Functional analysis of LjSUT4, a vacuolar sucrose transporter from Lotus japonicus

Anke Reinders, Alicia B. Sivitz, Colby G. Starker, J. Stephen Gantt, John M. Ward

Research output: Contribution to journalArticle

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Abstract

Sucrose transporters in the SUT family are important for phloem loading and sucrose uptake into sink tissues. The recent localization of type III SUTs AtSUT4 and HvSUT2 to the vacuole membrane suggests that SUTs also function in vacuolar sucrose transport. The transport mechanism of type III SUTs has not been analyzed in detail. LjSUT4, a type III sucrose transporter homolog from Lotus japonicus, is expressed in nodules and its transport activity has not been previously investigated. In this report, LjSUT4 was expressed in Xenopus oocytes and its transport activity assayed by two-electrode voltage clamping. LjSUT4 transported a range of glucosides including sucrose, salicin, helicin, maltose, sucralose and both α- and β-linked synthetic phenyl glucosides. In contrast to other sucrose transporters, LjSUT4 did not transport the plant glucosides arbutin, fraxin and esculin. LjSUT4 showed a low affinity for sucrose (K 0.5 = 16 mM at pH 5.3). In addition to inward currents induced by sucrose, other evidence also indicated that LjSUT4 is a proton-coupled symporter: 14C-sucrose uptake into LjSUT4-expressing oocytes was inhibited by CCCP and sucrose induced membrane depolarization in LjSUT4-expressing oocytes. A GFP-fusion of LjSUT4 localized to the vacuole membrane in Arabidopsis thaliana and in the roots and nodules of Medicago truncatula. Based on these results we propose that LjSUT4 functions in the proton-coupled uptake of sucrose and possibly other glucosides into the cytoplasm from the vacuole.

Original languageEnglish (US)
Pages (from-to)289-299
Number of pages11
JournalPlant molecular biology
Volume68
Issue number3
DOIs
StatePublished - Oct 2008

Fingerprint

Lotus corniculatus var. japonicus
Sucrose
transporters
sucrose
Glucosides
glucosides
Vacuoles
Oocytes
vacuoles
trichlorosucrose
oocytes
uptake mechanisms
protons
Membranes
Protons
Loteae
Arbutin
Carbonyl Cyanide m-Chlorophenyl Hydrazone
Esculin
Medicago truncatula

Keywords

  • Electrophysiology
  • Lotus japonicus
  • Sucrose transporter
  • Vacuole
  • Xenopus oocytes

Cite this

Functional analysis of LjSUT4, a vacuolar sucrose transporter from Lotus japonicus. / Reinders, Anke; Sivitz, Alicia B.; Starker, Colby G.; Gantt, J. Stephen; Ward, John M.

In: Plant molecular biology, Vol. 68, No. 3, 10.2008, p. 289-299.

Research output: Contribution to journalArticle

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AB - Sucrose transporters in the SUT family are important for phloem loading and sucrose uptake into sink tissues. The recent localization of type III SUTs AtSUT4 and HvSUT2 to the vacuole membrane suggests that SUTs also function in vacuolar sucrose transport. The transport mechanism of type III SUTs has not been analyzed in detail. LjSUT4, a type III sucrose transporter homolog from Lotus japonicus, is expressed in nodules and its transport activity has not been previously investigated. In this report, LjSUT4 was expressed in Xenopus oocytes and its transport activity assayed by two-electrode voltage clamping. LjSUT4 transported a range of glucosides including sucrose, salicin, helicin, maltose, sucralose and both α- and β-linked synthetic phenyl glucosides. In contrast to other sucrose transporters, LjSUT4 did not transport the plant glucosides arbutin, fraxin and esculin. LjSUT4 showed a low affinity for sucrose (K 0.5 = 16 mM at pH 5.3). In addition to inward currents induced by sucrose, other evidence also indicated that LjSUT4 is a proton-coupled symporter: 14C-sucrose uptake into LjSUT4-expressing oocytes was inhibited by CCCP and sucrose induced membrane depolarization in LjSUT4-expressing oocytes. A GFP-fusion of LjSUT4 localized to the vacuole membrane in Arabidopsis thaliana and in the roots and nodules of Medicago truncatula. Based on these results we propose that LjSUT4 functions in the proton-coupled uptake of sucrose and possibly other glucosides into the cytoplasm from the vacuole.

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