Heterologous expression of Na+-K+-ATPase in insect cells:: intracellular distribution of pump subunits

The Na+-K+-ATPase is a heterodimeric plasma membrane protein responsible for cellular ionic homeostasis in nearly all animal cells. It has been shown that some insect cells (e.g., High Five cells) have no (or extremely low) Na+-K+-ATPase activity. We expressed sheep kidney Na+-K+-ATPase alpha- and beta -subunits individually and together in High Five cells via the baculovirus expression system. We used quantitative slot-blot analyses to determine that the expressed Na+-K+-ATPase comprises between 0.5% and 2% of the total membrane protein in these cells. Using a five-step sucrose gradient (0.8-2.0 M) to separate the endoplasmic reticulum, Golgi apparatus, and plasma membrane fractions, we observed functional Na+ pump molecules in each membrane pool and characterized their properties. Nearly all of the expressed protein functions normally, similar to that found in purified dog kidney enzyme preparations. Consequently, the measurements described here were not complicated by an abundance of nonfunctional heterologously expressed enzyme. Specifically, ouabain-sensitive ATPase activity, [H-3]ouabain binding, and cation dependencies were measured for each fraction. The functional properties of the Na+-K+-ATPase were essentially unaltered after assembly in the endoplasmic reticulum. In addition, we measured ouabain-sensitive Rb-86(+) uptake in whole cells as a mean to specifically evaluate Na+-K+-ATPase molecules that were properly folded and delivered to the plasma membrane. We could not measure any ouabain-sensitive activities when either the alpha -subunit or beta -subunit were expressed individually. Immunostaining of the separate membrane fractions indicates that the alpha -subunit, when expressed alone, is degraded early in the protein maturation pathway (i.e., the endoplasmic reticulum) but that the beta -subunit is processed normally and delivered to the plasma membrane. Thus it appears that only the alpha -subunit has an oligomeric requirement for maturation and trafficking to the plasma membrane. Furthermore, assembly of the alpha-beta heterodimer within the endoplasmic reticulum apparently does not require a Na+ pump-specific chaperone.