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) with 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 the rabbit gastric H,K-ATPase (beta HK), by cRNA injection in Xenopus oocyte. We studied the K+ activation kinetics by measuring the Na,K-pump current induced by external K+ under voltage clamp conditions. The endogenous oocyte Na,K-ATPase was selectively inhibited, taking advantage of the large difference in ouabain sensitivity between Xenopus and Bufo Na,K pumps. The K+ half-activation constant (K1/2) was higher in the alpha 1 beta 3NaK than in the alpha 1 beta 1NaK groups in the presence of external Na+, but there was no significant difference in the absence of external Na+. Association of alpha 1 and beta HK subunits produced active Na,K pumps with a much lower apparent affinity for K+ both in the presence and in the absence of external Na+. The voltage dependence of the K1/2 for external K+ was similar with the three beta subunits. Our results indicate that the beta subunit has a significant influence on the ion transport activity of the Na,K pump. The small structural differences between the beta 1NaK and beta 3NaK subunits results 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 directly related to the K+ binding site. In contrast, association of an alpha 1 subunit with a beta HK subunit results in a Na,K pump in which the K+ binding or translocating mechanisms are altered since the apparent affinity for external K+ is affected even in the absence of external Na+.
Jaisser
F
F
Institut de Pharmacologie et de Toxicologie, Lausanne, Switzerland.
Jaunin
P
P
Geering
K
K
Rossier
B C
BC
Horisberger
J D
JD
D000818:Animals; D001711:Biotransformation / Q000187:drug effects; D002024:Bufo marinus; D004594:Electrophysiology; D007536:Isomerism; D007700:Kinetics; D009865:Oocytes / Q000201:enzymology; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D018075:RNA, Complementary / Q000096:biosynthesis; D011817:Rabbits; D012964:Sodium / Q000502:physiology; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects / Q000502:physiology*; D014981:Xenopus
D018075:RNA, Complementary; D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium