Apple sucrose transporter SUT1 and sorbitol transporter SOT6 interact with cytochrome b5 to regulate their affinity for substrate sugars.
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.
Fan
Ren-Chun
RC
State Key Laboratory of Plant Physiology and Biochemistry, China Agricultural University, Beijing 100094, China.
Peng
Chang-Cao
CC
Xu
Yan-Hong
YH
Wang
Xiao-Fang
XF
Li
Yan
Y
Shang
Yi
Y
Du
Shu-Yuan
SY
Zhao
Rui
R
Zhang
Xiao-Yan
XY
Zhang
Ling-Yun
LY
Zhang
Da-Peng
DP
D000222:Adaptation, Physiological; D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000166:cytology / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000378:metabolism; D050260:Carbohydrate Metabolism; D002241:Carbohydrates / Q000172:deficiency; D015786:Cytochromes b5 / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D027845:Malus / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D009928:Organ Specificity; D010940:Plant Proteins / Q000378:metabolism*; D011485:Protein Binding; D021381:Protein Transport; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000254:growth & development / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D013379:Substrate Specificity
D029681:Arabidopsis Proteins; D002241:Carbohydrates; D026901:Membrane Transport Proteins; D010940:Plant Proteins; D029701:Saccharomyces cerevisiae Proteins; C078729:sucrose transport protein, plant; D015786:Cytochromes b5