Transmembrane topology and subcellular distribution of the benzodiazepine receptor

The distribution and transmembrane topology of benzodiazepine receptors were investigated in situ using intact and saponin-treated neurons of brain cell cultures. Reversible ligand binding and photoaffinity labeling, using 3H-flunitrazepam as a probe, were employed in conjunction with trypsin-induced exhaustive proteolysis and competition binding using a novel benzodiazepine (Ro7-0213) that contains a quaternary ammonium moiety bearing a full positive charge. About 80% of the benzodiazepine receptors, with apparent subunit molecular weights of 48,000 and 51,000, are located in the surface membrane and contain one or more trypsin-sensitive sites exposed at the extracellular surface. The ligand recognition sites of the surface receptors are orientated toward the extracellular space and are sensitive to both competition by Ro7-0213 and trypsin attack. Approximately 20% of the receptors are intracellular or sequestered and are insensitive to both trypsin and competition by Ro7-0213 in intact cells. Strikingly, all of the 3H-flunitrazepam photolabeled sites inaccessible to competition by Ro7-0213 are also resistant to extracellular trypsin, but are sensitive to trypsin and Ro7-0213 in saponin-treated cells. This result provides strong evidence for protected receptors. Surface receptors are differentially sensitive to trypsin-induced inactivation such that 57% (of total) are inactivated, while 24% (of total) are cleaved but, surprisingly, retain the capacity to bind 3H-flunitrazepam. The observation that the ability of the receptor fragment to bind ligand is unaltered demonstrates that the integrity of the benzodiazepine binding site is not contingent upon the presence of about half of the intact polypeptide chain.(ABSTRACT TRUNCATED AT 250 WORDS)