A role for the beta-subunit in the expression of functional Na+-K+-ATPase in Xenopus oocytes.
In all cellular systems studied so far, the catalytic alpha- and the glycosylated beta-subunit of Na+-K+-ATPase are coordinately synthesized and are assembled into stoichiometric alpha, beta-complexes. In contrast to these data, in this study we show that the fully grown oocyte of Xenopus laevis synthesizes much less beta-subunit than alpha-subunit. The alpha-subunit produced in excess over the beta-subunit is membrane associated but highly trypsin sensitive and can be compared with the immature alpha-subunit population identified in epithelial cells immediately after synthesis (K. Geering, J. P. Kraehenbuhl, and B.C. Rossier, J. Cell Biol. 105: 2613-2619, 1987). The Xenopus oocyte thus turns out to be a unique system to study the functional role of the beta-subunit. Injection of beta-subunit-specific mRNA transcribed in vitro from a beta-cDNA clone (derived from Xenopus kidney, A6 cells) into oocytes results in translation of a glycosylated beta-subunit. The synthesis of this exogenous beta-subunit increases significantly the proportion of trypsin-resistant oocyte alpha-subunits able to perform cation-dependent conformational changes. In addition, 25-65% more ouabian binding sites are expressed at the plasma membrane in beta-mRNA-injected oocytes. In contrast, newly synthesized alpha-subunit translated after injection of size-fractionated mRNA enriched in alpha-mRNA remains trypsin sensitive as the oocyte alpha-subunit. These data suggest that association of the beta-subunit to the alpha-subunit provokes a structural rearrangement of the alpha-subunit that might be a first step toward the functional maturation of the Na+-K+-ATPase and its expression at the plasma membrane.
Geering
K
K
Institut de Pharmacologie de l'Université de Lausanne, Switzerland.
Theulaz
I
I
Verrey
F
F
Häuptle
M T
MT
Rossier
B C
BC
D000818:Animals; D002462:Cell Membrane / Q000201:enzymology; D004351:Drug Resistance; D005260:Female; D009865:Oocytes / Q000201:enzymology / Q000502:physiology*; D012333:RNA, Messenger / Q000235:genetics / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000096:biosynthesis / Q000502:physiology*; D014158:Transcription, Genetic; D014357:Trypsin / Q000494:pharmacology; D014981:Xenopus / Q000502:physiology*
D012333:RNA, Messenger; D014357:Trypsin; D000254:Sodium-Potassium-Exchanging ATPase