An N-terminal sequence targets and tethers Na+ pump α2 subunits to specialized plasma membrane microdomains

Sodium pumps (alpha B dimers) with the alpha 1 isoform of the catalytic (alpha) subunit are expressed in all cells. Additionally, most cells express Na+ pumps with a second alpha isoform. For example, astrocytes and arterial myocytes also express Na+ pumps with the alpha 2 isoform. The alpha 2 pumps localize to plasma membrane ( PM) microdomains overlying junctional sarco-/endoplasmic reticulum (S/ER), but the alpha 1 pumps are more uniformly distributed. To study alpha 2 targeting, we expressed alpha 1/alpha 2 and alpha 2/alpha 1 chimeras and 1-90 and 1-120 amino acid N-terminal peptides in primary cultured mouse astrocytes. Immunocytochemistry revealed that alpha 2/alpha 1 ( but not alpha 1/alpha 2) chimeras markedly reduced native alpha 2 (i.e. were dominant negatives). N-terminal (1-120 and 1-90 amino acids) alpha 2 ( and alpha 3), but not alpha 1 peptides also targeted to the PM-S/ER junctions and were dominant negative for native alpha 2 in astrocytes and arterial myocytes. Thus alpha 2 and alpha 3 have the same targeting sequence. Ca2+ (fura-2) signals in astrocytes expressing the 1-90 alpha 2 peptide were comparable to signals in cells from alpha 2 null mutants (i.e. functionally dominant negative): 1 mu M ATP-evoked Ca2+ transients were augmented, and 100 nM ouabain-induced amplification was abolished. Amino acid substitutions in the 1-120 alpha 1 and alpha 2 constructs, and in full-length alpha 1, revealed that Leu-27 and Ala-35 are essential for targeting/tethering the constructs to PM-S/ER junctions.