Hydrophobic Core of the Steroidogenic Acute Regulatory Protein for Cholesterol Transport

The steroidogenic acute regulatory protein (StAR), the first family member of START (StAR-related lipid transport) proteins, plays an essential role by facilitating the movement of cholesterol from the outer to inner mitochondrial membrane. Wild-type and mutant StAR binds cholesterol with similar intensity, but only wild-type StAR can transport it to mitochondria. Here, we report that the hydrophobic core is crucial for biological activity of proteins with START domains. Wild-type StAR increased steroidogenic activity by 7-9-fold compared to mutant R182L StAR, but both of them showed similar near-UV CD spectra. The fluorescence maximum of wild-type StAR is red shifted in comparison to mutant StAR under identical urea concentration. TFE increased the a-helical contribution of wild-type StAR more than the mutant protein. Acrylamide quenching for the wild-type protein (K-SV = 12.0 +/- 0.2-11.2 +/- 0.5 M-1) exceeded that of the mutant protein (K-SV = 4 +/- 0.2 M-1). Consistent with these findings, the hydrophobic probe ANS bound wild-type StAR (K-app = 8.1 x 10(5) M-1) to a greater degree than mutant StAR (K-app = 3.75 x 10(5) M-1). Partial proteolysis examined by mass spectrometry suggests that only wild-type StAR has a protease-sensitive C-terminus, but not the mutant. Stopped-flow CD revealed that the time of unfolding of mutant StAR was 0.017 s. In contrast, the wild-type StAR protein is unfolded in 16.3 s. In summary, these results demonstrate that wild-type StAR adopts a very flexible form due to the accommodation of more water molecules, while mutant StAR is generated by an alternate folding pathway making it inactive.