N-218 MLN64, a protein with StAR-like steroidogenic activity, is folded and cleaved similarly to StAR

The steroidogenic acute regulatory protein (StAR) facilitates the movement of cholesterol from the outer to inner mitochondrial membrane in adrenal and gonadal cells, fostering steroid biosynthesis. MLN64 is a 445-amino acid protein of unknown function. When 218 amino-terminal residues of MLN-64 are deleted, the resulting N-218 MLN64 has 37% amino acid identity with StAR and 50% of StAR's steroidogenic activity in transfected cells. Antiserum to StAR cross-reacts with N-218 MLN64, indicating the presence of similar epitopes in both proteins. Western blotting shows that MLN64 is proteolytically cleaved in the placenta to a size indistinguishable from N-218 MLN64. Bacterially expressed N-218 MLN64 exerts StAR-like activity to promote the transfer of cholesterol from the outer to inner mitochondrial membrane in vitro. CD spectroscopy indicates that N-218 MLN64 is largely alpha-helical and minimally affected by changes in ionic strength or the hydrophobic character of the solvent, although glycerol increases the beta-sheet content. However, decreasing pH diminishes structure, causing aggregation. Limited proteolysis at pH 8.0 shows that the C-terminal domain of N-218 MLN64 is accessible to proteolysis whereas the 244-414 domain is resistant, suggesting it is more compactly folded. The presence of a protease-resistant domain and a protease-sensitive carboxy-terminal domain in N-218 MLN64 is similar to the organization of StAR. However, as MLN64 never enters the mitochondria, the protease-resistant domain of MLN64 cannot be a mitochondrial pause-transfer sequence, as has been proposed for StAR. Thus the protease-resistant domain of N-218 MLN64, and by inference the corresponding domain of StAR, may have direct roles in their action to foster the flux of cholesterol from the outer to the inner mitochondrial membrane.