Evolution of multicellularity in animals

Evolution of an ancient protein function involved in organized multicellularity in animalsAnderson et al. eLife 2016;5:e10147

Very interesting study about the evolution of multicellularity in animals. As has long been known in plants, animal studies conducted within the last five years have shown that the orientations of successive cell divisions determine the nature of tissue formed, whether increasing in just one plane or more. The position of the mitotic spindle (which appears during cell division) determines this orientation. A protein complex interacts with the mitotic spindle and enables this movement and orientation of mitotic spindles.

In the present paper, the authors have presented evidence that GK-PID, a critical protein component of this complex, evolved by a series of steps–1) two successive gene duplications in descendants of the most recent common ancestor of metazoa and choanoflagellates and 2) loss of enzymatic function and gain of an ability to bind with a protein in the mitotic spindle. The first set of changes is seen in this figure of the evolutionary relationships of these proteins, showing the ancestral protein Anc-gk-dup (brown circle), Anc-GK1-PID (first blue circle) and Anc-GK2-PID (second blue circle) in the ancestor invertebrates and vertebrates:

Function and phylogeny of the guanylate kinase (gk) and GKPID protein family. (A) The GKPID of the protein Discs-large (Dlg, blue) serves as a scaffold for spindle orientation by physically linking the localized cortical protein Pins (green) to astral microtubules (red) via the motor protein KHC-73 (black). (B) Reduced phylogeny of the protein family containing gk enzymes (brown) and protein-binding GKPIDs (blue). Parentheses show the number of sequences in each clade. Reconstructed proteins Anc-gkdup (the preduplication ancestor of gk enzymes and GKPIDs in animals/choanoflagellates), Anc-GK1PID and Anc-GK2PID (the GKPID in the common ancestor of animals and choanoflagellates, and of animals, respectively) are marked as circles with approximate likelihood ratio support. Scale bar indicates number of substitutions per site. – See more at: http://elifesciences.org/content/5/e10147#.dpuf

The second series of changes were demonstrated by, first, the (theoretical)  reconstruction of the protein sequences of these successive ancestors and second, the biochemical synthesis of these reconstructed protein sequences, followed by functional study of these proteins. These experiments demonstrated that the first reconstructed protein Anc-gk-dup retained the enzymatic function of its closest relatives in fungi and bacteria; that the second protein Anc-GK1-PID lost its enzymatic activity and acquired the ability to interact with the spindle protein as well as to re-orient it; and that these same functions have largely been fine-tuned in the most recently evolved protein Anc-GK2-PID.



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