Among the recombinant TSPOs, the BcTSPO.

Overexpression of recombinant Bacillus cereus TSPO (BcTSPO) in E. coli bacteria leads to its recovery with a bound hemin both in bacterial membrane (MB) and inclusion bodies (IB). Unlike mouse TSPO, BcTSPO purified in SDS detergent from IB is well structured and can bind various ligands such as high-affinity PK 11195, protoporphyrin IX (PPIX) and δ-aminolevulinic acid (ALA). For each of the three ligands, 1H-15N HSQC titration NMR experiments suggest that different amino acids of BcTSPO binding cavity are involved in the interaction. PPIX, an intermediate of heme biosynthesis, binds to the cavity of BcTSPO and its fluorescence can be significantly reduced in the presence of light and oxygen. The light irradiation leads to two products that have been isolated and characterized as photoporphyrins. They result from the addition of singlet oxygen to the two vinyl groups hence leading to the formation of hydroxyaldehydes. The involvement of water molecules, recently observed along with the binding of heme in Rhodobacter sphaeroides (RsTSPO) is highly probable. Altogether, these results raise the question of the role of TSPO in heme biosynthesis regulation as a possible scavenger of reactive intermediates.

Enhanced structure/function of mTSPO translocator in lipid:surfactant mixed micelles.

TSPO is a ubiquitous transmembrane protein used as a pharmacological marker in neuroimaging. The only known atomic structure of mammalian TSPOs comes from the solution NMR of mouse TSPO (mTSPO) bound to the PK11195 ligand and in a DPC surfactant environment. No structure is available in a biomimetic environment and without PK11195 which strongly stiffens the protein. We measured the effect of different amphiphilic environments on ligand-free mTSPO to study its structure/function and find optimal solubilization conditions. By replacing the SDS surfactant, where the recombinant protein is purified, with mixed lipid:surfactant (DMPC:DPC) micelles at different ratios (0:1, 1:2, and 2:1, w:w), the α-helix content and interactions and the intrinsic tryptophan (Trp) fluorescence of mTSPO are gradually increased. Small-angle X-ray scattering (SAXS) shows a more extended mTSPO/belt complex with the addition of lipids: Dmax ∼95 Å in DPC alone versus ∼142 Å in DMPC:DPC (1:2). SEC-MALLS shows that the molecular composition of the mTSPO belt is ∼98 molecules for DPC alone and ∼58 DMPC and ∼175 DPC for DMPC:DPC (1:2). Additionally, DMPC:DPC micelles stabilize mTSPO compared to DPC alone, where the protein has a greater propensity to aggregate. These structural changes are consistent with the increased affinity of mTSPO for the PK11195 ligand in presence of lipids (Kd ∼70 µM in DPC alone versus ∼0.91 µM in DMPC:DPC, 1:2), as measured by microscale thermophoresis (MST). In conclusion, mixed lipid:surfactant micelles open new possibilities for the stabilization of membrane proteins and for their study in solution in a more biomimetic amphiphilic environment.