Woodland HR and Fry AM (2008), Pix proteins and the evolution of centrioles.

XB-ART-39096

PLoS One 2008 Jan 01;311:e3778. doi: 10.1371/journal.pone.0003778.

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Pix proteins and the evolution of centrioles.

Woodland HR , Fry AM .

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We have made a wide phylogenetic survey of Pix proteins, which are constituents of vertebrate centrioles in most eukaryotes. We have also surveyed the presence and structure of flagella or cilia and centrioles in these organisms, as far as is possible from published information. We find that Pix proteins are present in a vast range of eukaryotes, but not all. Where centrioles are absent so are Pix proteins. If one considers the maintenance of Pix proteins over evolutionary time scales, our analysis would suggest that their key function is to make cilia and flagella, and the same is true of centrioles. Moreover, this survey raises the possibility that Pix proteins are only maintained to make cilia and flagella that undulate, and even then only when they are constructed by transporting ciliary constituents up the cilium using the intraflagellar transport (IFT) system. We also find that Pix proteins have become generally divergent within Ecdysozoa and between this group and other taxa. This correlates with a simplification of centrioles within Ecdysozoa and a loss or divergence of cilia/flagella. Thus Pix proteins act as a weathervane to indicate changes in centriole function, whose core activity is to make cilia and flagella.

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Species referenced: Xenopus laevis
Genes referenced: poc1a tbl1x wdr5

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	Figure 1. Pix protein organization.A. A schematic diagram of the two human Pix proteins with amino acid numbers indicated. Pix proteins consist of an N-terminal domain containing seven WD40 repeats (green) and a highly conserved C-terminal coiled-coil (blue). B. A model of the Pix protein WD40 repeat domain folded to form a β-propellor. The structural model was built using MODELLER with the structure of the WDR5 protein (pdb-entry: 2GNQ) serving as a template. The figure was generated in pymol.
	Figure 2. Pix proteins across eukaryotes.A cladogram of a wide range of organisms is drawn according to the current, generally accepted consensus. Pix genes were identified by BLAST search of genomes and identified as hits to the N-terminal 7 WD-40 repeats and the C-terminal coiled-coil region. When these proteins were in turn used to search all genomes their closest vertebrate homologues were Pix proteins. The presence of Pix genes in the genome is indicated by black entries and their absence by red. Pix sequences were compared to Xenopus laevis Pix2 by pairwise BLAST and the P-value for the match is shown.
	Figure 3. Sequence alignments of Pix proteins from representatives of major animal groups compared by ClustalW.Nematostella vectensis (Cnidaria, Anthozoa); Capitella sp. (Lophotrochozoa, Annelida); Drosophila melanobaster (Ecdysozoa, Arthropoda); Xenopus laevis, with two Pix genes (Deuterostomata, Vertebrata); Monosiga brevicollis (Choanoflagellida). Identical amino acids blocked in black and domains are identified according to the Nematostella sequence, using the programs SMART and Coils at EMBL-EBI.
	Figure 4. Comparison of Pix proteins in Ecdysozoa.ClustalW was used to compare the Pix proteins of Drosophila melanogaster (Insecta, Diptera), Anopheles gambiae (Insecta, Diptera) Apis melifera (Insecta, Hymenoptera), Daphnia pulex (Crustacea, Cladocera). Details as in Figure 3.

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Afzelius, Asymmetry of cilia and of mice and men. 1999, Pubmed