Different ionic conditions prompt NHE2 and NHE3 translocation to the plasma membrane

We tested whether NHE3 and NHE2 Na(+)/H(+) exchanger isoforms were recruited to the plasma membrane (PM) in response to changes in ion homeostasis. NHE2-CFP or NHE3-CFP fusion proteins were functional Na(+)/H(+) exchangers when transiently expressed in NHE-deficient PS120 fibroblasts. Confocal morphometry of cells whose PM was labeled with FM4-64 measured the fractional amount of fusion protein at the cell surface. In resting cells, 10-20% of CFP fluorescence was at PM and stable over time. A protocol commonly used to activate the Na(+)/H(+) exchange function (NH(4)-prepulse acid load sustained in Na(+)-free medium), increased PM percentages of PM NHE3-CFP and NHE2-CFP. Separation of cellular acidification from Na(+) removal revealed that only NHE3-CFP translocated when medium Na(+) was removed, and only NHE2-CFP translocated when the cell was acidified. NHE2/NHE3 chimeric proteins demonstrate that the Na(+)-removal response element resides predominantly in the NHE3 cytoplasmic tail and is distinct from the acidification response sequence of NHE2.