Characterisation of the human Multidrug resistence Protein 3

The human multidrug resistance protein 3 (MDR3, ABCB4) belongs to the ATP binding cassette (ABC) transporter family found in all kingdoms of life. MDR3 is located in the liver more specifically in the apical membrane of hepatocytes and is required for translocation of phosphatidylcholine (PC) from the inner to the outer leaflet of the canalicular membrane. In the canaliculus PC forms mixed micelles with bile salts and cholesterol that protect the canalicular membrane. Mutations in MDR3 can cause dysfunction, leading to various liver diseases, for example progressive familial intrahepatic cholestasis type 3 (PFIC3).

Interestingly, MDR3 shares 75% identity with the well-characterized multidrug resistance protein 1 (MDR1/ABCB1). However, up to date MDR3 shows no phenotype with respect to multidrug resistance.

To investigate the function of MDR3 in vitro, we have expressed MDR3 in the yeast Pichia pastoris and were able to purify MDR3 with adequate yields via tandem-affinity chromatography. Purified MDR3 exhibited significant ATPase activity that can be stimulated by the lipids DPPC and DOPC.

So far our work shows that heterologously expressed MDR3 can be purified in a functional state and might provide the foundation to address questions like: why MDR3 is a PC floppase where as the homologue MDR1 is involved in multidrug resistance on a molecular level.