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J Biol Chem, Vol. 275, Issue 13, 9563-9571, March 31, 2000
From the We have cloned and functionally characterized a
novel, neuron-specific, H+-coupled oligopeptide
transporter (OPT3) from Caenorhabditis elegans that
functions predominantly as a H+ channel. The
opt3 gene is ~4.4 kilobases long and consists of 13 exons. The cDNA codes for a protein of 701 amino acids with 11 putative transmembrane domains. When expressed in mammalian cells and
in Xenopus laevis oocytes, OPT3 cDNA induces
H+-coupled transport of the dipeptide glycylsarcosine.
Electrophysiological studies of the transport function of OPT3 in
Xenopus oocytes show that this transporter, although
capable of mediating H+-coupled peptide transport,
functions predominantly as a H+ channel. The H+
channel activity of OPT3 is ~3-4-fold greater than the
H+/peptide cotransport activity as determined by
measurements of H+ gradient-induced inward currents in the
absence and presence of the dipeptide using the two-microelectrode
voltage clamp technique. A downhill influx of H+ was
accompanied by a large intracellular acidification as evidenced from
the changes in intracellular pH using an ion-selective microelectrode. The H+ channel activity exhibits a
K0.5H of 1.0 µM at a membrane potential of -50 mV. At the level of primary structure, OPT3 has moderate homology with OPT1 and OPT2, two
other H+-coupled oligopeptide transporters previously
cloned from C. elegans. Expression studies using the
opt3::gfp fusion constructs in transgenic C. elegans demonstrate that opt3 gene is
exclusively expressed in neurons. OPT3 may play an important
physiological role as a pH balancer in the maintenance of
H+ homeostasis in C. elegans.
A Novel H+-coupled Oligopeptide Transporter (OPT3)
from Caenorhabditis elegans with a Predominant Function as
a H+ Channel and an Exclusive Expression in
Neurons*
§,
,,,, and
Department of Biochemistry and Molecular
Biology, Medical College of Georgia, Augusta, Georgia 30912, the
¶ Department of Physiology and Biophysics, Case Western Reserve
University, School of Medicine, Cleveland, Ohio 44106-4970, and
Laboratory of Molecular Biology/NIDDK, National Institutes of
Health, Bethesda, Maryland 20892-0510
*
This work was supported by National Institutes of Health
Grant DK 28389 (to F. H. L.), by a grant from the Medical
College of Georgia Research Institute, Inc., and by a grant from the
Biomedical Research Support Grant Program, School of Medicine, Medical
College of Georgia (to Y. J. F.).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.
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