Solution structure of human steroidogenic acute regulatory protein STARD1 studied by small-angle X-ray scattering

Intracellular cholesterol transfer to mitochondria, a bottleneck of adrenal and gonadal steroidogenesis, relies on the functioning of the steroidogenic acute regulatory protein (StAR, STARD1), for which many disease-associated mutations have been described. Despite significant progress in the field, the exact mechanism of cholesterol binding and transfer by STARD1 still remains debatable and often considers significant structural rearrangements to achieve ligand binding. The crystal structure of STARD1, obtained recently at medium resolution, suggests that this protein has the same fold as other members of the START family. However, hydrodynamic properties and solution conformation of STARD1 are insufficiently characterized, partially due to poor solubility of this protein. Here, we used our recent protocol to obtain stable and soluble STARD1 and analyzed its hydrodynamic properties and solution conformation using a previously inapplicable small-angle X-ray scattering (SAXS). The SAXS data obtained exclusively from a monodisperse fraction of the monomeric protein suggest that, apart from movements of the flexible Ω1-loop, STARD1 unlikely undergoes significant spontaneous rearrangements proposed earlier as a gating mechanism for cholesterol binding. The consistency with the previously reported solution NMR structure of STARD6 suggests similarity of hydrodynamic behavior of other STARD-containing proteins.