ISOFORM-SPECIFIC INTERACTIONS OF NA,K-ATPASE SUBUNITS ARE MEDIATED VIA EXTRACELLULAR DOMAINS AND CARBOHYDRATES

The functional unit of the Na,K-ATPase consists of a catalytic alpha s ubunit noncovalently linked with a glycoprotein subunit, beta. Using o uabain binding assays and immunoprecipitation of rodent alpha/beta com plexes, we show here that all six possible isozymes between three alph a and two beta isoforms fan be formed in Xenopus oocytes. Two isoform- specific differences in alpha/beta interactions are observed: (i) alph a 1/beta 1 and alpha 2/beta 2 complexes, in contrast to alpha 1/beta 2 complexes, are stable against Triton X-100-mediated dissociation, and (ii) beta 2 subunits must carry N-glycans to combine with alpha 1 but not with alpha 2. The interacting surfaces are mainly exposed to the extracellular side because coexpression of a truncated beta 1 subunit comprising the ectodomain results in assembly with alpha 1 and alpha 2 , but not with alpha 3; the beta 2 ectodomain combines with alpha 2 on ly, A chimera consisting of 81% and 19% of the alpha 1 N terminus and alpha 2 C terminus, respectively, behaves like alpha 2 and coprecipita tes with the beta 2 ectodomain, In contrast, the reciprocal chimera do es not coprecipitate with the beta 2 ectodomain. These results provide evidence for a selective interaction of Na,K-ATPase alpha and beta su bunits.