Molten-globule structure and membrane binding of the N-terminal protease-resistant domain (63-193) of the steroidogenic acute regulatory protein (StAR)

The first step in steroidogenesis is the movement of cholesterol from the o uter to inner mitochondrial membrane, this movement is facilitated by the s teroidogenic acute regulatory protein (StAR). StAR has molten-globule prope rties at low pH and a protease-resistant N-terminal domain at pH 4 and pH 8 comprising residues 63-193, To explore the mechanism of action of StAR we investigated the structural properties of the bacterially expressed N-termi nal domain (63-193 StAR) using CD, limited proteolysis and NMR, Far- and ne ar-UV CD showed that the amount of secondary structure was greater at acidi c than at neutral pH, but there was little tertiary structure at any pH. Un like 63-193 StAR liberated from N-62 StAR by proteolysis, biosynthetic 63-1 93 StAR was no longer resistant to trypsin or proteinase K at pH 7, or to p epsin at pH 4, Addition of trifluoroethanol and SDS increased secondary str ucture at pH 7. and dodecylphosphocholine and CHAPS increased secondary str ucture at pH 2, pH 4 and pH 7, However, none of these conditions induced te rtiary structure, as monitored by near-UV CD or NMR. Liposomes of phosphati dylcholine. phosphatidylserine and their mixture increased secondary struct ure of 63-193 StAR at pH 7, as monitored by far-UV CD, and stable protein-l iposome complexes were identified by gel-permeation chromatography. These r esults provide further evidence that the N-terminal domain of StAR is a mol ten globule, and provide evidence that this conformation facilitates the in teraction of the N-terminal domain of StAR with membranes. We suggest that this interaction is the key to understanding the mechanism of StAR's action .