Examination of the molecular mechanism of SH reagent-induced inhibition of the intestinal brush-border membrane Na+/phosphate cotransporter

Modulation of phosphate uptake and amphotropic murine leukemia virus entry by posttranslational modifications of PIT-2

PIT-2 is a type III sodium phosphate cotransporter and the receptor for amphotropic murine leukemia viruses. We have investigated the expression and the functions of a tagged version of PIT-2 in CHO cells. PIT-2 remained equally abundant at the cell surface within 6 h following variation of the phosphate supply. In contrast, the efficiency of phosphate uptake and retrovirus entry was inversely related to the extracellular phosphate concentration, indicating that PIT-2 activities are modulated by posttranslational modifications of cell surface molecules induced by phosphate. Conformational changes of PIT-2 contribute to both activities, as shown by the inhibitory effect of sulfhydryl reagents known as inhibitors of type II cotransporters. A physical association of PIT-2 with actin was demonstrated. Modifications of the actin network were induced by variations of the concentrations of extracellular phosphate, cytochalasin D, or lysophosphatidic acid. They revealed that the formation of actin stress fibers determines the cell surface distribution of PIT-2, the internalization of the receptor in response to virus binding, and the capacity to process retrovirus entry. Thus, the presence of PIT-2 at the cell surface is not sufficient to ensure phosphate transport and susceptibility to amphotropic retrovirus infection. Further activation of cell surface PIT-2 molecules is required for these functions.

Simultaneous occlusion of Na+ and phosphate by the intestinal brush border membrane Na+/phosphate cotransporter

The molecular mechanism of Na+ and phosphate transport by the intestinal brush border membrane Na+/phosphate cotransporter was examined using anion and cation exchange columns interfaced to a light-activated microsecond timer. The nature of ion:cotransporter interaction was examined for sensitivity to itself versus non-substrates, and sensitivity to amino acid specific reagents. Two possible ion:cotransporter interactions were examined. (a) ion bound, where ion exchanges rapidly with medium ion but not non-substrates, and (b) ion occluded, where ion exchanges slowly with medium ion. The Na+/phosphate cotransporter was found to occlude Na+ or Na+ and H2PO4, but not H2PO4 alone or HPO4. Phosphate occlusion had an absolute requirement for Na+ with K+ or Cs+ unable to substitute. Phosphate occlusion distinguished between phosphate and sulfate. Deocclusion from the fully loaded cotransporter was consistent with the release of 2 Na+'s prior to phosphate. These results predict two ternary cotransporter conformations differing in the Na+ sensitivity of their phosphate affinity.

Effect of substrates and pH on the intestinal Na+/phosphate cotransporter: evidence for an intervesicular divalent phosphate allosteric regulatory site

Intervesicular divalent phosphate-induced inhibition of the intestinal brush-border membrane Na+/phosphate cotransporter was examined using Na(+)-dependent phosphate uptake, substrate-induced tryptophan fluorescence quenching, and the apparent pKa values for substrate-induced conformational changes. In right-side-out (RSO) reconstituted proteoliposomes, only monovalent phosphate inhibited Na(+)-dependent phosphate uptake in the absence of pre-equilibration. Addition of divalent phosphate to inside-out (ISO) proteoliposomes resulted in 80 +/- 5% inhibition of Na(+)-dependent phosphate uptake in the absence of pre-equilibration. The nature of divalent phosphate-induced inhibition of cotransporter function was examined using cotransporter partial reaction assays based on substrate-induced conformational changes reported as changes in tryptophan fluorescence. Na+ but not K+ induced a quenching of tryptophan fluorescence with a K0.5 of 25 mM and an apparent Hill coefficient of 1.8. Monovalent phosphate (difluorophosphate) induced a further quenching of tryptophan fluorescence with a K0.5 of 53 microM. Divalent phosphate (monofluorophosphate) had no effect on tryptophan fluorescence, but inhibited the difluorophosphate-induced quenching of tryptophan fluorescence. The Na+ to Na++ divalent phosphate (monofluorophosphate) conformation and the Na+ to Na++ monovalent phosphate (difluorophosphate) conformations were compared using tryptophan quench reagents. These transitions had different apparent pKa values and different phenylglyoxal sensitivities consistent with monovalent phosphate and divalent phosphate interacting with the cotransporter at separate sites.