Tryptophan-Rich Sensory Protein/Translocator Protein (TSPO) from Cyanobacterium Fremyella diplosiphon Binds a Broad Range of Functionally Relevant Tetrapyrroles.

Tryptophan-rich sensory protein/translocator protein (TSPO) is a membrane protein involved in stress adaptation in the cyanobacterium Fremyella diplosiphon. Characterized mammalian and proteobacterial TSPO homologues bind tetrapyrroles and cholesterol ligands. We investigated the ligand binding properties of TSPO from F. diplosiphon (FdTSPO1), which was functionally characterized in prior genetic studies. Two additional TSPO proteins (FdTSPO2 and FdTSPO3) are present in F. diplosiphon; they are similar in size to reported bacterial TSPOs and smaller than FdTSPO1. The longer cyanobacterial TSPO1 is found almost exclusively in filamentous cyanobacteria and has a relatively low degree of homology to bacterial and mammalian TSPO homologues with confirmed tetrapyrrole binding. To probe distinctions of long-form TSPOs, we tested the binding of porphyrin and bilin to FdTSPO1 and measured binding affinities in the low micromolar range, with the highest binding affinity detected for heme. Although tetrapyrrole ligands bound FdTSPO1 with affinities similar to those previously reported for proteobacterial TSPO, binding of cholesterol to FdTSPO1 was particularly poor and was not improved by introducing an amino acid motif known to enhance cholesterol binding in other bacterial TSPO homologues. Additionally, we detected limited binding of bacterial hopanoids to FdTSPO1. Cyanobacterial TSPO1 from the oxygenic photosynthetic F. diplosiphon, thus, binds a range of tetrapyrroles of functional relevance with efficiencies similar to those of mammalian and proteobacterial homologues, but the level of cholesterol binding is greatly reduced compared to that of mammalian TSPO. Furthermore, the ΔFdTSPO1 mutant exhibits altered growth in the presence of biliverdin compared to that of wild-type cells under green light. Together, these results suggest that TSPO molecules may play roles in bilin homeostasis or trafficking in cyanobacteria.