A chloride- and calcium-dependent glutamate-binding protein from rat brain. Identification as a ubiquitous constituent of the inner mitochondrial membrane

We have recently solubilized and enriched a chloride- and calcium-dependent glutamate-binding protein from rat brain (Brose, N., Halpain, S., Suchanek, C., and Jahn, R. (1989) J. Biol. Chem. 264, 9619-9625). The partially purified protein fraction, containing two major protein components of 51,000 Da and 105,000 Da, was used to generate a rabbit antiserum. This serum quantitatively precipitated the binding activity from membrane extracts. Small amounts of the antiserum inhibited glutamate binding when chloride was absent from the incubation medium. Three protein bands were labeled by the serum on immunoblots. From the affinity purified antibody fractions contained in the serum, only the antibodies directed against a 51,000-Da protein were able to immunoprecipitate the binding activity, indicating that this protein is an essential component of the binding site. A survey of a variety of rat tissues by immunoblot analysis revealed a ubiquitous distribution of the protein. After subcellular fractionation of liver and brain, the 51,000-Da protein copurified with mitochondrial markers. Furthermore, exclusive labeling of mitochondria was observed by light and electron microscopy immunocytochemistry. Subfractionation of purified liver mitochondria resulted in a selective association of the protein with inner mitochondrial membranes. Pharmacological characterization of glutamate binding to liver mitochondrial membranes revealed a pattern almost identical to that of the chloride- and calcium-dependent glutamate-binding site in rat brain.