Identification of the sodium-calcium exchanger as the major ricin-binding glycoprotein of bovine rod outer segments and its localization to the plasma membrane

Rapid purification and characterization of protein kinase C from bovine retinal rod outer segments

A rapid FPLC procedure for the purification of protein kinase C from bovine rod outer segments is described. The enzyme is essentially homogeneous after purification and exhibits a molecular mass of approximately 85 kDa, as determined by SDS/PAGE. From its chromatographic behaviour on hydroxyapatite, and from Western-blotting experiments using isoenzyme-specific antibodies, we were able to identify the bovine rod outer segment protein kinase C as being of the alpha or type-III form. The purified protein kinase C has a specific activity of 1066 nmol 32P.min-1.mg protein-1, and shows a 30-fold activation upon the addition of the effectors Ca2+, PtdSer and 1,2-diacylglycerol. Arachidonic acid and linoleic acid were also found to enhance significantly the activity of the purified enzyme.

Calcium and light adaptation in retinal photoreceptors

In the past several years there has been great progress in the understanding of the phototransduction process in retinal photoreceptors. Recently, this knowledge has expanded and we now understand the mechanism of background light adaptation in these cells and the role that calcium has to play.

The sodium-calcium exchanger of bovine rod photoreceptors: K(+)-dependence of the purified and reconstituted protein

The K(+)-dependence of the rod photoreceptor sodium-calcium exchanger was investigated using the Ca2(+)-sensitive dye arsenazo III after reconstitution of the purified protein into proteoliposomes. The uptake of Ca2+ by Na(+)-loaded liposomes was found to be greatly enhanced by the presence of external K+ (EC50 approximately 1 mM) in a Michaelis-Menten manner, suggesting that one K+ ion is involved in the transport of one Ca2+ ion. We also found a minimal degree of Ca2+ uptake in the total absence of K+. Other alkali cations, notably Rb+ and, to a lesser extent, Cs+, were also able to stimulate Na(+)-Ca2+ exchange. We also investigated the K(+)-dependence of the photoreceptor Na(+)-Ca2+ exchanger by determining the effects of electrochemical K+ gradients on the Na(+)-activated Ca2+ efflux from proteoliposomes. We found that, under conditions of membrane voltage clamp with FCCP, inwardly directed electrochemical K+ gradients (i.e., K0+ greater than Ki+) inhibited, whereas an outwardly directed electrochemical K+ gradient (i.e., Ki+ greater than K0+) enhanced, Na(+)-dependent Ca2+ efflux, consistent with the notion that K+ is cotransported in the same direction as Ca2+. The investigation of the reconstituted exchanger at physiological (i.e. Ki+ = 110 mM, K0+ = 2.5 mM) potassium concentrations revealed that the Na(+)-dependence of Ca2(+)-efflux was highly cooperative (n = 3.01 from Hill plots), indicating that at least three, but possibly four, Na+ ions are exchanged for one Ca2+ ion. Under these conditions the reconstituted exchanger showed a Km for Na+ of 26.1 mM, and a turnover number of 115 Ca2+.s-1 per exchanger molecule. Our results with the purified and reconstituted sodium-calcium exchanger from rod photoreceptors are therefore consistent with previous reports (Cervetto, L., Lagnado, L., Perry, R.J., Robinson, D.W. and McNaughton, P.A. (1989) Nature 337, 740-743; Schnetkamp, P.P.M., Basu, D.K. and Szerencsei, R.T. (1989) Am. J. Physiol. 257, C153-C157) that the sodium-calcium exchanger of rod photoreceptors cotransports K+ under physiological conditions with a stoichiometry of 4 Na+:1 Ca2+, 1K+.