Unusual degradation of α-βcomplexes in Xenopus oocytes by β-subunits of Xenopus gastric H-K-ATPase

The catalytic alpha-subunit of oligomeric P-type ATPases such as Na-K-ATPase and H-K-ATPase requires association with a beta-subunit after synthesis in the endoplasmic reticulum (ER) to become stably expressed and functionally active. In this study, we have expressed the beta-subunit of Xenopus gastric H-K-ATPase (beta HK) in Xenopus oocytes together with alpha-subunits of H-K-ATPase (alpha HK) or Na-K-ATPase (alpha NK) and have followed the biosynthesis, assembly, and cell surface expression of functional pumps. Immunoprecipitations of Xenopus beta HK from metabolically labeled oocytes show that it is well expressed and, when synthesized without alpha-subunits, can leave the ER and become fully glycosylated. Xenopus beta HK can associate with both coexpressed alpha HK and alpha NK, but the alpha-beta complexes formed are degraded rapidly in or close to the ER and do not produce functional pumps at the cell surface as assessed by Rb-86 uptake. A possible explanation of these results is that Xenopus beta HK may contain a tissue-specific signal that is important in the formation or correct targeting of functional alpha-beta complexes in the stomach but that cannot be recognized in Xenopus oocytes and in consequence leads to cellular degradation of the alpha-beta complexes in this experimental system.