Topological Analysis of the Peripheral Benzodiazepine Receptor in Yeast Mitochondrial Membranes Supports a Five-transmembrane Structure

The peripheral benzodiazepine receptor, implicated in the transport of cholesterol from the outer to the inner mitochondrial membrane, is predicted by hydropathy analysis to feature five membrane-spanning domains, with the amino terminus within the mitochondrial periplasm and the carboxyl terminus in the external cytoplasm. We have tested these structural predictions directly by immunodetection of c-Myc-tagged peripheral benzodiazepine receptor on intact yeast mitochondria and by specific labeling in yeast membranes of cysteine residues introduced by site-directed mutagenesis. The combined results support the model originally proposed with some minor but important modifications. The theoretical model predicted relatively short alpha-helical domains, only long enough to span a phospholipid monolayer, whereas the results presented here would support a model with extended alpha-helices sufficiently long to span an entire membrane bilayer, with concomitant shorter loop and tail regions.