Abstract
START domain proteins are conserved α/β helix-grip fold that play a role in the non-vesicular and intracellular transport of lipids and sterols. The mechanism and conformational changes permitting the entry of the ligand into their buried binding sites is not well understood. Moreover, their functions and the identification of cognate ligands is still an active area of research. Here, we report the solution structure of STARD6 and the characterization of its backbone dynamics on multiple time-scales through 15N spin-relaxation and amide exchange studies. We reveal for the first time the presence of concerted fluctuations in the Ω1 loop and the C-terminal helix on the microsecond-millisecond time-scale that allows for the opening of the binding site and ligand entry. We also report that STARD6 binds specifically testosterone. Our work represents a milestone for the study of ligand binding mechanism by other START domains and the elucidation of the biological function of STARD6.
Highlights
Whereas the three-dimensional structures of four of the five START domains of the STARD1 and STARD4 subfamilies; STARD1 and STARD510, STARD32 and STARD411 have been solved, the structure of STARD6 remains to be determined
Except for some loop deletions and insertions, the structures of all these START domains are very similar; the backbone superposition of STARD110, STARD32, STARD411, STARD510 and STARD6 is shown in Supplementary Fig. 8
We demonstrate the concomitant conformational exchange in the Ω1 loop and the N-terminal portion of α4 in the apo-state of STARD6
Summary
Whereas the three-dimensional structures of four of the five START domains of the STARD1 and STARD4 subfamilies; STARD1 and STARD510, STARD32 and STARD411 have been solved, the structure of STARD6 remains to be determined. Unbiased molecular dynamic simulations on the μs time-scale have sampled structural rearrangements of STARD6 consistent with those fluctuations and suggest that they induce the coalescence of the internal cavity and the surrounding solvent, allowing for the ligand entry. This constitutes the first experimental demonstration of a detailed molecular mechanism for ligand binding by a START domain
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