Ionic selectivity of the coupled and uncoupled currents carried by the amino acid transporter KAAT1.
The ability of the intestinal amino acid co-transporter KAAT-1 expressed in Xenopus oocytes to transport different cations in either amino acid coupled or uncoupled manner was studied using voltage-clamp conditions. KAAT1-expressing oocytes exhibit a transporter-related current in the absence of organic substrate (uncoupled current). In the presence of various alkali cations the amplitude of this current follows the sequence: ILi > INa > IK approximately equal to IRb approximately equal to ICs. Addition of 1 mM leucine causes large increases in K+ and Na+ currents, while the Li+ current undergoes a more complex change and Rb+ and Cs+ currents are only marginally affected. Pre-steady-state currents in the absence of organic substrate are apparent when Na+, K+, or Li+ are the bathing ions; analysis of these currents in terms of charge movement reveals that Na+, K+, and Li+ interact differently with the transporter. The uncoupled current in mixtures of Na+ and Li+ fails to exhibit anomalous mole-fraction behavior. Kinetic analysis of ion binding and uncoupled permeation argues against a multi-ion single-file mechanism in the KAAT1 cotransporter.
Bossi
E
E
Department of Structural and Functional Biology, University of Insubria, Varese, Italy.
Sacchi
V F
VF
Peres
A
A
D026921:Amino Acid Transport Systems, Neutral; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism / Q000502:physiology*; D004338:Drug Combinations; D004553:Electric Conductivity; D005260:Female; D019476:Insect Proteins; D007413:Intestinal Mucosa / Q000378:metabolism; D007477:Ions; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology; D008562:Membrane Glycoproteins / Q000378:metabolism / Q000502:physiology*; D008954:Models, Biological; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D012964:Sodium / Q000494:pharmacology; D014982:Xenopus laevis
D026921:Amino Acid Transport Systems, Neutral; D002352:Carrier Proteins; D004338:Drug Combinations; D019476:Insect Proteins; D007477:Ions; C112249:KAAT1 protein, insect; D008562:Membrane Glycoproteins; D008094:Lithium; D012964:Sodium