MODULATION OF THE NA,K-PUMP FUNCTION BY BETA-SUBUNIT ISOFORMS

To study the role of the Na,K-ATPase beta subunit in the ion transport . activity, we have coexpressed the Bufo alpha 1 subunit (alpha 1) wit h three different isotypes of beta subunits, the Bufo Na,K-ATPase beta 1 (beta 1NaK) or beta 3 (beta 3NaK) subunit or the beta subunit of th e rabbit gastric H,K-ATPase (beta HK), by cRNA injection in Xenopus oo cyte. We studied the K+ activation kinetics by measuring the Na,K-pump current induced by external K+ under voltage clamp conditions. The en dogenous oocyte Na,K-ATPase was selectively inhibited, taking advantag e of the large difference in ouabain sensitivity between Xenopus and B ufo Na,K pumps. The K+ half-activation constant (K-1/2) was higher in the alpha 1 beta 3NaK than in the alpha 1 beta 1NaK groups in the pres ence of external Na+, but there was no significant difference in the a bsence of external Na+. Association of alpha 1 and beta HK subunits pr oduced active Na,K pumps with a much lower apparent affinity for K+ bo th in the presence and in the absence of external Na+. The voltage dep endence of the K-1/2 for external K+ was similar with the three beta s ubunits. Our results indicate that the beta subunit has a significant influence on the ion transport activity of the Na,K pump. The small st ructural differences between the beta 1NaK and beta 3NaK subunits resu lts in a difference of the apparent affinity for K+ that is measurable only in the presence of external Na+, and thus appears not to be dire ctly related to the K+ binding site. In contrast, association of an al pha 1 subunit with a beta HK subunit results in a Na,K pump in which t he K+ binding or translocating mechanisms are altered since the appare nt affinity for external K+ is affected even in the absence of externa l Na+.