Rt. Szerencsei et al., Stoichiometry of the retinal cone Na/Ca-K exchanger heterologously expressed in insect cells: Comparison with the bovine heart Na/Ca exchanger, BIOCHEM, 40(20), 2001, pp. 6009-6015
The transport stoichiometry is an essential property of antiporter and symp
orter transport proteins. In this study, we determined the transport stoich
iometry of the retinal cone potassium-dependent Na/Ca exchanger (NCKX) expr
essed in sodium-loaded cultured insect cells. The Na/Ca and Rb/Ca coupling
ratios were obtained by direct measurements of the levels of Rb-86 and Ca-4
5 uptake and sodium release associated with reverse Na/Ca exchange. Rb/Ca c
oupling ratios of 0.98 [standard deviation (SD) of 0.12, 15 observations] a
nd 0.92 (SD of 0.12, 13 observations) were obtained for the chicken and hum
an retinal cone NCKX, respectively. Na/Ca coupling ratios of 4.11 (SD of 0.
24, 10 observations) and 3.98 (SD of 0.34, 15 observations) were obtained f
or the chicken and human retinal cone NCKX, respectively, whereas a lower a
verage coupling ratio of 3.11 (SD of 0.34, 10 observations) was obtained wi
th cells expressing the bovine Na/Ca exchanger (NCX1). These results are co
nsistent with a 4Na/1Ca + 1K stoichiometry for retinal cone NCKX. High Five
cells expressing full-length dolphin rod NCKX, Caenorhabditis elegans NCKX
, or bovine rod NCKX from which the two large hydrophilic loops were remove
d all showed a significant calcium-dependent 86Rb uptake, whereas no calciu
m-dependent 86Rb uptake was observed in cells expressing bovine NCX1. The c
alcium dependence of 45Ca uptake yielded values between 1 and 2.5 muM for t
he external calcium dissociation constant of the different NCKX proteins st
udied here.