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J. Biol. Chem., Vol. 276, Issue 36, 33413-33418, September 7, 2001
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From the CAATCH1
(cation-amino acid
transporter/channel) is a recently cloned
insect epithelial membrane protein related to mammalian Na+-, Cl
Amino Acid Transporter CAATCH1 Is Also an Amino Acid-gated Cation
Channel*
§ and Department of Physiology, School of
Medicine, University of California Los Angeles, Los Angeles,
California 90095-1751 and the ¶ Department of Physiology and
Functional Genomics, College of Medicine, University of Florida,
Gainesville, Florida 32610-0274
-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
CAATCH1-mediated amino acid transport and ion fluxes by utilizing the
Xenopus oocyte expression system in conjunction with
electrophysiology and radiotracer 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 external alkali
cations. Remarkably, CAATCH1-associated Na+ or
K+ currents are large and do not saturate with voltage nor
with cation concentration. These currents reverse in Nernstian fashion, thereby conferring channel activity in CAATCH1. Upon step-changes in
the membrane potential, CAATCH1-expressing oocytes exhibit transient
currents. Detailed analyses of these transients in the absence and
presence of amino acids reveal direct ligand-protein interaction,
demonstrating that binding by different amino acids (e.g.
proline, threonine, methionine) differentially affects the state
probability of CAATCH1 but has no effect on the maximal charge movement
(Qmax). Together these data suggest that
CAATCH1 is a multifunction membrane protein that mediates
thermodynamically uncoupled amino acid uptake but functions
predominantly as an amino acid-gated alkali cation channel.
*
This work was supported by National Institutes of Health
Grants DK19567 (to E. M. W.) and AI30464 (to W. R. H.) and by
American Heart Association Grant 50975-B (to B. R. S.).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
Copyright © 2001 by The American Society for Biochemistry and Molecular Biology, Inc.
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