A functional interaction between CHIF and Na-K-ATPase:: implication for regulation by FXYD proteins

Like the gamma- subunit of Na- K- ATPase, the corticosteroid hormone- induced factor (CHIF) is a member of the FXYD family of one- transmembrane- segment proteins. Both CHIF and two splice variants of gamma, gamma(a) and gamma(b), are expressed in the kidney. Immunolocalization experiments demonstrate mutually exclusive expression of CHIF and gamma in different nephron segments. Specific coimmunoprecipitation experiments demonstrate the existence in kidney membranes of the complexes alpha/beta/gamma(a), alpha/beta/gamma(b), and alpha/beta/CHIF and exclude mixed complexes such as alpha/beta/gamma(a)/gamma(b) and alpha/beta/gamma/CHIF. CHIF has been expressed in HeLa cells harboring the rat alpha(1)-subunit of Na- K- ATPase. Rb-86 flux experiments demonstrate that CHIF induces a two-to threefold increase in apparent affinity for cytoplasmic Na (K'(Na)) but does not affect affinity for extracellular K (Rb) ions (K'(K)) or V-max. Measurements of Na- K- ATPase using isolated membranes show similar but smaller effects of CHIF on K'(Na), whereas K'(K) and K'(ATP) are unaffected. The functional effects of CHIF differ from those of gamma. An implication of these findings is that other FXYD proteins could act as tissue-specific modulators of Na-K-ATPase.