Physical and Functional Links between Anion Exchanger-1 and Sodium Pump

[摘要] Anion exchanger-1 (AE1) mediates chloride-bicarbonate exchange across the plasma membranes of erythrocytes and, via a slightly shorter transcript, kidney epithelial cells. On an omnivorous human diet, kidney AE1 is mainly active basolaterally in α -intercalated cells of the collecting duct, where it is functionally coupled with apical proton pumps to maintain normal acid-base homeostasis. The C-terminal tail of AE1 has an important role in its polarized membrane residency. We have identified the β 1 subunit of Na+,K+-ATPase (sodium pump) as a binding partner for AE1 in the human kidney. Kidney AE1 and β 1 colocalized in renal α -intercalated cells and coimmunoprecipitated (together with the catalytic α 1 subunit of the sodium pump) from human kidney membrane fractions. ELISA and fluorescence titration assays confirmed that AE1 and β 1 interact directly, with a K d value of 0.81 μ M. GST-AE1 pull-down assays using human kidney membrane proteins showed that the last 11 residues of AE1 are important for β 1 binding. siRNA-induced knockdown of β 1 in cell culture resulted in a significant reduction in kidney AE1 levels at the cell membrane, whereas overexpression of kidney AE1 increased cell surface sodium pump levels. Notably, membrane staining of β 1 was reduced throughout collecting ducts of AE1-null mouse kidney, where increased fractional excretion of sodium has been reported. These data suggest a requirement of β 1 for proper kidney AE1 membrane residency, and that activities of AE1 and the sodium pump are coregulated in kidney.

[学科分类] 泌尿医学