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J. Biol. Chem., Vol. 281, Issue 14, 9145-9151, April 7, 2006

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AtUTr1, a UDP-glucose/UDP-galactose Transporter from Arabidopsis thaliana, Is Located in the Endoplasmic Reticulum and Up-regulated by the Unfolded Protein Response^*

Francisca Reyes^¶12, Lorena Marchant¹³, Lorena Norambuena¹⁴, Ricardo Nilo^¶, Herman Silva^¶, and Ariel Orellana^¶5

From the Plant Cell Biology Millennium Nucleus, Graduate School, Faculty of Sciences, University of Chile, Casilla 653, Santiago, Chile and the ^¶Center of Plant Biotechnology, Andrés Bello University, República 217, Santiago, Chile

The folding of glycoproteins in the endoplasmic reticulum (ER) depends on a quality control mechanism mediated by the calnexin/calreticulin cycle. During this process, continuous glucose trimming and UDP-glucose-dependent re-glucosylation of unfolded glycoproteins takes place. To ensure proper folding, increases in misfolded proteins lead to up-regulation of the components involved in quality control through a process known as the unfolded protein response (UPR). Reglucosylation is catalyzed by the ER lumenal located enzyme UDP-glucose glycoprotein glucosyltransferase, but as UDP-glucose is synthesized in the cytosol, a UDP-glucose transporter is required in the calnexin/calreticulin cycle. Even though such a transporter has been hypothesized, no protein playing this role in the ER yet has been identified. Here we provide evidence that AtUTr1, a UDP-galactose/glucose transporter from Arabidopsis thaliana, responds to stimuli that trigger the UPR increasing its expression around 9-fold. The accumulation of AtUTr1 transcript is accompanied by an increase in the level of the AtUTr1 protein. Moreover, subcellular localization studies indicate that AtUTr1 is localized in the ER of plant cells. We reasoned that an impairment in AtUTr1 expression should perturb the calnexin/calreticulin cycle leading to an increase in misfolded protein and triggering the UPR. Toward that end, we analyzed an AtUTr1 insertional mutant and found an up-regulation of the ER chaperones BiP and calnexin, suggesting that these plants may be constitutively activating the UPR. Thus, we propose that in A. thaliana, AtUTr1 is the UDP-glucose transporter involved in quality control in the ER.

Received for publication, November 14, 2005 , and in revised form, January 23, 2006.

^* This work was supported in part by Iniciativa Científica Milenio Grant P02-009-F and Fondecyt Grants 1030551, 2010038, and 2010066. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1.

¹ These authors contributed equally to this work.

² Recipient of a doctoral fellowship from Conicyt, Chile.

³ Present address: Dept. of Anatomy and Developmental Biology, University College London, Gower Street, London WC1E 6BT, UK.

⁴ Present address: Center for Plant Cell Biology, Dept. of Botany and Plant Sciences, 2130 Batchelor Hall, University of California, Riverside, CA 92521.

⁵ To whom correspondence should be addressed. Tel.: 56-2-6618448; Fax: 56-2-6615832; E-mail: aorellana{at}unab.cl.

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