Role of the intracellular domain of the beta subunit in Na,K pump function.

The catalytic alpha subunit of the (Na,K)- and (H,K)-ATPases needs to be coexpressed with a beta subunit in order to produce cation transport activity. Although the isoform of the beta subunit is known to influence the functional characteristics of the Na,K pump, the role of the different domains of the beta subunit is not fully understood. We have studied the function of a Na,K pump resulting from the expression of a wild-type alpha subunit with a N-terminally truncated mutant of the beta subunit using the two-electrode voltage clamp and the cut-open oocyte techniques. While the maximal activity, measured as the K+-activated outward current, was not significantly altered, the beta N-terminal truncation induced an ouabain-sensitive conductance in the absence of extracellular K+. The voltage dependence of the ouabain-sensitive charge distribution indicated that in the Na/Na exchange conditions, the E1-E2 conformation equilibrium was shifted towards the E2 conformation, a change resulting from alteration of both the forward and the backward reaction rate. Removal of the intracellular domain of the beta subunit modifies several aspects of the whole enzyme function by a mechanism that must imply the state of the extracellular and/or transmembrane parts of the alpha/beta subunit complex.