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J. Biol. Chem., Vol. 279, Issue 29, 30150-30157, July 16, 2004

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Mechanisms and Functional Properties of Two Peptide Transporters, AtPTR2 and fPTR2^*

Chien-Sung Chiang, Gary Stacey, and Yi-Fang Tsay¶

From the Institute of Molecular Biology, Academia Sinica, Taipei 11529, Taiwan and the Department of Plant Microbiology and Pathology, University of Missouri, Columbia, Missouri 65211

The Arabidopsis AtPTR2 and fungal fPTR2 genes, which encode H⁺/dipeptide cotransporters, belong to two different subgroups of the peptide transporter (PTR) (NRT1) family. In this study, the kinetics, substrate specificity, stoichiometry, and voltage dependence of these two transporters expressed in Xenopus oocytes were investigated using the two-microelectrode voltage-clamp method. The results showed that: 1) although AtPTR2 belongs to the same PTR family subgroup as certain H⁺/nitrate cotransporters, neither AtPTR2 nor fPTR2 exhibited any nitrate transporting activity; 2) AtPTR2 and fPTR2 transported a wide spectrum of dipeptides with apparent affinity constants in the range of 30 µM to 3 mM, the affinity being dependent on the side chain structure of both the N- and C-terminal amino acids; 3) larger maximal currents (I_max) were evoked by positively charged dipeptides in AtPTR2- or fPTR2-injected oocytes; 4) a major difference between AtPTR2 and fPTR2 was that, whereas fPTR2 exhibited low Ala-Asp^– transporting activity, AtPTR2 transported Ala-Asp^– as efficiently as some of the positively charged dipeptides; 5) kinetic analysis suggested that both fPTR2 and AtPTR2 transported by a random binding, simultaneous transport mechanism. The results also showed that AtPTR2 and fPTR2 were quite distinct from PepT1 and PepT2, two well characterized animal PTR transporters in terms of order of binding of substrate and proton(s), pH sensitivity, and voltage dependence.

Received for publication, May 10, 2004

^* This work was supported by National Science Council Grants NSC-89-2311-B-001-033 and NSC-89-2311-B-001-124 (to Y.-F. T.) and National Science Foundation Grant MCB-0235286 (to G. S.). 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 Figs. S1–S3 and Tables S1 and S2.

^¶ To whom correspondence should be addressed. E-mail: yftsay{at}gate.sinica.edu.tw.

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