Impaired protein maturation of the conjugate export pump multidrug resistance protein 2 as a consequence of a deletion mutation in Dubin-Johnson syndrome

The Dubin-Johnson syndrome is an inherited disorder characterized by conjug ated hyperbilirubinemia, The deficient hepatobiliary transport of anionic c onjugates is caused by the absence of a functional multidrug-resistance pro tein 2 (MRP2, symbol ABCC2) from the apical (canalicular) membrane of hepat ocytes. Mechanisms underlying this deficiency may include rapid degradation of mutated MRP2 messenger RNA (mRNA) or impaired MRP2, protein maturation and trafficking. We investigated the consequences of the mutation MRP2 Delt a (R,M), which leads to the loss of 2 amino acids from the second ATP-bindi ng domain of MRP2, The MRP2 Delta (R,M) mutation is associated with the abs ence of the MRP2 glycoprotein from the apical membrane of hepatocytes. Tran sfection of mutated MRP2 complementary DNA (cDNA) led to an MRP2 Delta (R,M ) protein that was only core glycosylated, sensitive to endoglycosidase H d igestion, and located in the endoplasmic reticulum (ER) of transfected HEK2 93 and HepG2 cells. This indicated that deletion of Arg1392 and Met1393 lea ds to impaired maturation and trafficking of the protein from the ER to the Golgi complex. Inhibition of proteasome function resulted in a paranuclear accumulation of the MRP2 Delta (R,M) protein, suggesting that proteasomes are involved in the degradation of the mutant protein. This is the first mu tation in Dubin-Johnson syndrome shown to cause deficient MRP2, maturation and impaired sorting of this glycoprotein to the apical membrane.