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Regions of the Catalytic α Subunit of Na,K-ATPase Important for Functional Interactions with FXYD 2*

A. Zouzoulas,R. Blostein

Published 2006 in Journal of Biological Chemistry

ABSTRACT

The γ modulator (FXYD 2) is a member of the FXYD family of single transmembrane proteins that modulate the kinetic behavior of Na,K-ATPase. This study concerns the identification of regions in the α subunit that are important for its functional interaction with γ. An important effect of γ is to increase K+ antagonism of cytoplasmic Na+ activation apparent as an increase in KNa′ at high [K+]. We show that although γ associates with α1, α2, and α3 isoforms, it increases the KNa′ of α1 and α3 but not α2. Accordingly, chimeras of α1 and α2 were used to identify regions of α critical for the increased KNa′. As with α1 and α2, all chimeras associate with γ. Kinetic analysis of α2front/α1back chimeras indicate that the C-terminal (Lys907-Tyr1018) region of α1, which includes transmembrane (TM)9 close to γ, is important for the increase in KNa′. However, similar experiments with α1front/α2back chimeras indicate a modulatory role of the loop between TMs 7 and 8. Thus, as long as the α1 L7/8 loop is present, replacement of TM9 of α1 with that of α2 does not abrogate the γ effect on KNa′. In contrast, as long as TM9 is that of α1, replacement of L7/8 of α1 with that of α2 does not abolish the effect. It is suggested that structural association of the TM regions of α and FXYD 2 is not the sole determinant of this effect of FXYD on KNa′ but is subject to long range modulation by the extramembranous L7/8 loop of α.

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