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 alpha-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.
Renal reabsorption appears to play a major role in d-mannose homeostasis. Here we show that in rat kidney, the transport of d-mannose by brush border membrane vesicles from tubular epithelial cells involves an uphill and rheogenic Na-dependent system, which is fully inhibited by d-mannose itself, incompletely inhibited by d-glucose, d-fructose, phloridzin, and phloretin, and noninhibited by l-mannose or disaccharides. In addition, this system exhibits both low capacity (112.9+/-15.6 pmol/mg/second) and high affinity (0.18+/-0.04 mm), with a 2:1 stoichiometry for the Na:d-mannose interaction, and low affinity for sodium (16.6+/-3.67 mm). We also show expression of d-mannose transport by Xenopus laevis oocytes injected with rat renal polyA(+) RNA. Kinetic analysis of the expressed transport was performed after RNA enrichment by fractionation through a sucrose density gradient and was shown to be identical to that measured in membrane vesicles. The RNA species encoding the expressed transport has a small mean size, 1 kb approximately, and shows no homology with the SGLT family of Na-dependent d-glucose transporters, as shown by low stringent RT-PCR and northern analysis. The expressed transport is specific for d-mannose, since in spite of a significant inhibition by d-glucose and d-fructose, neither of these two substrates was transported above the level of the water-injected oocytes.
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