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Cloning of the PPAR genes and cDNAs from plaice (pleuronectes platessa) and atlantic salmon (salmo salar)

Genes and corresponding cDNAs encoding three distinct PPAR isotypes have been identified in plaice (Pleuronectes platessa). In Atlantic salmon (Salmo salar) an second PPAR beta isoform has been identified in addition to the three other isotypes. Of the three fish isotypes the deduced protein of PPAR beta exhibits the highest homology with its mammalian counterpart in both the DNA and ligand binding domains (DBD and LBD, respectively). Plaice PPAR alpha and gamma are also highly homologous to their mammalian and salmon counterparts in the DBD. However, both plaice isotypes have an extended LBD, as compared either to mammalian or salmon PPARs.

Furthermore, in both palice and salmon PPAR gamma two important amino acid substitutions have been observed in relation to their mammalian homologue, both involving residues implicate in ligand binding in the human receptor. The above indicate the ligand binding properties of the fish PPAR gamma are potentially different from those of its mammalian homologue.

The genomic organization of the fish PPARs is remarkably different from that observed in mammalian genes in that the LBD in the plaice alpha and beta and salmon beta receptors is encoded by three exons; the LBD of plaice PPAR gamma is encoded by four exons; while those of salmon PPAR alpha and gamma are encoded by only two exons as is also the case for all mammalian PPARs.

This result represents the first demonstration of the existence of three PPAR isotypes in fish and suggest that the structure and function of these receptors has evolved before the divergence of the osteichtyan and amphibian/mammalian lineages. Therefore, the study of these receptors in fish can result in significant knowledge concerning fatty acid and lipid metabolism in lower vertebrates.

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United Kingdom
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