Fig. 6: Proposed model for ISP precursor translocation by Bcs1L.

Three nucleotide-binding states of mBcs1L are depicted schematically: Apo, ATP, and ADP. The ATP state is represented by two conformations: ATP state-1 and ATP state-2. Bound ATP is shown as T and ADP as D. The TM domain is shown in blue, Bcs1-specific domain in orange and AAA domain in green for the large domain and blue for the small domain. The mitochondrial inner membrane (MIM) is represented by two parallel horizontal lines and shaded gray. The substrate ISP precursor is also shown. The two substrate cavities were shown and labeled as Matrix cavity and MIM cavity, respectively. Also depicted are two seals: the IMS-seal and the Matrix-seal. In the apo/ADP conformation, both IMS-seal and Matrix-seal are closed. In the ATP state-1 conformation, both Matrix-seal and IMS-seal are open transiently, allowing passage of ISP-ED. In the ATP state-2 conformation, both Matrix-seal and IMS-seal are retracted but not fully closed, wrapping around the TM helix. The ATP hydrolysis resets the mBcs1 to the ADP conformation, presumably allowing the escape of the TM helix. The nucleotide exchange also permits transient release of ISP precursor.