Subcellular distribution of the 18 kDa translocator protein and transcript variant PBR-S in human cells

Despite continued interest in the 18kDa translocator protein (PBR/TSPO) as a biomarker and a therapeutic target for a range of diseases, its functional role, such as in the steroid synthesis pathway and energy metabolism has either become contentious or remains to be described more precisely. The PBR/TSPO gene consists of four exons, while a shorter isoform termed PBR-S lacks exon 2. The PBR-S 102-codon open reading frame differs to that of PBR/TSPO, resulting in a protein that is completely unrelated to PBR/TSPO. To our knowledge, PBR-S protein has never been described and has no known or proposed function. To obtain possible clues on the role of this uncharacterised protein, we compared the subcellular distribution of PBR-S to that of PBR/TSPO. By expressing fluorescently tagged PBR/TSPO, we confirmed that full-length PBR/TSPO co-localises with mitochondria in HeLa, HEK-293, MDA-MB-231, BJ and U87-MG human cell lines. Unlike the strictly mitochondrial localisation of PBR/TSPO, PBR-S has a punctate distribution throughout the cytosol that co-localises with lysosomes in HeLa, HEK-293, MDA-MB-231, BJ and U87-MG cells. In summary, within the cell lines examined we confirm mitochondria rather than occasionally reported other localisations, such as the cell nucleus, to be the only site where PBR/TSPO resides. Due to the lack of any shared protein sequences and the different subcellular locations, we suggest that the previously uncharacterised PBR-S protein variant of the PBR/TSPO gene is likely to serve a different yet to be discovered function compared to PBR/TSPO.