Prolonged exposure (5h) of oocytes overexpressing the intestinal peptide transporter PEPT1 to the dipeptide Gly-Gln (GQ, 5 mM) resulted in a decrease of PEPT1-mediated transport current and of membrane capacitance (surface area). These effects could be effectively blocked by co-exposure of oocytes to cytochalasin D suggesting endocytotic removal of PEPT1 from the plasma membrane. The intracellular accumulation of GQ or the depolarisation of the cell membrane resulting from the cotransport of protons could be excluded as possible trigger mechanisms of endocytosis. pH measurements after long-term exposure to substrate showed a decrease of the cytosolic pH by 0.5 unit, the resulting decrease of driving force had however only a minor direct effect on the transport rate. Coexpression of PEPT1 with the sodium-coupled glucose transporter SGLT1 indicated that the activity of the latter remained unaltered after GQ incubation, whereas the coexpressed proton-coupled amino acid transporter PAT1 was inhibited similar to PEPT1. The data show that moderate cytosolic acidification triggered by an unknown mechanism selective endocytosis of proton-coupled transporters.

This work was supported by the DFG (KO1605/2-3 to G. Kottra).