Cholesterol fill-in model: mechanism for substrate recognition by ABC proteins

Area of Science:

• Biochemistry
• Molecular Biology
• Human Physiology

Background:

• Human ATP-binding cassette (ABC) proteins are crucial for lipid homeostasis and defense against hydrophobic xenobiotics.
• Dysfunction of ABC proteins is linked to various human diseases, highlighting their importance in health.
• The precise mechanism by which ABC proteins transport diverse hydrophobic compounds and cellular lipids remains largely unknown.

Purpose of the Study:

• To elucidate the substrate recognition mechanism of lipid-transporting versus drug-transporting ABC proteins.
• To investigate the role of cholesterol and phospholipids in the binding and transport functions of ABC proteins.
• To propose a novel model for ABC protein substrate interaction.

Main Methods:

• Comparative functional analysis of drug-transporting and lipid-transporting ABC proteins.
• Investigation of substrate binding site characteristics and interactions with phospholipids and cholesterol.
• Examination of the role of cholesterol in substrate binding, particularly for ABCB1/MDR1.

Main Results:

• ABC proteins, regardless of their primary function (lipid or drug transport), share a common substrate binding site capable of recognizing phospholipids, cholesterol, and drugs.
• Cholesterol plays a significant role in substrate recognition by binding to ABC proteins alongside phospholipids or drugs.
• Cholesterol was observed to fill unoccupied space in the substrate binding site of ABCB1/MDR1 when small drugs were bound, indicating a flexible interaction.

Conclusions:

• ABC proteins exhibit flexible, one-to-many substrate recognition capabilities, deviating from traditional one-to-one models.
• The binding of cholesterol is integral to the substrate recognition mechanism of various ABC proteins.
• A novel mechanism, termed the "cholesterol fill-in model," is proposed to explain the flexible substrate handling by ABC proteins.