Molecular insights into the human ABCB6 transporter

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• ABCB6 protein is vital for porphyrin transport, drug resistance, and blood group antigen expression.
• The precise mechanism of ABCB6-mediated porphyrin transport remains largely unknown.
• Understanding ABCB6 function is crucial for various biological processes and disease mechanisms.

Purpose of the Study:

• To elucidate the structural basis of ABCB6 function, particularly its role in porphyrin transport.
• To determine the high-resolution structures of human ABCB6 in different functional states.
• To propose a mechanism for ATP-driven substrate translocation by ABCB6.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structures of nanodisc-reconstituted human ABCB6.
• Structures were obtained for both the apo-state and the ATP-bound state at high resolution (3.6 and 3.5 Å).
• Functional characterization and structural analyses were integrated to understand the mechanism.

Main Results:

• The cryo-EM structures revealed a unique loop within the transmembrane domain (TMD) of ABCB6.
• This loop divides the TMD into two cavities and regulates substrate access, being removed upon ATP binding.
• The ATP-bound state lacked ligand cavities, suggesting a post-substrate release state before ATP hydrolysis, supporting an "ATP-switch" model.

Conclusions:

• The study proposes an "ATP-switch" model for ABCB6 function, highlighting the role of ATP in regulating conformational changes.
• The identified unique loop and its dynamic behavior are key to understanding ABCB6-mediated transport.
• These findings provide critical insights into the molecular mechanism of porphyrin transport and ABCB6 regulation.