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 io n-transport and ouabain-binding properties of a chimeric ATPase (alpha (1)-CBD; Ishii acid Takeyasu (1995) EMBO J. 14, 58-67) formed by the a lpha(1)-subunit of chicken Na+,K+-ATPase (alpha(1)) and the calmodulin binding domain (CBD) of the rat plasma membrane Ca2+-ATPase, alpha(1) -CBD can be expressed and transported to the oocyte plasma membrane wi thout the beta-subunit, and shows ouabain binding, In contrast to ouab ain binding, this chimera requires the beta-subunit for its cation (Na + and K+) transport activity, alpha(1)-CBD exhibits an altered stoichi ometry of Na+-K+ exchange. A detailed analysis of Na-22(+) efflux, Rb- 86(+) uptake, pump current and ouabain binding suggests that the chime ric molecule can operate in an electrically silent 2Na(+)-2K(+) exchan ge mode and, with much lower probability, in its normal 3Na(+)-2K(+) e xchange mode. (C) 1997 Federation of European Biochemical Societies.