The epithelial brush-border Na/H Exchanger NHE3 associates with the actin cytoskeleton by binding to ezrin directly and via PDZ domain containing NHERF proteins

1. The Na+/H+ exchanger NHE3 associates with the actin cytoskeleton by binding ezrin both directly and indirectly. Both types of interaction are necessary for acute regulation of NHE3. Most NHE3 acute regulation is by changes in trafficking by effects on exocytosis and for endocytosis. However, NHE3 activity also can be regulated without changing the surface expression of NHE3 (change in turnover number). 2. Direct ezrin binding to NHE3: A positive amino acid cluster in the α-helical juxtamembrane region in the COOH-terminus of NHE3 (aa K516, R520, R527) is necessary for binding to the FERM domain III of ezrin. The direct ezrin binding of NHE3 is necessary for many aspects of basal trafficking, including basal exocytosis, delivery from the synthetic pathway and movement of NHE3 in BB, which probably contributes to endocytosis over a prolonged period. 3. Indirect ezrin binding to NHE3: PDZ domain containing proteins NHERF1 or NHERF2 as intermediates in linking NHE3 to ezrin are necessary for many aspects of NHE3 regulation. NHERF/ERM binding to NHE3 occurs in the cytosolic domain of NHE3 between aa 475-689. This NHERF binding is involved in NHE3 complex formation and restricts NHE3 mobility in the BB. However, it is dynamic, for instance changing in some cases of signaling. Also NHERF binding is necessary for LPA stimulation of NHE3 and inhibition of NHE3 by Ca2+, cAMP and cGMP.