MRP3, a new ATP-binding cassette protein localized to the canalicular domain of the hepatocyte

Bile secretion in liver is driven in large part by ATP-binding cassette (AB C)-type proteins that reside in the canalicular membrane and effect ATP-dep endent transport of bile acids, phospholipids, and non-bile acid organic an ions. Canalicular ABC-type proteins can be classified into two subfamilies based on membrane topology and sequence identity: MDR1, MDR3, and SPGP rese mble the multidrug resistance (MDR) P-glycoprotein, whereas MRP2 is similar in structure and sequence to the multidrug resistance protein MRP1 and tra nsports similar substrates. We now report the isolation of the rMRP3 gene f rom rat liver, which codes for a protein 1522 amino acids in length that ex hibits extensive sequence similarity with MRP1 and MRP2. Northern blot anal yses indicate that rMRP3 is expressed in lung and intestine of Sprague-Dawl ey rats as well as in liver of Eisai hyperbilirubinemic rats and TR- mutant rats, which are deficient in MRP2 expression. rMRP3 expression is also tra nsiently induced in liver shortly after birth and during obstructive choles tasis. Antibodies raised against MRP3 recognize a polypeptide of 190-200 kD a, which is reduced in size to 155-165 kDa after treatment with endoglycosi dases. Immunoblot analysis and immunoconfocal microscopy indicate that rMRP 3 is present in the canalicular membrane, suggesting that it may play a rol e in bile formation.