Biochemical and Molecular Characterization of the Sodium‐Calcium Exchanger from Bovine Rod Photoreceptors^a

Abstract

A major obstacle in the purification of membrane transport or receptor proteins is the requirement for a method of detection of the protein in question after solubilization 'This work was supported by the Deutsche Forschungsgemeinschaft (SFB 169 Projekt 0%) and a Leibniz-Programm Grant to Prof. Hartmut Michel.

Molecular cloning and characterization of the human cardiac Na+/Ca2+ exchanger cDNA.

“…Although Na+/Ca2' exchange activity is very high in the retina (28), the relative amount of the exchanger mRNA in the retina appeared very low. This is probably due to the existence of an additional, retina-specific, Na+/Ca2+ exchanger that has little sequence homology to the cardiac-type exchanger (31). The molecular mass of the exchanger of cardiac sarcolemma (120 kDa) is different from that of the retinal exchanger (215 kDa) (13,28), and antibodies against the cardiac Na+/Ca2+ exchanger crossreact poorly with the purified rod exchanger (29).…”

Sodium-Calcium Exchanger in Cultured Human Retinal Pigment Epithelium

“…Reeves et al, 1994 ;Friedman and Gesek, 1995), has not been studied in human RPE, although there is evidence for its presence in frog, dogfish and bovine RPE (Salceda, 1989 ;Fijisawa et al, 1993). There is no information available as to whether the Na + \Ca# + exchanger expressed in RPE is of the cardiac type (Nicoll, Longoni and Philipson, 1990), or is of the unique type expressed by the adjacent rod photoreceptors, i.e., a K + -dependent antiporter (Achilles et al, 1991).…”

“…A second type which is K + -dependent and operates with a stoichiometry of 4 Na + : (1 Ca# + j1 K + ), appears to be expressed exclusively in the retinal rod photoreceptors (Achilles et al, 1991). Genes that encode these two classes of Na + \Ca# + exchanger exhibit minimal sequence homology (Nicoll et al, 1990 ;Achilles et al, 1991). Because of the significant differences in both structure and function, it is important to identify which type of exchanger is present in human RPE cells.…”

“…The most widely distributed is the cardiac type, which operates with a stoichiometry of 3 Na + :1 Ca# + , and is expressed in a broad variety of cell types . A second type which is K + -dependent and operates with a stoichiometry of 4 Na + : (1 Ca# + j1 K + ), appears to be expressed exclusively in the retinal rod photoreceptors (Achilles et al, 1991). Genes that encode these two classes of Na + \Ca# + exchanger exhibit minimal sequence homology (Nicoll et al, 1990 ;Achilles et al, 1991).…”