ROLE OF BETA-SUBUNIT DOMAINS IN THE ASSEMBLY, STABLE EXPRESSION, INTRACELLULAR ROUTING, AND FUNCTIONAL-PROPERTIES OF NA,K-ATPASE

The beta-subunit of Na,K-ATPase (beta NK) interacts with the catalytic alpha-subunit (alpha NK) in the ectodomain, the transmembrane, and th e cytoplasmic domain. The functional significance of these different i nteractions was studied by expressing alpha NK in Xenopus oocytes alon g with N-terminally modified beta NK or with chimeric beta NK/ beta H, K-ATPase (beta HK), Complete truncation of the beta NK N terminus allo ws for cell surface-expressed, functional Na,K-pumps that exhibit, how ever, reduced apparent K+ and Na+ affinities as assessed by electrophy siological measurements. A mutational analysis suggests that these fun ctional effects are not related to a direct interaction of the beta N terminus with the alpha NK but rather that N-terminal truncation induc es a conformational change in another functionally relevant beta domai n. Comparison of the functional properties of alpha NK.beta NK, alpha NK.beta HK, or alpha NK.beta NK/beta HK complexes shows that the effec t of the beta NK on K+ binding is mainly mediated by its ectodomain. F inally, beta HK/NK containing the transmembrane domain of PHR produces stable but endoplasmic reticulum-retained alpha NK.beta complexes, wh ile alpha NK/beta HK complexes can leave the ER but exhibit reduced ou abain binding capacity and transport function. Thus, interactions of b oth the transmembrane and the ectodomain of beta NK with alpha NK are necessary to form correctly folded Na,K-ATPase complexes that can be t argeted to the plasma membrane and/or become functionally competent. F urthermore, the beta N terminus plays a role in the beta-subunit's fol ding necessary for correct interactions with the alpha-subunit.