Phosphorylation of ABCB4 impacts its function: insights from disease-causing mutations.

Area of Science:

• Hepatology
• Molecular Biology
• Genetics

Background:

• ABCB4 transporter is crucial for phosphatidylcholine (PC) secretion in hepatocytes.
• Genetic defects in ABCB4 are linked to biliary diseases.
• The N-terminal domain of ABCB4 is poorly conserved and hypothesized to confer functional specificity.

Purpose of the Study:

• To investigate the role of ABCB4's N-terminal domain in its function.
• To identify and characterize ABCB4 variants associated with biliary diseases.
• To elucidate the impact of N-terminal variations on PC secretion and regulation.

Main Methods:

• Screening of a patient database for ABCB4 N-terminal domain variations.
• Expression of mutated ABCB4 variants in cell models (MDCK and HepG2).
• Assessment of protein maturation, targeting, stability, and PC secretion activity.
• In silico analysis, mass spectrometry, and pharmacological modulation of protein kinases.

Main Results:

• Two missense variants, T34M and R47G, were identified in patients with biliary diseases.
• Mutated ABCB4 proteins showed normal maturation and targeting but significantly decreased PC secretion.
• N-terminal variants impaired ABCB4 phosphorylation, which was further supported by in vitro and in vivo data.
• ABCB4 activity was modulated by protein kinases A and C, with mutants showing reduced responsiveness.

Conclusions:

• Disease-associated ABCB4 variants disrupt N-terminal phosphorylation, leading to reduced PC secretion.
• ABCB4 transporter activity is regulated by phosphorylation, particularly at N-terminal residues.
• These findings highlight the importance of N-terminal domain phosphorylation in ABCB4 function and biliary health.