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IN BRIEF

ATP Transporters in Peroxisomal Membranes

Science Editor

nhofmann{at}aspb.org

Upon seed germination, lipids contained in oil bodies are mobilized via β-oxidation, the glyoxylate cycle, and gluconeogenesis to provide energy for seedling growth in the form of sucrose (reviewed in Graham, 2008). The β-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, much less is known about the source of ATP for processes taking place in peroxisomes. Now, two groups have independently identified and characterized adenine nucleotide transporters from plant peroxisomal membranes. Arai et al. (pp. 3227–3240) identified Glycine max PEROXISOMAL ADENINE NUCLEOTIDE CARRIER1 (Gm PNC1) via a proteomics approach, isolating peroxisomal membrane proteins from soybean cotyledons. Gm PNC1, and two related Arabidopsis thaliana proteins, At PNC1 and PNC2, were then found to belong to a clade including peroxisomal adenine nucleotide transporters from other species. Independently, Linka et al. (pp. 3241–3257) identified At PNC1 and PNC2 using homology searches based on Sc Ant1p. Both groups used fluorescent fusion proteins to demonstrate that the 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 proteins to facilitate ATP uptake. The results demonstrate that the PNC proteins can exchange ATP or ADP for AMP, with a preference for ATP over ADP. This is consistent with a physiological role for PNCs in providing ATP for activation of β-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 β-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 exhibit seedling growth defects (see figure ) that are partially rescued by sucrose feeding, consistent 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, demonstrating that β-oxidation in peroxisomes is compromised. These phenotypes suggest that PNC transport of ATP into peroxisomes is required for lipid mobilization and β-oxidation upon germination. The strikingly similar studies of Linka et al. and Arai et al. thus provide strong evidence that, as in yeast, ATP is transported via dedicated adenine nucleotide transporters into the lumen of plant peroxisomes.

View larger version (59K): [in this window] [in a new window] [as a PowerPoint slide]   Phenotype of the pnc1/2 RNAi Knockdown Mutant. Wild-type (A) and pnc1/2i mutant (B) seedlings grown on medium without sucrose under constant illumination. (Image provided by Yuko Arai and Tomoko Kurata [National Institute for Basic Biology, Okazaki, Japan].)

www.plantcell.org/cgi/doi/10.1105/tpc.108.201210

REFERENCES

Arai, Y., Hayashi, M., and Nishimura, M. (2008). Proteomic identification and characterization of a novel peroxisomal adenine nucleotide transporter supplying ATP for fatty acid β-oxidation in soybean and Arabidopsis. Plant Cell 20: 3227–3240.[Abstract/Free Full Text]

Graham, I.A. (2008). Seed storage oil mobilization. Annu. Rev. Plant Biol. 59: 115–142.[CrossRef][Medline]

Linka, N., Theodoulou, F.L., Haslam, R.P., Linka, M., Napier, J.A., Neuhaus, H.E., and Weber, A.P.M. (2008). Peroxisomal ATP import is essential for seedling development in Arabidopsis thaliana. Plant Cell 20: 3241–3257.[Abstract/Free Full Text]

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Proteomic Identification and Characterization of a Novel Peroxisomal Adenine Nucleotide Transporter Supplying ATP for Fatty Acid β-Oxidation in Soybean and Arabidopsis

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Plant Cell 2008 20: 3227-3240. [Abstract] [Full Text]  

Peroxisomal ATP Import Is Essential for Seedling Development in Arabidopsis thaliana

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