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J. Biol. Chem., Vol. 283, Issue 36, 24673-24681, September 5, 2008

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Closely Related Members of the Medicago truncatula PHT1 Phosphate Transporter Gene Family Encode Phosphate Transporters with Distinct Biochemical Activities^*

Jinyuan Liu¹, Wayne K. Versaw, Nathan Pumplin, S. Karen Gomez, Laura A. Blaylock, and Maria J. Harrison²

From the Boyce Thompson Institute for Plant Research, Cornell University, Ithaca, New York 14853 and the Department of Biology, Texas A&M University, 3258 TAMU, College Station, Texas 77843

Phosphorus is one of the essential mineral nutrients required by all living cells. Plants assimilate phosphate (P_i) from the soil, and their root systems encounter tremendous variation in P_i concentration, both temporally and spatially. Genome sequence data indicate that plant genomes contain large numbers of genes predicted to encode P_i transporters, the functions of which are largely unexplored. Here we present a comparative analysis of four very closely related P_i transporters of the PHT1 family of Medicago truncatula. Based on their sequence similarity and locations in the genome, these four genes probably arose via recent gene duplication events, and they form a small subfamily within the PHT1 family. The four genes are expressed in roots with partially overlapping but distinct spatial expression patterns, responses to P_i and expression during arbuscular mycorrhizal symbiosis. The proteins are located in the plasma membrane. Three members of the subfamily, MtPT1, MtPT2, and MtPT3, show low affinities for P_i. MtPT5 shares 84% amino acid identity with MtPT1, MtPT2, and MtPT3 but shows a high affinity for P_i with an apparent K_m in yeast of 13 µM. Sequence comparisons and protein modeling suggest that amino acid residues that differ substantially between MtPT5 and the other three transporters are clustered in two regions of the protein. The data provide the first clues as to amino acid residues that impact transport activity of plant P_i transporter proteins.

Received for publication, April 8, 2008 , and in revised form, June 30, 2008.

^* The work was supported by United States National Science Foundation Grant IBN-0343975 and The Samuel Roberts Noble Foundation. Microscopy analyses were carried out in the BTI Plant Cell Imaging Center using instrumentation purchased with an award from National Science Foundation Grant DBI-0618969. 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 Figs. S1–S7 and Table S1.

¹ Present address: Dept. of Plant Pathology, 334 Plant Science Bldg., Cornell University, Ithaca, NY 14853.

² To whom correspondence should be addressed: Tower Road, Ithaca, NY 14853. Tel.: 607-254-6472; Fax: 607-254-6779; E-mail: mjh78{at}cornell.edu.

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