| Abstract
| - Na,K- and H,K-ATPase (X,K-ATPase) α subunits need association with a β subunit for theirmaturation, but the authentic β subunit of nongastric H,K-ATPase α subunits has not been identified. Tobetter define α−β interactions in these ATPases, we coexpressed human, nongastric H,K-ATPase α (AL1)and Na,K-ATPase α1 (α1NK) as well as AL1−α1 and α1−AL1 chimeras, which contain exchanged M9and M10 membrane domains, together with each of the known β subunits in Xenopus oocytes and followedtheir resistance to cellular and proteolytic degradation and their ER exit. We show that all β subunits(gastric βHK, β1NK, β2NK, β3NK, or Bufo bladder β) can associate efficiently with α1NK, but onlygastric βHK, β2NK, and Bufo bladder β can form stably expressed AL1−β complexes that can leave theER. The trypsin resistance and the forces of subunit interaction, probed by detergent resistance, are lowerfor AL1−β complexes than for α1NK−β complexes. Furthermore, chimeric α1−AL1 can be stabilizedby β subunits, but α1-AL1−gastric βHK complexes are retained in the ER. On the other hand, chimericAL1−α1 cannot be stabilized by any β subunit. In conclusion, these results indicate that (1) none of theknown β subunits is the real partner subunit of AL1 but an as yet unidentified, authentic β should havestructural features resembling gastric βHK, β2NK, or Bufo bladder β and (2) β-mediated maturation ofα subunits is a multistep process which depends on the membrane insertion properties of α subunits aswell as on several discrete events of intersubunit interactions.
|
