Peroxisomal transporters associated with β-oxidation

Abstract

Peroxisomes perform a range of different functions, including β-oxidation of fatty acids and synthesis and degradation of bioactive lipid-derived molecules. A key feature of peroxisomes is their role in metabolic pathways which are shared between several subcellular compartments, including mitochondria, chloroplasts and cytosol. Transport across the peroxisomal membrane is therefore essential for the coordination of metabolism. Although transport proteins are very likely required for import of substrates and cofactors, export of intermediates and products and the operation of redox shuttles, relatively few peroxisomal transporters have been identified to date. We have identified and characterised two peroxisomal transport systems which are required for β-oxidation in the model plant, Arabidopsis thaliana. Peroxisomal Nucleotide Carrier 1 and 2 were identified by homology with the yeast peroxisomal adenine nucleotide carrier and were shown by complementation and in vitro uptake assays to catalyse the counter exchange of ATP with AMP [1]. Inducible RNAi lines demonstrated that import of ATP into peroxisomes is essential for activation of fatty acids during seedling establishment and plays a role in other β-oxidation reactions such as auxin metabolism. Arabidopsis also contains a single peroxisomal ABC transporter, COMATOSE (CTS), which has been identified in at least four independent forward genetic screens. Analysis of cts null mutants has demonstrated that CTS plays key roles in a number of developmental and physiological processes, including germination, seedling establishment, fertility and root growth [2]. We demonstrate that the different roles of CTS in planta are separable by mutagenesis [3] and can be related to different biochemical roles, specifically the ability to metabolise distinct substrates such as fatty acids and hormone precursors via β-oxidation. Taken together, these findings strongly suggest that CTS is a broad specificity transporter which mediates uptake of substrates for β-oxidation into the peroxisome. Here, we present biochemical characterisation of heterologously-expressed CTS, provide evidence for its role as a transporter of fatty acyl-CoAs and compare its activity to that of yeast and mammalian homologues.