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Departments of 1Toxicology, 2Pharmacology, 3Physiology, 4Genetics, and 5Anatomy, University of Zaragoza, Zaragoza E50013, Spain
Submitted 16 April 2003 ; accepted in final form 11 June 2003
Renal reabsorption is the main mechanism that controls mannose homeostasis.
This takes place through a specific Na-coupled uphill transport system, the
molecular identity of which is unknown. We prepared and screened a
size-selected rat kidney cortex cDNA library through the expression of mannose
transport in Xenopus laevis oocytes. We have identified a membrane
protein that induces high-affinity and specific Na-dependent transport of
D-mannose and D-glucose in X. laevis oocytes,
most likely through stimulation of the capacity of an endogenous transport
system of the oocyte. Sequencing has revealed that the cDNA encodes the
counterpart of the human membrane-associated protein MAP17, previously known
by its overexpression in renal, colon, lung, and breast carcinomas. We show
that MAP17 is a 12.2-kDa nonglycosylated membrane protein that locates to the
brush-border plasma membrane and the Golgi apparatus of transfected cells and
that it is expressed in the proximal tubules of the kidney cortex and in the
spermatids of the seminiferous tubules. It spans twice the cell membrane, with
both termini inside the cell, and seems to form homodimers through
intracellular Cys55, a residue also involved in transport
expression. MAP17 is responsible for mannose transport expression in oocytes
by rat kidney cortex mRNA. The induced transport has the functional
characteristics of a Na-glucose cotransporter (SGLT), because
D-glucose and
-methyl-D-glucopyranoside are also
accepted substrates that are inhibited by phloridzin. The corresponding
transporter from the proximal tubule remains to be identified, but it is
different from the known mammalian SGLT-1, -2, and -3.
expression cloning; kidney cortex; mannose reabsorption
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