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A corticosteroid-induced gene expressing an "IsK-like" K+ channel activity in Xenopus oocytes
B Attali1, H Latter, N Rachamim
1Department of Membrane Research and Biophysics, Weizmann Institute of Science, Rehovot, Israel.
Abstract:
Screening a rat colon cDNA library for aldosterone-induced genes resulted in the molecular cloning of a cDNA whose corresponding mRNA is strongly induced in the colon by dexamethasone, aldosterone, and a low NaCl diet. A similar mRNA was detected in kidney papilla but not in brain, heart, or skeletal muscle. Xenopus laevis oocytes injected with cRNA synthesized from this clone, designated CHIF (channel-inducing factor), express a K(+)-specific channel activity. The biophysical, pharmacological, and regulatory characteristics of this channel are very similar to those reported before for IsK (minK). These include: slow (tau > 20 s) activation by membrane depolarization with a threshold potential above -50 mV, blockade by clofilium, inhibition by phorbol ester, and activation by 8-bromoadenosine 3',5'-cyclic monophosphate and high cytoplasmic Ca2+. The primary structure of this clone, however, shows no homology to IsK. Instead, CHIF exhibits > 50% similarity to two other short bitopic membrane proteins, phospholemman and the gamma subunit of Na+K(+)-ATPase. The data are consistent with the possibility that CHIF is a member of a family of transmembrane regulators capable of activating endogenous oocyte transport proteins.
Insights
Researchers identified a new gene, channel-inducing factor (CHIF), in rat colon that is induced by aldosterone and dexamethasone. This factor forms a potassium channel in oocytes, suggesting a role in regulating ion transport.
Area of Science:
- Molecular biology
- Physiology
- Ion channel research
Background:
- Aldosterone and dexamethasone regulate gene expression in the colon.
- Ion transport is crucial for maintaining electrolyte balance in the kidney and colon.
Purpose of the Study:
- To identify and characterize novel genes induced by aldosterone in the rat colon.
- To investigate the function of a newly cloned aldosterone-induced factor.
Main Methods:
- cDNA library screening of rat colon.
- mRNA expression analysis in various tissues.
- Xenopus laevis oocyte expression system.
- Electrophysiological recordings to characterize channel activity.
Main Results:
- A cDNA encoding channel-inducing factor (CHIF) was cloned, with its mRNA induced by aldosterone, dexamethasone, and low NaCl diet in colon and kidney papilla.
- CHIF expression in Xenopus oocytes resulted in K(+)-specific channel activity.
- The channel's properties (slow activation, blockade by clofilium, inhibition by phorbol ester, activation by cAMP and Ca2+) resemble IsK (minK).
- CHIF shares structural similarity with phospholemman and Na+/K+-ATPase gamma subunit, but not IsK.
Conclusions:
- CHIF represents a novel K(+)-specific channel or channel regulator.
- CHIF's characteristics suggest a role in regulating ion transport in the colon and kidney.
- CHIF may belong to a family of transmembrane proteins that modulate endogenous transport proteins.