Bacterial Ortholog of Mammalian Translocator Protein (TSPO) with Virulence Regulating Activity

Abstract

The translocator protein (TSPO), previously designated as peripheral-type benzodiazepine receptor, is a protein mainly located in the outer mitochondrial membrane of eukaryotic cells. TSPO is implicated in major physiological functions and functionally associated with other proteins such as the voltage-dependent anionic channel, also designated as mitochondrial porin. Surprisingly, a TSPO-related protein was identified in the photosynthetic bacterium Rhodobacter sphaeroides but it was initially considered as a relict of evolution. In the present study we cloned a tspO gene in Pseudomonas fluorescens MF37, a non-photosynthetic eubacterium and we used bioinformatics tools to identify TSPO in the genome of 97 other bacteria. P. fluorescens TSPO was recognized by antibodies against mouse protein and by PK 11195, an artificial ligand of mitochondrial TSPO. As in eukaryotes, bacterial TSPO appears functionally organized as a dimer and the apparent Kd for PK 11195 is in the same range than for its eukaryotic counterpart. When P. fluorescens MF37 was treated with PK 11195 (10−5 M) adhesion to living or artificial surfaces and biofilm formation activity were increased. Conversely, the apoptotic potential of bacteria on eukaryotic cells was significantly reduced. This effect of PK11195 was abolished in a mutant of P. fluorescens MF37 deficient for its major outer membrane porin, OprF. The present results demonstrate the existence of a bacterial TSPO that shares common structural and functional characteristics with its mammalian counterpart. This protein, apparently involved in adhesion and virulence, reveals the existence of a possible new inter kingdom signalling system and suggests that the human microbiome should be involuntarily exposed to the evolutionary pressure of benzodiazepines and related molecules. This discovery also represents a promising opportunity for the development of alternative antibacterial strategies.

Structural and Functional Evolution of the Translocator Protein (18 kDa)

“…This function for Tspo genes in erythropoiesis is supported by experimental evidence from avian and zebrafish models [9][10][11]. TSPO homologs have also been identified in plants and bacteria, where they have developed specialized functions [12,13]. In this review, we will summarize the evolutionary divergence of TSPO structure and its function(s).…”

“…In a recent study, the TSPO of the non-photosynthetic eubacterium Pseudomonas fluorescens was shown to share structural and functional characteristics with mammalian TSPO. The shared characteristics include binding for the high affinity diagnostic isoquinoline carboxamide ligand PK 11195, and involvement in bacterial adhesion and virulence [12]. Adhesion and expression of virulence factors are closely regulated by iron availability.…”

“…1A) [12,29]. As more prokaryotic genome sequences become available, we can further clarify the TSPO distribution among different species.…”

“…Nevertheless from currently available data it can be concluded that some prokaryotes such as Escherichia coli or Pseudomonas aeruginosa lack this gene in their genomes [12,30]. In P. fluorescens and P. syringae, the presence of transposases encoded in the vicinity of TspO that lack the usual CG enrichment relative to neighboring genes suggests that if eubacterial TspO resulted from a transfer, then the transfer event was ancient [12]. It is not clear whether E. coli and other species that lack a Tspo gene lost Tspo during evolution or never had a Tspo gene.…”

“…TSPOs also have multiple binding sites for benzodiazepines [97], even though TSPO binding to benzodiazepines is dependent upon the presence of other partners such as the voltage-dependent anion channel (VDAC) [91,98,99]. A functional relationship between TSPO and the major porin OprF exists in the bacterium P. fluorescens [12]. Gram-negative bacteria have two membranes, with OprF located in the outer membrane, and TSPO located in the cytoplasmic (or inner) membrane.…”