AJP - Cell Physiology, Vol 257, Issue 5 C851-C858, Copyright © 1989 by American Physiological Society
A role for the beta-subunit in the expression of functional Na+-K+-ATPase in Xenopus oocytes
K. Geering, I. Theulaz, F. Verrey, M. T. Hauptle and B. C. Rossier
Institut de Pharmacologie de l'Universite de Lausanne, Switzerland.
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.
