title: Steady-state and presteady-state kinetics of the H+/hexose cotransporter (STP1) from Arabidopsis thaliana expressed in Xenopus oocytes.
abstract: We have investigated the steady-state and presteady-state kinetics of the cloned H+/hexose cotransporter from Arabidopsis thaliana (STP1) expressed in Xenopus oocytes using the two-electrode voltage-clamp method. Steady-state sugar-dependent currents were measured between -150 and +50 mV as a function of external [3-O-methyl-D-glucose] (3OMG) and [H+]. At pH 6.5 (316 nM H+) the maximal current for sugar, i3OMGmax, was voltage-dependent, increasing from 40 nA at -30 mV to 95 nA at -150 mV. The apparent affinity of sugar, K3OMG0.5, at pH 6.5 decreased from 30 microM at -30 mV to 11 microM at -70 mV and was then voltage-independent between -70 and -150 mV. Increasing the extracellular [H+] to 3160 nM (pH 5.5) increased i3OMGmax to 65 nA at -30 mV and 212 nA at -150 mV. K3OMG0.5 at pH 5.5 also increased and was voltage-dependent: 15 microM at -50 mV rising to 25 microM at -150 mV. At pH 6.5 and 5.5, the Hill coefficient n for 3OMG was voltage-independent and averaged 1.2 and 1.4, respectively. At saturating [3OMG] iHmax was voltage-dependent and KH0.5 was voltage-independent, averaging 370 nM (pH 6.4). The Hill coefficient n for H+ was voltage-independent and averaged 1. The sugar specificity of STP1 was D-mannose > or = 2-deoxyglucose > D-galactose > or = 3OMG > D-xylose > D-glucose > D-fucose > D-fructose > L-glucose > L-arabinose > D-arabinose, demonstrating that STP1 has a low substrate specificity. Transient currents recorded after rapid steps in membrane potential relaxed with time constants tau, between 3 and 14 ms. The charge movement Q (the integral of the current transients) fitted to a Boltzmann relation with maximal charge Qmax of 3.4 nanocoulombs and an apparent valence z approximately 1 corresponding to a transporter density of 2 x 10(10)/oocyte. Potential for 50% Qmax (V0.5) was -28 mV. At saturating 3OMG and at low external [H+] (pH 7.5), the transient STP1 currents were eliminated. The presteady-state data indicate that STP1 can bind H+ in the absence of sugar, and the steady-state data suggest that H+/hexose cotransport occurs via a sequential mechanism.
authors.affiliation: Department of Physiology, UCLA School of Medicine 90024-1751.
authors.forename: K J
authors.initials: KJ
authors.lastname: Boorer
authors.forename: D D
authors.initials: DD
authors.lastname: Loo
authors.forename: E M
authors.initials: EM
authors.lastname: Wright
mesh_terms: D000818:Animals; D017360:Arabidopsis / Q000378:metabolism*; D001692:Biological Transport; D005260:Female; D006601:Hexoses / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials; D009004:Monosaccharide Transport Proteins / Q000235:genetics* / Q000378:metabolism; D009865:Oocytes; D013379:Substrate Specificity; D014982:Xenopus laevis
chemical_list: D006601:Hexoses; D009004:Monosaccharide Transport Proteins; D006859:Hydrogen