Validation of the mechanism of cholesterol binding by StAR using short molecular dynamics simulations

We previously proposed an original two-state cholesterol binding mechanism by StAR, in which the C-terminal α-helix of StAR gates the access of cholesterol to its binding site cavity. This cavity, which can accommodate one cholesterol molecule, was proposed to promote the reversible unfolding of the C-terminal α-helix and allow for the entry and dissociation of cholesterol. In our molecular model of the cholesterol–StAR complex, the hydrophobic moiety of cholesterol interacts with hydrophobic amino acid side-chains located in the C-terminal α-helix and at the bottom of the cavity. In this study, we present a structural in silico analysis of StAR. Molecular dynamics simulations showed that point mutations of Phe267, Leu271 or Leu275 at the α-helix 4 increased the gyration radius (more flexibility) of the protein's structure, whereas the salt bridge double mutant E169M/R188M showed a decrease in flexibility (more compactness). Also, in the latter case, an interaction between Met169 and Phe267 disrupted the hydrophobic cavity, rendering it impervious to ligand binding. These obtained results are in agreement with previous in vitro experiments, and provide further validation of the two-state binding mode of action.