Structural basis of substrate recognition and translocation by human ABCA4

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• ATP-binding cassette (ABC) subfamily A (ABCA) transporters are crucial for lipid transport.
• Mutations in ABCA transporters cause hereditary lipid disorders.
• The precise mechanisms of substrate recognition and translocation by ABCA transporters remain largely unknown.

Purpose of the Study:

• To elucidate the molecular mechanism of human ABCA4 transporter function.
• To determine the structural basis of substrate recognition and translocation by ABCA4.
• To provide insights into the general mechanism of ABCA-mediated lipid transport.

Main Methods:

• Three cryo-electron microscopy (cryo-EM) structures of human ABCA4 were determined.
• Structures were resolved at 3.3-3.4 Å resolution.
• Distinct functional states of the transporter were captured.

Main Results:

• The nucleotide-free state reveals a lateral-opening conformation of the transmembrane domains (TMDs), facilitating substrate entry from the lipid bilayer.
• The physiological substrate, N-retinylidene-phosphatidylethanolamine (NRPE), is bound between the TMDs in the luminal leaflet.
• The ATP-bound state shows a closed conformation of the TMDs, preventing substrate binding.

Conclusions:

• The study provides a molecular understanding of ABCA4-mediated NRPE recognition and translocation.
• A common 'lateral access and extrusion' mechanism is proposed for ABCA-mediated lipid transport.
• These findings offer a foundation for understanding ABCA transporter function in health and disease.