ROLE OF THE TRANSMEMBRANE AND EXTRACYTOPLASMIC DOMAIN OF BETA-SUBUNITS IN SUBUNIT ASSEMBLY, INTRACELLULAR-TRANSPORT, AND FUNCTIONAL EXPRESSION OF NA,K-PUMPS

The ubiquitous Na,K- and the gastric H,K-pumps are heterodimeric plasm a membrane proteins composed of an alpha and a beta subunit. The H,K-A TPase beta subunit (betaHK) can partially act as a surrogate for the N a,K-ATPase beta subunit (betaNK) in the formation of functional Na,K-p umps (Horisberger et al., 1991. J Biol. Chem. 257:10338-10343). We hav e examined the role of the transmembrane and/or the ectodomain of beta NK in (a) its ER retention in the absence of concomitant synthesis of Na,K-ATPase alpha subunits (alphaNK) and (b) the functional expression of Na,K-pumps at the cell surface and their activation by external K. We have constructed chimeric proteins between Xenopus betaNK and rab bit betaHK by exchanging their NH2-terminal plus transmembrane domain with their COOH-terminal ectodomain (betaNK/HK, betaHK/NK). We have ex pressed these constructs with or without coexpression of alphaNK in th e Xenopus oocyte. In the absence of alphaNK, Xenopus betaNK and all ch imera that contained the ectodomain of betaNK were retained in the ER while betaHK and all chimera with the ectodomain of betaHK could leave the ER suggesting that ER retention of unassembled Xenopus betaNK is mediated by a retention signal in the ectodomain. When coexpressed wit h alphaNK, only betaNK and betaNK/HK chimera assembled efficiently wit h alphaNK leading to similar high expression of functional Na,K-pumps at the cell surface that exhibited, however, a different apparent K+ a ffinity. BetaHK or chimera with the transmembrane domain of betaHK ass embled less efficiently with alphaNK leading to lower expression of fu nctional Na,K-pumps with a different apparent K+ affinity. The data in dicate that the transmembrane domain of betaNK is important for effici ent assembly with alphaNK and that both the transmembrane and the ecto domain of beta subunits play a role in modulating the transport activi ty of Na,K-pumps.