Native MA-10 mouse Leydig tumor cell mitochondrial preparations were examined by transmission electron (TEM) and atomic force (AFM) microscopic procedures in order to investigate the topography and organization of the peripheral-type benzodiazepine receptor (PBR). Mitochondria were immunolabeled with an anti-PBR antiserum coupled to gold-labeled secondary antibodies. Results obtained indicate that the 18 000 MW PBR protein is organized in clusters of 4–6 molecules. Moreover, on many occasions, the interrelationship among the PBR molecules was found to favor the formation of a single pore. Taking into account recent observations that the 18 000 MW PBR protein is functionally associated with the pore forming 34 000 MW voltage-dependent anion channel (VDAC) these results suggest that (i) the mitochondrial PBR complex could function as a pore, thus allowing the translocation of cholesterol and other molecules to the inner mitochondrial membrane, and (ii) the native receptor is a multimeric complex of an approximate 140 000 MW composed on an average of five 18 000 PBR subunits, one 34 000 VDAC subunit, and associated lipids.
