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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Apple Sucrose Transporter SUT1 and Sorbitol Transporter SOT6 Interact with Cytochrome b5 to Regulate Their Affinity for Substrate Sugars^1,[W],[OA]

State Key Laboratory of Plant Physiology and Biochemistry, China Agricultural University, Beijing 100094, China (R.-C.F., C.-C.P., Y.-H.X., X.-F.W., Y.L., R.Z., X.-Y.Z., L.-Y.Z.); Department of Biological Sciences and Biotechnology, Tsinghua University, Beijing 100084, China (Y.S., S.-Y.D., D.-P.Z.); and College of Forestry, South China Agricultural University, Guangzhou 510640, China (C.-C.P.)

Sugar transporters are central machineries to mediate cross-membrane transport of sugars into the cells, and sugar availability may serve as a signal to regulate the sugar transporters. However, the mechanisms of sugar transport regulation by signal sugar availability remain unclear in plant and animal cells. Here, we report that a sucrose transporter, MdSUT1, and a sorbitol transporter, MdSOT6, both localized to plasma membrane, were identified from apple (Malus domestica) fruit. Using a combination of the split-ubiquitin yeast two-hybrid, immunocoprecipitation, and bimolecular fluorescence complementation assays, the two distinct sugar transporters were shown to interact physically with an apple endoplasmic reticulum-anchored cytochrome b5 MdCYB5 in vitro and in vivo. In the yeast systems, the two different interaction complexes function to up-regulate the affinity of the sugar transporters, allowing cells to adapt to sugar starvation. An Arabidopsis (Arabidopsis thaliana) homolog of MdCYB5, AtCYB5-A, also interacts with the two sugar transporters and functions similarly. The point mutations leucine-73 proline in MdSUT1 and leucine-117 proline in MdSOT6, disrupting the bimolecular interactions but without significantly affecting the transporter activities, abolish the stimulating effects of the sugar transporter-cytochrome b5 complex on the affinity of the sugar transporters. However, the yeast (Saccharomyces cerevisiae) cytochrome b5 ScCYB5, an additional interacting partner of the two plant sugar transporters, has no function in the regulation of the sugar transporters, indicating that the observed biological functions in the yeast systems are specific to plant cytochrome b5s. These findings suggest a novel mechanism by which the plant cells tailor sugar uptake to the surrounding sugar availability.

² These authors contributed equally to the article.

³ Present address: State Key Laboratory of Plant Cell and Chromosome Engineering, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China.

⁴ Present address: College of Forestry, Beijing Forestry University, Beijing 100083, China.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Da-Peng Zhang (zhangdp{at}tsinghua.edu.cn).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.141374

^* Corresponding author; e-mail zhangdp{at}tsinghua.edu.cn.

Received May 13, 2009; accepted June 3, 2009; published June 5, 2009.

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