ATP Transporters in Peroxisomal Membranes

Upon seed germination, lipids contained in oil bodies are mobilized via b-oxidation, the glyoxylate cycle, and gluconeogenesis to pro- vide energy for seedling growth in the form of sucrose (reviewed in Graham, 2008). The b-oxidation of fatty acids and some steps of the gyloxylate cycle occur in peroxisomes. In Saccharomyces cerevisiae, ATP is imported into the peroxisomal lumen by ADENINE NUCLEOTIDE TRANSPORTER1, Ant1p, which exchanges ATP for AMP. In plants, however,muchlessisknownaboutthesource of ATP for processes taking place in perox- isomes. Now, two groups have independently identified and characterized adenine nucleo- tidetransportersfromplantperoxisomalmem- branes. Arai et al. (pp. nnn) identified Glycine max PEROXISOMAL ADENINE NUCLEOTIDE CARRIER1 (Gm PNC1) via a proteomics ap- proach, isolating peroxisomal membrane pro- teinsfromsoybeancotyledons.GmPNC1,and two related Arabidopsis thaliana proteins, At PNC1 andPNC2,were thenfound to belongto a clade including peroxisomal adenine nucle- otide transporters from other species. Inde- pendently, Linka et al. (pp. nnn) identified At PNC1 and PNC2 using homology searches based on Sc Ant1p. Both groups used fluo- rescentfusionproteinstodemonstratethatthe PNC proteins localize to peroxisomes. Linka et al. found that both At PNC proteins could complement a yeast mutant lacking ANT1p, suggesting that At PNC 1 and PNC2 are functional ATP transporters. Using slightly different approaches, both sets of researchers directly measured the abilities of these pro- teins to facilitate ATP uptake. The results demonstrate that the PNC proteins can exchange ATP or ADP for AMP, with a prefer- ence for ATP over ADP. This is consistent with a physiological role for PNCs in providing ATP for activation of b-oxidation substrates inside the peroxisome and transporting AMP to the cytosol for regeneration of ATP. In addition, both groups concluded that expression data suggests that the PNC proteins are involved in b-oxidation and the glyoxylate cycle upon germination. Both groups used RNA interference to generate Arabidopsis mutants with reduced levels of PNC1 and PNC2. These mutants ex- hibit seedling growth defects (see figure) that are partially rescued by sucrose feeding, con- sistent with a defect in fatty acid catabolism such that lipid reserves are not converted into sucrose. In addition, the mutants have fatty acid accumulation and herbicide resistance phenotypes,demonstratingthat b-oxidationin peroxisomes is compromised. These pheno- types suggest that PNC transport of ATP into peroxisomes is required for lipid mobilization and b-oxidation upon germination. The strik-