Acute regulation of NHE3 by protein kinase A requires a multiprotein signal complex

Biochemical and cellular experiments in fibroblasts have established the re quirement for a member of the PDZ motif Na+/H+ exchanger regulatory factor family of proteins (NHERF and NHERF2) in cAMP-mediated phosphorylation and inhibition of NHE3 activity. NHERF interacts with the actin cytoskeleton th rough the scaffolding protein ezrin to target a multiprotein signal complex to the plasma membrane. Recent experiments have focused on elements of thi s model. First. using specific antibodies. NHERF was identified in the rena l proximal tubule, where it colocalized with ezrin and NHE3. NHERF2 was see n in glomeruli. the renal vasculature. and collecting duct cells, where it colocalized with ROMK. This distinct nephron localization suggests differen t physiologic roles for NHERF and NHERF2. Second, the signal-complex model of protein kinase A regulation of NHE3 developed in fibroblasts has been ex tended to epithelial cells by the development of a dominant-negative opossu m kidney cell line expressing an ezrin binding domain-deficient truncation of NHERF. Preliminary studies indicate that these cells have normal basal N a+/H+ exchanger activity but a blunted inhibitory response to cAMP. Third. biochemical. biophysical, and cell experiments have indicated that NHERF hi nds to itself in a head-to-head configuration. raising the possibility that dimerization may alter the availability of active NHERF. The potential rol e of the NHERF proteins in the kidney has been expanded by recent studies i ndicating their involvement in the membrane targeting, trafficking. sorting , and regulation of a range of other transporters. receptors. and signaling proteins. NHERF and related PDZ-containing proteins may serve as adapters for regulation of renal transporters.