STARD3: Proteins and Enzymes

The steroidogenic acute regulatory (StAR) protein facilitates the movement of cholesterol from the outer to inner mitochondrial membrane in adrenal and gonadal cells, fostering steroid biosynthesis. MLN 64 is a 445-amino acid protein of unknown function. When 218 amino-terminal residues of MLN 64 are deleted, the resulting N-218 MLN 64 has 37% amino acid identity with StAR and 50% of StAR's steroidogenic activity in transfected cells. Bacterially expressed N-218 MLN 64 exerts StAR-like activity to promote the transfer of cholesterol from the outer to inner mitochondrial membrane in vitro. The presence of a protease-resistant domain and a protease-sensitive carboxy-terminal domain in N-218 MLN 64 is similar to the organization of StAR. However, as MLN 64 never enters the mitochondria, the protease-resistant domain of MLN 64 cannot be a mitochondrial pause-transfer sequence, as has been proposed for StAR.