InterPro domain: IPR038330

Members of this group are involved in transmembrane signalling. In both prokaryotes and mitochondria they are localized to the outer membrane, and have been shown to bind and transport dicarboxylic tetrapyrrole intermediates of the haem biosynthetic pathway [ 1 , 2 ]. They are associated with the major outer membrane porins (in prokaryotes) and with the voltage-dependent anion channel (in mitochondria) [ 3 ].

Rhodobacter sphaeroides TspO (previously CrtK) is involved in signal transduction, functioning as a negative regulator of the expression of some photosynthesis genes (PpsR/AppA repressor/antirepressor regulon). This down-regulation is believed to be in response to oxygen levels. TspO works through (or modulates) the PpsR/AppA system and acts upstream of the site of action of these regulatory proteins [ 4 ]. It has been suggested that the TspO regulatory pathway works by regulating the efflux of certain tetrapyrrole intermediates of the haem/bacteriochlorophyll biosynthetic pathways in response to the availability of molecular oxygen, thereby causing the accumulation of a biosynthetic intermediate that serves as a corepressor for the regulated genes [ 5 ]. A homologue of the TspO protein in Rhizobium meliloti (Sinorhizobium meliloti) is involved in regulating expression of the ndi locus in response to stress conditions [ 6 ]. There is evidence that the S. meliloti TspO acts through, or in addition to, the FixL regulatory system.

In animals, translocator protein (TSPO), previously known as peripheral-type benzodiazepine receptor (PBR, MBR) is a mitochondrial protein (located in the outer mitochondrial membrane) where it forms a complex with several proteins of the mitochondrial permeability transition pore (MPTP). TSPO is involved in multiple processes, including regulation of cell death, cholesterol transport and steroid biosynthesis, mitochondrial respiration and oxidation and mitochondrial protein import [ 7 , 8 ].

These observations suggest that fundamental aspects of this receptor and the downstream signal transduction pathway are conserved in bacteria and higher eukaryotic mitochondria. The alpha-3 subdivision of the purple bacteria is considered to be a likely source of the endosymbiont that ultimately gave rise to the mitochondrion. Therefore, it is possible that the mammalian PBR remains both evolutionarily and functionally related to the TspO of R. sphaeroides.

1. Isolation of the mitochondrial benzodiazepine receptor: association with the voltage-dependent anion channel and the adenine nucleotide carrier. Proc. Natl. Acad. Sci. U.S.A. 89, 3170-4
2. A sensory transducer homologous to the mammalian peripheral-type benzodiazepine receptor regulates photosynthetic membrane complex formation in Rhodobacter sphaeroides 2.4.1. J. Biol. Chem. 270, 21167-75
3. In vitro reconstitution of a functional peripheral-type benzodiazepine receptor from mouse Leydig tumor cells. Mol. Pharmacol. 45, 201-11
4. TspO as a modulator of the repressor/antirepressor (PpsR/AppA) regulatory system in Rhodobacter sphaeroides 2.4.1. J. Bacteriol. 183, 6355-64
5. A novel mechanism for the regulation of photosynthesis gene expression by the TspO outer membrane protein of Rhodobacter sphaeroides 2.4.1. J. Biol. Chem. 274, 21234-43
6. A homologue of the tryptophan-rich sensory protein TspO and FixL regulate a novel nutrient deprivation-induced Sinorhizobium meliloti locus. Appl. Environ. Microbiol. 66, 5353-9
7. The translocator protein (TSPO): a novel target for cancer chemotherapy. Int. J. Biochem. Cell Biol. 45, 1212-6
8. The 18 kDa translocator protein (TSPO): a new perspective in mitochondrial biology. Curr. Mol. Med. 12, 356-68