The Ca2+/calmodulin binding domain of the Ca2+-ATPase linked to the Na+,K+-ATPase alters transport stoichiometry.

Using Xenopus oocytes as an expression system, we have investigated ion-transport and ouabain-binding properties of a chimeric ATPase (alpha1-CBD; Ishii and Takeyasu (1995) EMBO J. 14, 58-67) formed by the alpha1-subunit of chicken Na+,K(+)-ATPase (alpha1) and the calmodulin binding domain (CBD) of the rat plasma membrane Ca2(+)-ATPase. alpha1-CBD can be expressed and transported to the oocyte plasma membrane without the beta-subunit, and shows ouabain binding. In contrast to ouabain binding, this chimera requires the beta-subunit for its cation (Na+ and K+) transport activity. alpha1-CBD exhibits an altered stoichiometry of Na(+)-K+ exchange. A detailed analysis of 22Na+ efflux, 86Rb+ uptake, pump current and ouabain binding suggests that the chimeric molecule can operate in an electrically silent 2Na(+)-2K+ exchange mode and, with much lower probability, in its normal 3Na(+)-2K+ exchange mode.