Structures of ABCB4 provide insight into phosphatidylcholine translocation

Area of Science:

• Biochemistry
• Structural Biology
• Cell Biology

Background:

• Hepatocyte ABCB4 protein facilitates phosphatidylcholine translocation into bile canaliculi.
• Understanding ABCB4's lipid recruitment mechanism is vital for mitigating bile salt cytotoxicity.

Purpose of the Study:

• To elucidate the structural mechanisms of human ABCB4 in phospholipid translocation.
• To investigate the role of specific residues and drug interactions in ABCB4 function.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine high-resolution structures of ABCB4 in multiple conformations.
• Proteoliposome-based translocation assays using fluorescent phosphatidylcholine analogs.
• In vitro functional assays to confirm substrate specificity and residue importance.

Main Results:

• Three distinct functional conformations of human ABCB4 were resolved by cryo-EM.
• Structural data revealed the mechanism of phospholipid recruitment and identified key interactions, including cation-π interactions with tryptophan, conferring phosphatidylcholine specificity.
• A posaconazole-bound structure demonstrated drug-induced inhibition by blocking substrate access.
• In vitro assays confirmed ABCB4's substrate specificity and the critical role of the identified tryptophan residue.

Conclusions:

• The study provides unprecedented structural insights into the essential translocation of phosphatidylcholine by ABCB4 during bile generation.
• The findings rationalize ABCB4's specificity for phosphatidylcholine and its inhibition by azole drugs like posaconazole.
• This work lays the foundation for understanding bile salt-related liver disorders and developing targeted therapies.