Plant peroxisomal ABC transporters: flexible and unusual

ABC transporters of subfamily D mediate import of substrates for β-oxidation into peroxisomes. Whilst mammals possess three peroxisomal ABCD proteins which homodimerise to form transporters with distinct substrate specificities, Baker’s yeast has a single transporter formed by heterodimerisation, which imports long-chain fatty acyl CoAs. Plants have a single-fused heterodimer transporter that exhibits broad substrate specificity, reflecting the wide range of β-oxidation substrates processed by plants. The fusion appears to have occurred early in the evolution of land plants and was followed by an early duplication event in the monocot lineage. Plant ABCD proteins function in all stages of the life cycle and their physiological roles reflect the ability to transport diverse substrates including saturated and unsaturated fatty acids and aromatic compounds such as precursors of hormones and secondary metabolites. Recent work suggests that transport of CoA substrates involves their cleavage and re-esterification within the peroxisome, thus interaction with appropriate acyl adenylate-activating enzymes potentially provides a mechanism for regulating entry of different substrates into β-oxidation. The mechanism of ABCD transporter targeting is broadly conserved across kingdoms but evidence suggests the regulation of protein turnover differs.