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Identification of an Endoplasmic Reticulum ATP/ADP Transporter

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In eukaryotic cells, the endoplasmc reticulum (ER) is the site of protein translation and many crucial posttranslational protein modifications, including disulfide bond formation, protein folding and assembly, glycosylation, phosphorylation, and formation of integral membrane proteins. These processes all require energy in the form of ATP, and although ATP transport across ER membranes has been shown to occur in yeast and animal cells, the corresponding carrier proteins have not been identified in any eukaryote (reviewed in Hirschberg et al., 1998). The membrane-bound phosphatidylinositol phosphate phosphatase Sac1p was identified as a putative ATP transporter in yeast, but it was shown that it is unable to catalyze ATP transport itself and likely has a regulatory function in association with an as yet unidentified adenylate transporter (Kochendörfer et al., 1999).

In this issue of The Plant Cell, Leroch et al. (pages 438–451) identify an ATP/ADP transporter, called ER-ANT1, which is localized to the ER membrane of Arabidopsis. ER-ANT1 is a member of the mitochondrial carrier family and is phylogenetically closely related to plant mitochondrial ADP/ATP carriers (AACs; Picault et al., 2004) but does not possess a putative N-terminal transit peptide required for targeting to either the inner mitochondrial membrane or the chloroplast membrane. Using heterologous expression in Escherichia coli, the authors show that ER-ANT1 is integrated into the bacterial membrane and exhibits ATP/ADP transport activity. Similar to other AACs, transport activity was consistent with that of an antiporter, which conducts strict counterexchange of ATP for ADP across the membrane.

They then show that the protein is localized to the ER in Arabidopsis, first using a method of sucrose density gradient centrifugation and secondly using immunogold labeling of ER-ANT1 in pollen grains, which show high levels of ER-ANT1 expression (see figure). In addition, Arabidopsis transformed with the GUS reporter gene driven by the ER-ANT1 promoter showed high GUS expression in ER-active tissues, such as pollen, seeds, root tips, apical meristems, and vascular bundles. Functional analysis was conducted using ER-ANT1 knockout T-DNA mutant lines, which showed drastic reduction in plant growth as well as impaired root and seed development, demonstrating the critical function of the protein in plant growth and development. Significantly, the knockout lines showed a decrease in the expression of several genes encoding ER proteins that are dependent on a sufficient supply of ATP, including three BiP chaperones, a calreticulin chaperone, and a Ca²⁺-dependent protein kinase. This work provides strong evidence that ER-ANT1 functions as an ER membrane–localized ATP/ADP antiporter in Arabidopsis.

View larger version (83K): [in this window] [in a new window] [as a PowerPoint slide]   Immunogold labeling with ER-ANT1–specific antiserum and transmission electron microscopy imaging of Arabidopsis pollen grain tissue shows ER-ANT1 localized specifically to the ER (black spots) and not to mitochondria (M) or dictyosomes (D). (Reproduced from Leroch et al. [2008].)

www.plantcell.org/cgi/doi/10.1105/tpc.108.200211

REFERENCES

Hirschberg, C.B., Robbins, P.W., and Abeijon, C. (1998). Transporters of nucleotide sugars, ATP, and nucleotide sulfate in the endoplasmic reticulum and Golgi apparatus. Annu. Rev. Biochem. 67: 49–69.[CrossRef][ISI][Medline]

Kochendörfer, K.U., Then, A.R., Kearns, B.G., Bankaitis, V.A., and Mayinger, P. (1999). Sac1p plays a crucial role in microsomal ATP transport, which is distinct from its function in Golgi phospholipid metabolism. EMBO J. 18: 1506–1515.[CrossRef][ISI][Medline]

Leroch, M., Neuhaus, H.E., Kirchberger, S., Zimmermann, S., Melzer, M., Gerhold, J., and Tjaden, J. (2008). Identification of a novel adenine nucleotide transporter in the endoplasmic reticulum of Arabidopsis. Plant Cell 20: 438–451.[Abstract/Free Full Text]

Picault, N., Hodges, M., Palmieri, L., and Palmieri, F. (2004). The growing family of mitochondrial carriers in Arabidopsis. Trends Plant Sci. 9: 138–146.[CrossRef][ISI][Medline]

Related articles in Plant Cell:

Identification of a Novel Adenine Nucleotide Transporter in the Endoplasmic Reticulum of Arabidopsis

Michaela Leroch, H. Ekkehard Neuhaus, Simon Kirchberger, Sandra Zimmermann, Michael Melzer, Joachim Gerhold, and Joachim Tjaden
Plant Cell 2008 20: 438-451. [Abstract] [Full Text]  

This Article Full Text (PDF) All Versions of this Article: 20/2/245    most recent tpc.108.200211v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in Plant Cell Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Eckardt, N. A. PubMed PubMed Citation Articles by Eckardt, N. A. Agricola Articles by Eckardt, N. A.