Pharmacological and molecular heterogeneity of the

Abstract

The rat liver peripheral-type benzodiazepine receptor (PBR) was characterized by ligand binding with [³H]Ro5-4864 and by photolabeling using [³H]PK 14105. The native liver receptor could be solubilized with digitonin. The Mr of the native photolabeled receptor was 170 kDa while a single Mr 19 kDa protein was identified under denaturing conditions. Radioligand binding to rat liver subcellular fractions showed protoporphyrin IX had a 6.2-fold greater affinity for [³H]Ro5-4864 binding sites in mitochondria than in microsomes. Although heterogeneity of the rat liver benzodiazepine binding site, but not the isoquinoline binding site, was observed, a single 19 kDa protein band was identified by photolabeling with an isoquinoline ligand. Bovine and rat PBR have a similar tissue and subcellular distribution, but are pharmacologically and biochemically distinct. The bovine PBR had low affinity for Ro5-4864 and diazepam while [³H]PK 11195 binding was insensitive to modification of histidine residues. The native Mr the receptor was 200 kDa by gel filtration. Photolabeling identified a 17 kDa protein from both rat and bovine adrenal mitochondria under denaturing conditions. An affinity matrix was constructed to purify the native components of the PBR from both species, but the PBR could not be eluted from the matrix. To compare receptor components, the cDNA encoding the rat isoquinoline binding protein was used to screen a fetal calf adrenal cDNA library. A 822 base pair bovine cDNA was identified that encoded a polypeptide of 169 amino acids which had 78% positional identity to the rat protein and was 97% similar after accounting for conserved replacements. Comparison of the amino acid sequences indicates the rat and bovine proteins are homologs and the species differences in ligand binding may not be due to differences in the primary sequence of the isoquionoline binding proteins. Our results indicate the common characteristics of PBR is their ability to bind isoquinoline ligands, not benzodiazepine ligands, with high affinity. A conserved 17-19 kDa protein is required for demonstration of this receptor. A new nomenclature is presented which designates these receptors as τ (tau) receptors.