Structural Insights into the Binding and Degradation Mechanisms of Protoporphyrin IX by the Translocator Protein TSPO

Pei-Shan Yeh; Chieh-Chin Li; Yi-Shan Lu; Yun-Wei Chiang

doi:10.1021/jacsau.3c00514

Structural Insights into the Binding and Degradation Mechanisms of Protoporphyrin IX by the Translocator Protein TSPO

JACS Au. 2023 Oct 6;3(10):2918-2929. doi: 10.1021/jacsau.3c00514. eCollection 2023 Oct 23.

Authors

Pei-Shan Yeh¹, Chieh-Chin Li¹, Yi-Shan Lu¹, Yun-Wei Chiang¹

Affiliation

¹ Department of Chemistry, National Tsing Hua University, Hsinchu 300-044, Taiwan.

Abstract

The 18 kDa translocator protein (TSPO) has gained considerable attention as a clinical biomarker for neuroinflammation and a potential therapeutic target. However, the mechanisms by which TSPO associates with ligands, particularly the endogenous porphyrin ligand protoporphyrin IX (PpIX), remain poorly understood. In this study, we employed mutagenesis- and spectroscopy-based functional assays to investigate TSPO-mediated photo-oxidative degradation of PpIX and identify key residues involved in the reaction. We provide structural evidence using electron spin resonance, which sheds light on the highly conserved intracellular loop (LP1) connecting transmembrane 1 (TM1) and TM2. Our findings show that LP1 does not act as a lid to regulate ligand binding; instead, it interacts strongly with the TM3-TM4 linker (LP3) to stabilize the local structure of LP3. This LP1-LP3 interaction is crucial for maintaining the binding pocket structure, which is essential for proper ligand binding. Our results also demonstrate that PpIX accesses the pocket through the lipid bilayer without requiring conformational changes in TSPO. This study provides an improved understanding of TSPO-mediated PpIX degradation, highlighting potential therapeutic strategies to regulate the reaction.