Three-dimensional structure of TspO by electron cryomicroscopy of helical crystals

The 18 kDa TSPO protein is a polytopic mitochondrial outer membrane protein involved in a wide range of physiological functions and pathologies, including neurodegeneration and cancer. The pharmacology of TSPO has been extensively studied, but little is known about its biochemistry, oligomeric state, and structure. We have expressed, purified, and characterized a homologous protein, TspO from Rhodobacter sphaeroides, and reconstituted it as helical crystals. Using electron cryomicroscopy and single-particle helical reconstruction, we have determined a three-dimensional structure of TspO at 10 A resolution. The structure suggests that monomeric TspO comprises five transmembrane alpha helices that form a homodimer, which is consistent with the dimeric state observed in detergent solution. Furthermore, the arrangement of transmembrane domains of individual TspO subunits indicates a possibility of two substrate translocation pathways per dimer. The structure provides the first insight into the molecular architecture of TSPO/PBR protein family that will serve as a framework for future studies.