find their place on DNA ( target gene regulatory elements) perfectly.
Recently Barnard Degnan’s group in collaboration with Michel Vervoort group showed that sponge Amphimedon’s proneural basic helix loop helix protein (bHLH) produce strong proneural activity in Xenopus and Drosophila ,just like neurogenin and atonal genes.
Its really amazing that despite over billion years of independent evolution homologous proteins often exhibit functional equivalence,suggesting very little has changed in protein structure.
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