THE GAMMA-SUBUNIT IS A SPECIFIC COMPONENT OF THE NA,K-ATPASE AND MODULATES ITS TRANSPORT FUNCTION

The role of small, hydrophobic peptides that are associated with ion p umps or channels is still poorly understood. By using the Xenopus oocy te as an expression system, we have characterized the structural and f unctional properties of the gamma peptide which co-purifies with Na,K- ATPase. Immuno-radiolabeling of epitope-tagged gamma subunits in intac t oocytes and protease protection assays show that the gamma peptide i s a type I membrane protein lacking a signal sequence and exposing the N-terminus to the extracytoplasmic side, Go-expression of the rat or Xenopus gamma subunit with various proteins in the oocyte reveals that it specifically associates only with isozymes of Na,K-ATPase. The gam ma peptide does not influence the formation and cell surface expressio n of functional Na,K-ATPase alpha-beta complexes, On the other hand, t he gamma peptide itself needs association with Na,K-ATPase in order to be stably expressed in the oocyte and to be transported efficiently t o the plasma membrane, gamma subunits do not associate with individual alpha or beta subunits but only interact with assembled, transport-co mpetent alpha-beta complexes, Finally, electrophysiological measuremen ts indicate that the gamma peptide modulates the K+ activation of Na,K pumps, These data document for the first time the membrane topology, the specificity of association and a potential functional role for the gamma subunit of Na,K-ATPase.