Membrane curvature alters the activation kinetics of the epithelial Na+/H+ exchanger, NHE3.
The epithelial Na(+)/H(+) exchanger, NHE3, was found to activate slowly following an acute cytosolic acidification. The sigmoidal course of activation could not be explained by the conventional two-state model, which postulates that activation results from protonation of an allosteric modifier site. Instead, mathematical modeling predicted the existence of three distinct states of the exchanger: two different inactive states plus an active form. The interconversion of the inactive states is rapid and dependent on pH, whereas the conversion between the second inactive state and the active conformation is slow and pH-independent but subject to regulation by other stimuli. Accordingly, exposure of epithelial cells to hypoosmolar solutions activated NHE3 by accelerating this latter transition. The number of surface-exposed exchangers and their association with the cytoskeleton were not affected by hypoosmolarity. Instead, NHE3 is activated by the membrane deformation, a result of cell swelling. This was suggested by the stimulatory effects of amphiphiles that induce a comparable positive (convex) deformation of the membrane. We conclude that NHE3 exists in multiple states and that different physiological parameters control the transitions between them.
Alexander
R Todd
RT
Department of Pediatrics, Program in Cell Biology, and Program in Computational Biology, Hospital for Sick Children, Toronto, Ontario M5G 1X8, Canada.
Malevanets
Anatoly
A
Durkan
Anne M
AM
Kocinsky
Hetal S
HS
Aronson
Peter S
PS
Orlowski
John
J
Grinstein
Sergio
S
D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000737:chemistry*; D004285:Dogs; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008954:Models, Biological; D009893:Opossums; D009994:Osmolar Concentration; D010766:Phosphorylation; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors / Q000378:metabolism*
D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; C069139:growth factor-activatable Na-H exchanger NHE-1