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J. Biol. Chem., Vol. 277, Issue 38, 35219-35224, September 20, 2002
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From the Groupe de Recherche en Transport Membranaire,
Université de Montréal,
Montréal, Quebec H3T 1J4, Canada
rkST1, an orphan cDNA of the SLC5 family
(43% identical in sequence to the sodium myo-inositol
cotransporter SMIT), was expressed in Xenopus
laevis oocytes that were subsequently voltage-clamped and
exposed to likely substrates. Whereas superfusion with glucose and
other sugars produced a small inward current, the largest current was
observed with myo-inositol. The expressed protein, which we
have named SMIT2, cotransports myo-inositol with a
Km of 120 µM and displays a
current-voltage relationship similar to that seen with SMIT (now called
SMIT1). The transport is Na+-dependent, with a
Km of 13 mM. SMIT2 exhibits
phlorizin-inhibitable presteady-state currents and
substrate-independent "Na+ leak" currents similar to
those of related cotransporters. The steady-state cotransport current
is also phlorizin-inhibitable with a Ki of 76 µM. SMIT2 exhibits stereospecific cotransport of both
D-glucose and D-xylose but does not transport
fucose. In addition, SMIT2 (but not SMIT1) transports
D-chiro-inositol. Based on previous
publications, the tissue distribution of SMIT2 is different from that
of SMIT1, and the existence of this second cotransporter may explain
much of the heterogeneity that has been reported for inositol transport.
Identification of a Novel
Na+/myo-Inositol Cotransporter*
,
*
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.
To whom correspondence should be addressed: Groupe de Recherche en
Transport Membranaire, P.O. Box 6128 succ. "Centre-Ville" Montréal, PQ H3C 3J7, Canada. Tel.: 514-343-6111 (ext.
3289); Fax: 514-343-7146; E-mail: coady@magellan.umontreal.ca.
Copyright © 2002 by The American Society for Biochemistry and Molecular Biology, Inc.
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