| Summary: | <p>Abstract</p> <p>Background</p> <p>A <it>Paramecium tetraurelia </it>pilot genome project, the subsequent sequencing of a Megabase chromosome as well as the <it>Paramecium </it>genome project aimed at gaining insight into the genome of <it>Paramecium</it>. These cells display a most elaborate membrane trafficking system, with distinct, predictable pathways in which actin could participate. Previously we had localized actin in <it>Paramecium</it>; however, none of the efforts so far could proof the occurrence of actin in the cleavage furrow of a dividing cell, despite the fact that actin is unequivocally involved in cell division. This gave a first hint that <it>Paramecium </it>may possess actin isoforms with unusual characteristics. The genome project gave us the chance to search the whole <it>Paramecium </it>genome, and, thus, to identify and characterize probably all actin isoforms in <it>Paramecium</it>.</p> <p>Results</p> <p>The ciliated protozoan, <it>P. tetraurelia</it>, contains an actin multigene family with at least 30 members encoding actin, actin-related and actin-like proteins. They group into twelve subfamilies; a large subfamily with 10 genes, seven pairs and one trio with > 82% amino acid identity, as well as three single genes. The different subfamilies are very distinct from each other. In comparison to actins in other organisms, <it>P. tetraurelia </it>actins are highly divergent, with identities topping 80% and falling to 30%. We analyzed their structure on nucleotide level regarding the number and position of introns. On amino acid level, we scanned the sequences for the presence of actin consensus regions, for amino acids of the intermonomer interface in filaments, for residues contributing to ATP binding, and for known binding sites for myosin and actin-specific drugs. Several of those characteristics are lacking in several subfamilies. The divergence of <it>P. tetraurelia </it>actins and actin-related proteins between different <it>P. tetraurelia </it>subfamilies as well as with sequences of other organisms is well represented in a phylogenetic tree, where <it>P. tetraurelia </it>sequences only partially cluster.</p> <p>Conclusion</p> <p>Analysis of different features on nucleotide and amino acid level revealed striking differences in isoforms of actin and actin-related proteins in <it>P. tetraurelia</it>, both within the organism and in comparison to other organisms. This diversification suggests unprecedented specification in localization and function within a unicellular eukaryote.</p>
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