The research team led by Prof. CHEN Yuxing, Prof. ZHOU Congzhao and Prof. SUN Linfeng from Hefei National Laboratory for Physical Sciences at the Microscale and School of Life Sciences of University of Science and Technology of China (USTC) of Chinese Academy of Sciences (CAS), presented a high-resolution 3D-structure of human bile salts exporter ABCB11, by using cryogenic electron microscopy (cryo-EM). This structure provides a basic to clearly understanding the transport mechanism of these membrane proteins and the pathogenesis caused by their mutations. The study was published in Cell Research on March 20th.
Bile salts are the major part in bile, which was synthesized in liver cells by cholesterol as a precursor and transported to the bile canaliculi by membrane proteins to participate in the formation of bile. Bile is mainly used in the small intestine to assist in the digestion and absorption of fatty substances but the obstruction of the efflux of bile salt will lead to a series of cholestasis related disease, such as progressive familial intrahepatic cholestasis (PFIC), benign recurrent intrahepatic cholestasis (BRIC), intrahepatic cholestasis (ICP) and drug-induced cholestasis (DIC). PFIC is the severest disease among them, which is usually found in children, with liver enlargement, liver cirrhosis and ultimately liver failure leading to death before adulthood. Most patients with PFIC need a liver transplant to survive.
The ATP binding cassette (ABC) transporter, Human ABCB11, localizing on the apical canalicular domain of hepatocytes, is the most important protein for the secretion of bile salt to the bile canaliculi. However, due to the mutations in the gene that encodes this protein, the various cholestasis disease mention above will be caused. In the past 20 years since this gene was discovered, the researches on ABCB11 have been reported continuously, but the mechanism of the transportation of bile salt by this protein is still unknown. Using cryo-EM, the team solved the apo-form structure of human ABCB11 at the resolution of 3.5??. ABCB11 is composed of 1321 amino acid residues and functions as a monomer. It is a full ABC transporter that consists of two transmembrane domains (TMDs) and two nucleotide-binding domains (NBDs) in one polypeptide chain and with an ɑ helix at N-terminus, showing an inward-open conformation.
According to the 3-D structure of human ABCB11, the pathogenic mechanism of this protein was analyzed. The researchers found that mutations in clinical samples disrupted interactions within protein molecules or misfolded proteins, leading to reduced or total loss of protein transport function, eventually to related diseases. A series of bile salts and two inhibitors (Rifampicin and Glibenclamide) were used to stimulate the hydrolysis activity of ATP. The two inhibitors were found to competitively inhibit the activity of the protein, which is one of the main reasons of the damage of liver caused by such drugs.
This study not only has important reference significance for the study of the molecular structure of these proteins, but also provides theoretical guidance for the pathogenesis analysis of related diseases and rational drug design.
Paper link: https://www.nature.com/articles/s41422-020-0302-0
(Written by LU Hongyu, edited by YE Zhenzhen, USTC News Center)