Structure and activity of tryptophan-rich TSPO proteins

Abstract

TSPO translocator proteins bind steroids and porphyrins, and they are implicated in many human diseases, for which they serve as biomarkers and therapeutic targets. TSPOs have tryptophan-rich sequences that are fhighly conserved from bacteria to mammals. We report crystal structures for Bacillus cereus TSPO (BcTSPO) down to 1.7Å resolution, including a complex with the benzodiazepine-like inhibitor PK11195. We also describe BcTSPO-mediated protoporphyrin IX (PpIX) reactions, including catalytic degradation to a previously undescribed heme derivative. We used structure-inspired mutations to investigate reaction mechanisms, and we showed 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.

Structural Integrity of the A147T Polymorph of Mammalian TSPO

“…RsTspO shares the same fold as mTSPO and preserves several interactions among conserved residues. However, the structures of the two proteins also differ in important aspects, and differ from the structure of the tryptophan-rich sensory protein from Bacillus cereus (BcTspO) [24]. The conformational differences are most pronounced in the packing of TM1 and the TM1-TM2 loop ( Figure 4C).…”

Current status and future perspectives: TSPO in steroid neuroendocrinology

“…Using the bacterial Chlorobium tepidum TSPO purified in detergent, it was demonstrated that TSPO could induce rapid spectral changes to added PPIX indicative of chemical catalysis (Ginter et al 2013). A similar observation has been reported for Bacillus cereus TSPO (Guo et al 2015). However, the reaction rate was not proportional to TSPO concentration, but dependent on the availability of light and oxygen.…”

“…It has been suggested that TSPO could interact with VDAC1 or NOX2 to induce production of ROS (Gatliff et al 2014, Guilarte et al 2016. On the contrary, the association of TSPO with VDAC1 (Joo et al 2012, Joo et al 2015, or TSPO by itself (Guo et al 2015), could facilitate ROS neutralization. Although TSPO regulation of ROS is conceivable, at least as a secondary effect, functional evidence for the interactions and specific role of TSPO remains to be demonstrated.…”

“…(2) TSPO as an enzyme for protoporphyrin IX degradation. Chemical catalysis of PPIX degradation by bacterial TSPO has been proposed as a function for TSPO (Ginter et al 2013, Guo et al 2015. The reaction rate was dependent on availability of light and oxygen.…”

“…According to this hypothesis, the abundance of tryptophans in TSPO might react with ROS to generate tryptophan radicals (Guo et al 2015). Third, a putative TSPO-NADPH oxidase 2 (NOX2) association has been proposed to play a role in ROS generation.…”