Amino acid transporter CAATCH1 is also an amino acid-gated cation channel

CAATCH1 (cation-(a) under bar mino (a) under bar cid (t) under bar ransport er/(c) under bar hannel) is a recently cloned insect epithelial membrane pr otein related to mammalian Na+-, Cl--coupled neurotransmitter transporters (Feldman, D. H., Harvey, W. R., and Stevens, B. R. (2000) J. BioL Chem. 275 ,24518-24526). In the present study we analyze the relationship between CAA TCH1-mediated amino acid transport and ion fluxes by utilizing the Xenopus oocyte expression system in conjunction with electrophysiology and radiotra cer uptake. Simultaneous flux measurements reveal that electrical currents and amino acid transport are thermodynamically uncoupled. This observation is supported by measuring significant uptake even in the absence of externa l alkali cations. Remarkably, CAATCH1-associated Na+ or K+ currents are lar ge and do not saturate with voltage nor with cation concentration. These cu rrents reverse in Nernstian fashion, thereby conferring channel activity in CAATCH1. Upon step-changes in the membrane potential, CAATCH1-expressing o ocytes exhibit transient currents. Detailed analyses of these transients in the absence and presence of amino acids reveal direct ligand-protein inter action, demonstrating that binding by different amino acids (e.g. proline, threonine, methionine) differentially affects the state probability of CAAT CH1 but has no effect on the maximal charge movement (Q(max)). Together the se data suggest that CAATCH1 is a multifunction membrane protein that media tes thermodynamically uncoupled amino acid uptake but functions predominant ly as an,amino acid-gated alkali cation channel.