Fig. 3.

Evolution of TSPO functions. TSPO’s earliest function in porphyrin export from bacterial cells may have evolved into import of PPIX into and export of heme out of mitochondria and chloroplasts in eukaryotes. Functions acquired later in evolution include functioning as a SAR in plants and participating in a large protein complex that imports cholesterol into mitochondria of steroidogenic tissues in mammals. Purple squares denote porphyrins; yellow triangles denote cholesterol. Proteins involved in cholesterol transport are shaded blue; heme-containing proteins are shaded purple