Na(+)-Ca2+,K+ exchange in bovine retinal rod outer segments: quantitative characterization of normal and reversed mode

Binding of the retinal rod Na+/Ca2+-K+ exchanger to the cGMP-gated channel indicates local Ca(2+)-signaling in vertebrate photoreceptors

Ca(2+) ions enter the outer segment of rod or cone photoreceptors exclusively through the cGMP-gated channel and are extruded by the Na(+)/Ca(2+)-K(+) exchanger. Recent evidence indicates that in the plasma membrane, the Na(+)/Ca(2+)-K(+) exchanger is associated with the cGMP-gated channel. In this contribution, the possible physiologic significance of this protein complex is considered. Based on recent experimental evidence, the possibility of a direct functional interaction between the cGMP-gated channel and the Na(+)/Ca(2+)-K(+) exchanger is discussed. Furthermore, a quantitative estimation of the cytoplasmic Ca(2+) diffusion at the cGMP-gated channel indicates that Ca(2+) diffusion is largely confined to the complex of the cGMP-gated channel and the associated Na(+)/Ca(2+)-K(+) exchanger molecules.

Mutual inhibition of the dimerized Na/Ca-K exchanger in rod photoreceptors

In the dark, rod photoreceptors sustain a continuous influx of Na and Ca ions through the cGMP-gated channels of the rod outer segments (ROS). Whereas Na ions are extruded in the inner segment by the Na-pump, Ca ions are extruded already in the ROS by Na/Ca-K exchange. Our previous findings indicate that in the ROS plasma membrane, exchanger and channel form a complex of two exchangers associated per channel. Here, we report evidence of a novel regulatory mechanism of the dimerized exchanger, based on the following findings: (1), thiol-specific cross-linking with dimaleimides resulted in an increase of the Na/Ca-K exchange activity which correlated with the size of the cross-linking reagent, i.e., with increasing separation of the monomers in a dimerized exchanger; (2), partial proteolysis of the exchanger also increased the exchange rate by about a factor of two; (3), disintegration of the channel-exchanger complex by solubilization of the ROS membranes and preparation of proteoliposomes resulted in a twofold enhancement of the exchange rate; however (4), partial proteolysis of proteoliposomes, in which the exchanger molecules exist as monomers, did not result in any enhancement of the exchange rate. These findings suggest an inhibitory protein domain at the contact site of the dimerized exchanger. The physiological implication of this inference will be discussed in terms of a potential allosteric regulation of the exchanger in the channel-exchanger complex.

Direct evidence of Na+/Ca2+ exchange in squid rhabdomeric membranes

Na+/Ca2+ exchange has been investigated in squid (Loligo pealei) rhabdomeric membranes. Ca2+-containing vesicles have been prepared from purified rhabdomeric membranes by extrusion through polycarbonate filters of 1-micrometer pore size. After removal of external Ca2+, up to 90% of the entrapped Ca2+ could be specifically released by the addition of Na+; this finding indicates that most of the vesicles contained Na+/Ca2+ exchanger. The Na+-induced Ca2+ efflux had a half-maximum value (K1/2) of approximately 44 mM and a Hill coefficient of approximately 1.7. The maximal Na+-induced Ca2+ efflux was approximately 0.6 nmol Ca2+. s-1. mg protein-1. Similar Na+-induced Ca2+ effluxes were measured if K+ was replaced with Li+ or Cs+. Vesicles loaded with Ca2+ by Na+/Ca2+ exchange also released this Ca2+ by Na+/Ca2+ exchange, suggesting that Na+/Ca2+ exchange operated in both forward and reverse modes. Limited proteolysis by trypsin resulted in a rate of Ca2+ efflux enhanced by approximately fivefold when efflux was activated with 95 mM NaCl. For vesicles subjected to limited proteolysis by trypsin, Na+/Ca2+ exchange was characterized by a K1/2 of approximately 25 mM and a Hill coefficient of 1.6. For these vesicles, the maximal Na+-induced Ca2+ efflux was about twice as great as in control vesicles. We conclude that Na+/Ca2+ exchange proteins localized in rhabdomeric membranes mediate Ca2+ extrusion in squid photoreceptors.

The Na/Ca-K exchanger of rod photoreceptor exists as dimer in the plasma membrane

The oligomeric state of the Na/Ca-K exchanger in the plasma membrane of bovine photoreceptors was investigated using chemical cross-linking techniques. In the natural membrane, virtually all Na/Ca-K exchanger could be cross-linked mainly to a complex having an apparent molecular mass of 490 kDa by cupric phenanthroline catalyzed disulfide bonding as evidenced by Western blotting. Stable cross-links of the exchanger were also obtained with the thiol-specific reagent N,N'-p-phenylidenedimaleimide. Neuraminidase treatment reduced the apparent molecular mass of the highly glycosylated Na/Ca-K exchanger and of the 490 kDa cross-link product by 50 and 85 kDa, respectively. DL-1,4-Bismaleimido-2,3-butanediol (BMBD), a novel cleavable dimaleimide, was synthesized in order to produce cross-links that were stable to reductive conditions. Purification of the BMBD cross-linked exchanger followed by two-dimensional SDS polyacrylamide electrophoresis identified the cross-linked homodimers of the exchanger. There was no indication of higher oligomers, suggesting that the exchanger exists as a dimer in the plasma membrane. Hydrodynamic properties of the detergent-solubilized exchanger were determined by velocity sedimentation and gel filtration chromatography. The Triton X-100-solubilized exchanger ran as a single species having a Stokes radius of 10.0 nm, a sedimentation coefficient of 5.4 S, and a partial specific volume of 0.74 mL/g in Triton X-100. Similar results were obtained for the CHAPS-solubilized exchanger. A molecular mass of 236 and 205 kDa was calculated for the exchanger-detergent complex and the detergent-free protein, respectively. Neuraminidase treatment further reduced the molecular mass of the exchanger indicating that glycosylation contributes significantly to the mass of the exchanger. Cross-links of the exchanger were not detected if cross-linking was attempted after solubilization in 10 mM CHAPS. However, after reconstitution of the purified exchanger into soybean phosphatidylcholine vesicles, chemical cross-linking yielded again dimers. On the basis of these cross-linking and hydrodynamic studies, we conclude that the exchanger exists as a homodimer in the rod outer segment plasma membrane but dissociates into a monomer when solubilized in detergent.