Structures of the CcmABCD heme release complex at multiple states.

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Cytochromes c are essential electron carriers in respiration and photosynthesis, utilizing heme as a cofactor.
• The cytochrome c maturation system I (CcmABCDEFGH) facilitates the covalent attachment of heme to cytochrome c proteins in the periplasm.
• Efficient heme delivery to periplasmic proteins, particularly the heme chaperone CcmE, is crucial for cytochrome c function.

Purpose of the Study:

• To elucidate the structural basis and mechanistic details of heme transport by the CcmABCD complex.
• To understand the role of ATP hydrolysis in the heme transfer process mediated by CcmABCD.
• To characterize the interaction between CcmABCD and the periplasmic heme chaperone CcmE.

Main Methods:

• High-resolution cryo-electron microscopy (cryo-EM) was employed to determine the structures of CcmABCD.
• Structures were obtained for the unbound complex, in complex with the AMP-PNP inhibitor, and in complex with ATP and heme.
• Functional studies were integrated with structural data to propose a mechanistic model.

Main Results:

• High-resolution cryo-EM structures of the CcmABCD complex were determined in various functional states.
• The ATP-binding site was localized to the CcmA subunit, and the heme-binding site was identified within the CcmC subunit.
• A model for ATP-dependent heme trafficking, transfer to CcmE, and release of holo-CcmE from CcmABCD was proposed.

Conclusions:

• CcmABCD functions as an ATP-binding cassette (ABC) transporter complex.
• The complex utilizes ATP hydrolysis to facilitate the transfer of heme from CcmC to CcmE.
• This study provides critical structural insights into the mechanism of c-type cytochrome heme delivery.