A functional interaction between CHIF and Na-K-ATPase: implication for regulation by FXYD proteins
Haim Garty1, Moshit Lindzen, Rosemarie Scanzano
1Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel.
American Journal of Physiology. Renal Physiology
|September 10, 2002
Summary
The corticosteroid hormone-induced factor (CHIF) modulates sodium-potassium ATPase (Na-K-ATPase) by increasing its affinity for sodium. This FXYD protein exhibits distinct functional effects compared to the gamma subunit, suggesting tissue-specific regulation.
Area of Science:
- Biochemistry
- Cell Biology
- Physiology
Background:
- The FXYD protein family includes the gamma subunit of Na-K-ATPase and corticosteroid hormone-induced factor (CHIF).
- CHIF and gamma subunit variants (gamma(a), gamma(b)) are expressed in the kidney, with mutually exclusive localization in different nephron segments.
- Na-K-ATPase is a crucial ion pump involved in maintaining cellular homeostasis.
Purpose of the Study:
- To investigate the functional role of CHIF in modulating Na-K-ATPase activity.
- To compare the effects of CHIF with those of the gamma subunit on Na-K-ATPase.
- To explore the implications of CHIF's function for tissue-specific regulation of Na-K-ATPase.
Main Methods:
- Immunolocalization to determine protein expression patterns in nephron segments.
- Coimmunoprecipitation assays to identify protein complex formation in kidney membranes.
- ⁸⁶Rb flux experiments in HeLa cells expressing Na-K-ATPase alpha(1)-subunit and CHIF.
- Enzyme kinetic measurements of Na-K-ATPase activity using isolated kidney membranes.
Main Results:
- CHIF and gamma subunits exhibit distinct, non-overlapping expression patterns within kidney nephrons.
- Coimmunoprecipitation confirmed the formation of specific alpha/beta/gamma and alpha/beta/CHIF complexes, excluding mixed complexes.
- CHIF significantly increased the apparent affinity for cytoplasmic Na (K'(Na)) of Na-K-ATPase by two- to threefold, without altering affinity for extracellular K (K'(K)) or Vmax.
- Isolated membrane studies showed similar, though less pronounced, effects of CHIF on K'(Na), with no impact on K'(K) or K'(ATP) affinity.
Conclusions:
- CHIF acts as a functional modulator of Na-K-ATPase, primarily by altering its affinity for intracellular sodium.
- The functional profile of CHIF differs from that of the gamma subunit, highlighting distinct regulatory roles.
- These findings suggest that other FXYD proteins may serve as tissue-specific regulators of Na-K-ATPase activity.
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