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ARS Home » Northeast Area » Ithaca, New York » Robert W. Holley Center for Agriculture & Health » Research » Publications at this Location » Publication #35491
Title: POTASSIUM TRANSPORT KINETICS OF KAT1 EXPRESSED IN XENOPUS OOCYTES: A PROPOSED MOLECULAR STRUCTURE AND FIELD EFFECT MECHANISM FOR MEMBRANE TRANSPORT

Author
item CHILCOTT T C - UNIV OF CA AT DAVIS
item FROST-SHARTZER S - UNIV OF CA AT DAVIS
item IVERSON M W - UNIV OF CA AT DAVIS
item GARVIN D - 1907-05-00
item KOCHIAN L V - 1907-05-00
item LUCAS W J - UNIV OF CA AT DAVIS

Submitted to: C R Academic Science Paris Life Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: The absorption of mineral nutrients by plants is important for crop plant productivity and nutritional quality. Potassium (K) is the mineral nutrient that is maintained in plant cells at the highest levels. It is essential for plant growth and development, yet its transport is still poorly understood. This paper describes the characterization of the transport properties of the system encoded by KAT1, which is the first K+ transport gene cloned in higher plants. The messenger RNA for the gene was synthesized and injected into egg cells of Xenopus (African frog). These cells are commonly used to express transport genes from other organisms, in order to study the function of the transport protein. It was found that KAT1 exhibited properties similar to the K+ transport system we have previously studied in maize roots. Based on the transport properties of this system as studied in Xenopus eggs, a molecular model explaining the function of ion channels was proposed. This approach allows us to study the molecular mechanisms of K+ transport, which should provide a better understanding of how plants acquire this essential nutrient from the soil.

Technical Abstract: In this study, we present a detailed characterization of the K+ transport properties associated with the Arabidopsis thaliana cDNA, KAT1, expressed in Xenopus oocytes. Whole-cell voltage clamp records obtained from oocytes injected with KAT1 cRNA, in which the records were corrected for leakage using a P/4 subtraction protocol, confirmed that KAT1 encodes an inwardly rectifying K+ transporter. However, uncorrected records revealed that an ohmic component of KAT1-induced K+ current was removed using this P/4 protocol. Kinetic analysis of uncorrected records revealed that this single transport system yielded complex K+ transport kinetics that could be resolved into a saturable and a first-order kinetic component. The magnitude of the kinetic constants (Km, Vmax & k) increased exponentially for hyperpolarizing membrane potentials more negative than -50+/- 10 mV. These results indicate that the KAY1 transporter protein can function as a molecular mechanism for variable-affinity, saturable K+ uptake (Km varied from about 25 mM at -160 mV to 1 mM at -60 mV) as well as for nonsaturable first-order kinetic transport. Drawing from molecular structures proposed for other cation transporters we advance a structural model for KAT1. This molecular model of KAT1, in conjunction with electro-diffusion and field effect theories, provides a framework for the analysis of K+ transport. We propose that the membrane potential regulates local electric fields within the transport protein to effect voltage-gating, current-rectification and complex kinetics, and that such a mechanism can explain the characteristics of KAT1 and structurally related transport proteins.