MUTATION OF A CONSERVED PROLINE RESIDUE IN THE BETA-SUBUNIT ECTODOMAIN PREVENTS NA-K+-ATPASE OLIGOMERIZATION()

A highly conserved sequence motif (4 tyrosines and 1 proline: YYPYY) o f the Na+-K+-adenosinetriphosphatase (ATPase) beta1-subunit ectodomain has been mutagenized to study its possible role in alpha/beta-assembl y and sodium pump function. Single as well as double tyrosine mutants (tyrosine to phenylalanine: Y to F) of Xenopus laevis beta1-subunits a re able to associate with alpha1-subunits and form functional Na-K pum ps at the plasma membrane that are indistinguishable from wild-type al pha1,beta1-Na-K pumps (as assessed by measurements of ouabain binding, Rb-86 flux, Na-K pump current, and activation by external potassium). In contrast, a single proline mutation (proline to glycine: P244G) re duced by >90% the proper assembly and function of Na+-K+-ATPase, despi te a normal rate of synthesis and core glycosylation. Our data indicat e that proline-244 plays a critical role in the proper folding of the beta-subunit and its ability to associate efficiently with the alpha1- subunit in the endoplasmic reticulum.