9.A.24 The Mitochondrial Cholesterol/Porphyrin Uptake Translocator Protein (TSPO) Family

The central channel Tom40 of the preprotein translocase of the outer membrane (TOM) complex is thought to be responsible for the import of virtually all preproteins synthesized outside the mitochondria. Otera et al. (2007) analyzed the topogenesis of the peripheral benzodiazepine receptor (PBR), which integrates into the mitochondrial outer membrane (MOM) through five hydrophobic transmembrane segments (TMSs) and functions in cholesterol and porphyrin import into the inner membrane. Analyses of in vitro and in vivo import into TOM component-depleted mitochondria reveal that PBR import (1) depends on the import receptor Tom70 but requires neither the Tom20 and Tom22 import receptors nor the import channel Tom40, (2) shares the post-Tom70 pathway with the C-tail-anchored proteins, and (3) requires factors in the mitochondrial intermembrane space. Furthermore, membrane integration of mitofusins and mitochondrial ubiquitin ligase, MOM proteins with two and four TMSs, respectively, proceeds through the same initial pathway. Thus, an unidentified pathway for the membrane integration of MOM proteins with multiple TMSs was identified (Otera et al., 2007).

Translocator protein (TSPO; PBR) has been reported to be an 18 kDa high affinity cholesterol, porphyrin uptake and drug-binding protein found primarily in the outer mitochondrial membrane. Although TSPO is found in many tissue types, it is expressed at the highest levels under normal conditions in tissues that synthesize steroids (Batarseh and Papadopoulos, 2010). TSPO, the 3-d structure of which is known (PDB# 2MGY) has been associated with cholesterol import into mitochondria, a key function in steroidogenesis, and directly or indirectly with multiple other cellular functions including apoptosis, cell proliferation, differentiation, anion transport, porphyrin transport, heme synthesis, and regulation of mitochondrial function (Jaremko et al. 2014). Aberrant expression of TSPO has been linked to multiple diseases, including cancer, brain injury, neurodegeneration, Parkinson''s and Alheimer''s diseases, and ischemia-reperfusion injury.  It forms a large complex that includes VDAC-1, TSPO-associated protein-7 (PAP7; ACBD3), a protein kinase regulatory subunit, PKAR1A, and the StAR regluatory protein (Miller 2013).  TSPO is conformationally flexible (Jaremko et al. 2015).

Neurosteroids are able to rapidly control the excitability of the central nervous system, acting as regulators of type A receptors for GABA. Neurosteroid level alterations occur in psychiatric disorders, including anxiety disorders. Investigators have manipulated neurosteroidogenesis in an effort to correct neuronal excitation and inhibition imbalances, which may lie at the root of neuropsychiatric conditions. A promising target for therapy of anxiety disorders is the Translocator Protein (TSPO). TSPO is expressed predominantly in steroid-synthesizing tissues and is localized to contact sites between the outer and inner mitochondrial membranes.  It may mediate the rate-limiting step of neurosteroidogenesis. Brain concentrations of neurosteroids can be affected by selective TSPO activation. Indeed, TSPO drug ligands are able to stimulate primary neurosteroid formation that enhances GABAA receptor activity, pregnenolone and allopregnenalone, in both in vitro steroidogenic cells and in vivo animal models. A spectrum of TSPO ligands has been shown to exert anxiolytic actions when administered in rodents. The selective TSPO ligand, XBD173 (AC-5216, Emapunil), exerts anxiolytic effects not only in animal models, but also in humans. (Costa et al., 2012) reviewed the literature regarding the central nervous system biology of TSPO.

The TSPO (18 kDa translocator protein) is involved in cholesterol transport in organs that synthesize steroids and bile salts. Different natural and synthetic high-affinity TSPO ligands have been characterized through their ability to stimulate cholesterol transport, but they also stimulate other physiological processes including cell proliferation, apoptosis and calcium-dependent transepithelial ion secretion. TSPO is present in enterocyte mitochondria but not rat intestinal goblet cells (Ostuni et al. 2009). Enterocyte cytoplasm also contains the endogenous TSPO ligand, polypeptide DBI (diazepam-binding inhibitor). Whereas intestinal TSPO had high affinity for the synthetic ligand PK 11195, the pharmacological profile of TSPO in the duodenum was distinct from that in the jejunum and ileum. Specifically, benzodiazepine Ro5-4864 and protoporphyrin IX showed 5-13-fold lower affinity for duodenal TSPO. PK 11195 stimulated calcium-dependent chloride secretion in the duodenum and calcium-dependent chloride absorption in the ileum, but did not affect jejunum ion transport. Thus, the functional differences in subpopulations of TSPO in different regions of the intestine could be related to the structural organization of mitochondrial protein complexes that mediate the ability of TSPO to modulate either chloride secretion or absorption in the duodenum and ileum, respectively.

A 5 TMS bacterial homologue called the tryptophan-rich sensory protein of Rhodobacter spheroides binds retinoic acid, cucumin and an inhibitor of Bcl-2 actioin called gossypol (Li et al. 2013).  It appears to function in porphyrin degredation in a light- and oxygen-dependent process (Ginter et al. 2013). The protein and its function(s) have been reviewed. TSPO is believed to be involved either directly or indirectly in numerous biological functions, including mitochondrial cholesterol transport and steroid hormone biosynthesis, porphyrin transport and heme synthesis, apoptosis, cell proliferation, and anion transport. Localized to the outer mitochondrial membrane of steroidogenic cells, TSPO has been shown to associate with cytosolic and mitochondrial proteins as part of a large multiprotein complex involved in mitochondrial cholesterol transport, the rate-limiting step in steroidogenesis. It has been concluded that TSPO is a unique mitochondrial pharmacological target for diseases that involve increased mitochondrial activity, including steroidogenesis, but the specific function is not clear (Papadopoulos et al. 2017).

This family belongs to the .



Austin CJ., Kahlert J., Kassiou M. and Rendina LM. (2013). The translocator protein (TSPO): a novel target for cancer chemotherapy. Int J Biochem Cell Biol. 45(7):1212-6.

Batarseh, A. and V. Papadopoulos. (2010). Regulation of translocator protein 18 kDa (TSPO) expression in health and disease states. Mol. Cell Endocrinol 327: 1-12.

Costa, B., E. Da Pozzo, and C. Martini. (2012). Translocator protein as a promising target for novel anxiolytics. Curr Top Med Chem 12: 270-285.

Ginter, C., I. Kiburu, and O. Boudker. (2013). Chemical catalysis by the translocator protein (18 kDa). Biochemistry 52: 3609-3611.

Guillaumot, D., S. Guillon, T. Déplanque, C. Vanhee, C. Gumy, D. Masquelier, P. Morsomme, and H. Batoko. (2009). The Arabidopsis TSPO-related protein is a stress and abscisic acid-regulated, endoplasmic reticulum-Golgi-localized membrane protein. Plant J. 60: 242-256.

Guo, Y., R.C. Kalathur, Q. Liu, B. Kloss, R. Bruni, C. Ginter, E. Kloppmann, B. Rost, and W.A. Hendrickson. (2015). Protein structure. Structure and activity of tryptophan-rich TSPO proteins. Science 347: 551-555.

Hug, L.A., B.J. Baker, K. Anantharaman, C.T. Brown, A.J. Probst, C.J. Castelle, C.N. Butterfield, A.W. Hernsdorf, Y. Amano, K. Ise, Y. Suzuki, N. Dudek, D.A. Relman, K.M. Finstad, R. Amundson, B.C. Thomas, and J.F. Banfield. (2016). A new view of the tree of life. Nat Microbiol 1: 16048.

Jaremko L., Jaremko M., Giller K., Becker S. and Zweckstetter M. (2015). Conformational Flexibility in the Transmembrane Protein TSPO. Chemistry. 21(46):16555-63.

Jaremko, L., M. Jaremko, K. Giller, S. Becker, and M. Zweckstetter. (2014). Structure of the mitochondrial translocator protein in complex with a diagnostic ligand. Science 343: 1363-1366.

Korkhov, V.M., C. Sachse, J.M. Short, and C.G. Tate. (2010). Three-dimensional structure of TspO by electron cryomicroscopy of helical crystals. Structure 18: 677-687.

Li, F., J. Liu, Y. Zheng, R.M. Garavito, and S. Ferguson-Miller. (2015). Protein structure. Crystal structures of translocator protein (TSPO) and mutant mimic of a human polymorphism. Science 347: 555-558.

Li, F., Y. Xia, J. Meiler, and S. Ferguson-Miller. (2013). Characterization and modeling of the oligomeric state and ligand binding behavior of purified translocator protein 18 kDa from Rhodobacter sphaeroides. Biochemistry 52: 5884-5899.

Miller, W.L. (2013). Steroid hormone synthesis in mitochondria. Mol. Cell Endocrinol 379: 62-73.

Ostuni, M.A., G. Péranzi, R.A. Ducroc, M. Fasseu, B. Vidic, J. Dumont, V. Papadopoulos, and J.J. Lacapere. (2009). Distribution, pharmacological characterization and function of the 18 kDa translocator protein in rat small intestine. Biol Cell 101: 573-586.

Otera, H., Y. Taira, C. Horie, Y. Suzuki, H. Suzuki, K. Setoguchi, H. Kato, T. Oka, and K. Mihara. (2007). A novel insertion pathway of mitochondrial outer membrane proteins with multiple transmembrane segments. J. Cell Biol. 179: 1355-1363.

Papadopoulos, V., J. Fan, and B. Zirkin. (2017). Translocator protein (18 kDa): an update on its function in steroidogenesis. J Neuroendocrinol. [Epub: Ahead of Print]

Riond, J., M.G. Mattei, M. Kaghad, X. Dumont, J.C. Guillemot, G. Le Fur, D. Caput, and P. Ferrara. (1991). Molecular cloning and chromosomal localization of a human peripheral-type benzodiazepine receptor. Eur J Biochem 195: 305-311.

Taylor, J.M., A.M. Allen, and A. Graham. (2014). Targeting mitochondrial 18 kDa translocator protein (TSPO) regulates macrophage cholesterol efflux and lipid phenotype. Clin Sci (Lond) 127: 603-613.

Yeliseev, A.A. and S. Kaplan. (1999). 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: 21234-21243.

Yeliseev, A.A., K.E. Krueger, and S. Kaplan. (1997). A mammalian mitochondrial drug receptor functions as a bacterial "oxygen" sensor. Proc. Natl. Acad. Sci. USA 94: 5101-5106.


TC#NameOrganismal TypeExample

The peripheral benzodiazepine receptor (PBR), which can also bind isoquinoline carboxamides (Riond et al. 1991) and integrates into the mitochondrial outer membrane (MOM) through five hydrophobic transmembrane segments (TMSs). Also called "translocator protein", TSPO). It is a mitochondrial cholesterol and porphyrin uptake transporter (Jaremko et al. 2014; Taylor et al. 2014) but is also part of the mitochondrial permeability transition pore (MPTP) which includes cyclophilin D, VDAC (TC#1.B.8) and the adenine nucleotide translocator (Austin et al. 2013).  The 3-d structure has been determined at 2.4 Å resolution bound to its high affinity ligand, PK11195 which causes the otherwise loose 5 helix bundle to form a tight bundle with a hydrophobic pocket for PK11195 (Jaremko et al. 2014).


PBR of Homo sapiens (Q6ICF9)


Uncharacterized protein of 171 aas and 5 TMSs


UP of Loa loa (Eye worm) (Filaria loa)


TspO protein of 141 aas and 4-5 TMSs


TspO of Phaeocystis globosa virus


TspO-like; MBR-like protein of 163 aas

Red algae

TspO-like protein of Galdieria sulfuraria


TspO/MBR family member of 151 aas and 5 TMSs.  The crystal structure has been determined at 1.7 Å resolution (Guo et al. 2015).  The protein was solved in complex with the benzodiazepine-like inhibitor, PK11195. TspO-mediated protoporphyrin IX (PpIX) reactions were also described, including catalytic degradation to a previously undescribed heme derivative. Structure-inspired mutations allowed the investigation of the reaction mechanisms, showing that TSPOs from Xenopus and man have similar PpIX-directed activities. Although TSPOs have been regarded as transporters, the catalytic activity in PpIX degradation suggests physiological importance for TSPOs in protection against oxidative stress (Guo et al. 2015).


TspO of Bacillus cereus


Uncharacterized protein of 134 aas and 4 TMSs (Hug et al. 2016).

UP of Candidatus Peribacter riflensis


Uncharacterized protein of 161 aas and 4 TMSs.

UP of Luteimonas mephitis


Tryptophan-rich sensory protein. TSPO, of 167 aas and 5 TMSs

TSPO of Halococcus sediminicola


The outer membrane tryptophan-rich sensory protein (TspO) of the TSPO/MBR family of 159 aas and 5 TMSs (Yeliseev et al. 1997; Yeliseev and Kaplan 1999).  The 10 Å cryo electron microscopy structure is known (Korkhov et al. 2010) as are 1.8, 2.4 and 2.5 Å structures solved by x-ray crystallography (Li et al. 2015).  Crystals obtained in the lipidic cubic phase reveal the binding site of an endogenous porphyrin ligand. The three crystal structures reveal a dimer, providing insights into the controversial physiological role of TSPO and how a mutation in the human homologue causes psychiatric disorders and reduced pregnenolone production (Li et al. 2015).


TspO of Rhodobacter spheroides


The Endoplasmic reticulum/Golgi TSPO protein is mainly detected in dry seeds, but can be induced in vegetative tissues by osmotic or salt stress or abscisic acid (ABA) treatment (Guillaumot et al. 2009).


TSPO of Arabidopsis thaliana



TspO of Nostoc sp.

 TspO/MBR family protein of 186 aas and 5 TMSs


 TspO/MBR family protein of Acanthamoeba castellanii


TspO of Niastella koreensis


TspO of 159 aas


TspO of Lactococcus lactis

Peripheral-type benzodiazepine receptor of 188 aas


Peripheral-type benzodiazepine receptor of Zea mays

TspO homologue of 193 aas


TspO of Oryza sativa


TC#NameOrganismal TypeExample

TspO homologue of 171 aas


TspO homologue of Maricaulis maris


CrtK protein of 166 aas and 5 TMSs.


CrtK of Oceanicaulis sp.


TC#NameOrganismal TypeExample

Uncharacterized protein of 177 aas


UP of Mycobacterium vanbaalenii


Tryptophan-rich sensory proteinof 160 aas and 5 TM

TpsO of Nocardia soli


TC#NameOrganismal TypeExample

Uncharacterized protein, WcoO of 272 aas and 8 TMSs.


WcoO of Clavibacter michiganensis


Uncharacterized protein of 155 aas and 5 TMSs.

UP of Methanosarcina mazei


Uncharacterized protein of 258 aas


UP of Bacillus selenitireducens


Uncharacterized protein of 290 aas and 8 TMSs


UP of Coriobacterium glomerans


Uncharacterized protein of 281 aas and 8 TMSs.


UP of Phytophthora infestans (Potato late blight fungus)


Uncharacterized protein of 264 aas and 8 TMSs.


UP of Rhizophagus irregularis (Arbuscular mycorrhizal fungus) (Glomus intraradices)


Uncharacterized protein of 350 aas and 8 TMSs.


UP of Guillardia theta


Uncharacterized protein of 277 aas and 7 TMSs

UP of Microbacterium yannicii


Uncharacterized protein of 236 aas and 8 TMSs.

UP of Paracoccus zeaxanthinifaciens


Uncharacterized protein of 250 aas and 8 TMSs.

UP of Henriciella aquimarina