Processing, intracellular transport, and functional expression of endogenous and exogenous alpha-beta 3 Na,K-ATPase complexes in Xenopus oocytes.
The minimal functional Na,K-ATPase unit is composed of a catalytic alpha-subunit and a glycosylated beta-subunit. So far three putative beta-isoforms have been described, but only beta 1-isoforms have been identified clearly as part of a purified active enzyme complex. In this study we provide evidence that a putative beta 3-isoform might be the functional component of Xenopus oocyte Na,K-ATPase. beta 3-isoforms are expressed in the oocyte plasma membrane together with alpha-subunits, but beta 3-isoforms are synthesized to a lesser extent than alpha-subunits. The unassembled oocyte alpha-subunits accumulate in an immature trypsin-sensitive form most likely in the endoplasmic reticulum (ER). Injection of both beta 1- and beta 3-cRNA into oocytes abolishes the transport constraint of the oocyte alpha-subunit, renders it trypsin-resistant, and finally leads to an increased number of functional pumps at the plasma membrane. In addition, beta 3-isoforms as beta 1-isoforms depend on the concomitant synthesis of alpha-subunits to be able to leave the ER and to become fully glycosylated. Finally, alpha-beta 1 and alpha-beta 3 complexes expressed at the plasma membrane appear to have similar transport properties as assessed by ouabain binding, rubidium uptake, and electrophysiological measurements in oocytes coexpressing exogenous alpha 1- and beta 1- or beta 3-isoforms. Thus our data indicate that beta 3-isoforms have functional qualities similar to beta 1-isoforms. They can assemble and impose a structural reorganization to newly synthesized alpha-subunits which permits the exit from the ER and the expression of functional Na,K-pumps at the plasma membrane.
Jaunin
P
P
Institute of Pharmacology and Toxicology, University of Lausanne, Switzerland.
Horisberger
J D
JD
Richter
K
K
Good
P J
PJ
Rossier
B C
BC
Geering
K
K
D000818:Animals; D001692:Biological Transport; D002458:Cell Fractionation; D004591:Electrophoresis, Polyacrylamide Gel; D004721:Endoplasmic Reticulum / Q000201:enzymology; D005260:Female; D006031:Glycosylation; D006868:Hydrolysis; D007527:Isoenzymes / Q000096:biosynthesis / Q000378:metabolism*; D010063:Ovum / Q000201:enzymology*; D011233:Precipitin Tests; D011499:Protein Processing, Post-Translational; D000254:Sodium-Potassium-Exchanging ATPase / Q000096:biosynthesis / Q000378:metabolism*; D014357:Trypsin / Q000737:chemistry; D014981:Xenopus
D007527:Isoenzymes; D014357:Trypsin; D000254:Sodium-Potassium-Exchanging ATPase