Based on the known amino-acid sequence of the human and mouse 18 kDa PBR protein, a three-dimensional model of this receptor protein was recently developed using molecular dynamics simulations (42,43). According to this model, the five transmembrane domains of PBR were modeled as five a helices that span one phospholipid bilayer of the outer mitochondrial membrane. This receptor model was then tested as a carrier for a number of molecules and it was shown that it can accommodate a cholesterol molecule and function as a channel. Thus, it was suggested that the receptor's function is to carry cholesterol molecules from the outer lipid monolayer to the inner lipid monolayer of the outer membrane, thus acting as "shield," hiding the cholesterol from the hydrophobic membrane inner medium. Considering the PBR complex formation at the level of the contact sites, this cholesterol movement could end in the inner mitochondrial membrane. Thus, this theoretical model further supports our experimental data, presented in the section "PBR in Steroid Biosynthesis," on the role of PBR in the intramitochondrial cholesterol transport.
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