Human ABCB5, an ATP-binding cassette (ABC) transporter gene, has two major mRNA species. One transcript encodes an 812 amino acid polypeptide, ABCB5 beta, with a transmembrane domain and a nucleotide-binding domain. We isolated the cDNA of another ABCB5 mRNA that encodes a 1257 amino acid polypeptide. The translated ABCB5 protein is a full-sized ABC transporter that has an internally duplicated structure with two transmembrane domains and two nucleotide-binding domains. The 5' and 3' parts of the ABCB5 mRNA were expressed in the prostate and testis. HEK293 cells transfected with the ABCB5 cDNA expressed a 150-160 kDa protein. The ABCB5 transfectants showed approximately 1.5-fold higher resistance to doxorubicin, and 2- to 3-fold higher resistance to paclitaxel and docetaxel. Cellular uptake of radiolabeled paclitaxel and docetaxel in the transfectants was lower than that in the parental HEK293 cells. Treatment of the transfectants with ABCB5-targeted siRNA lowered their resistance to docetaxel. Revertant cells that express a reduced amount of ABCB5 also showed a lowered level of docetaxel resistance. These results indicated that the expression of ABCB5 conferred resistance to taxanes and anthracyclines. Membrane vesicles prepared from ABCB5 baculovirus-infected Sf21 cells showed higher vanadate-sensitive ATPase activity than the Sf21 control vesicles. The km and 14, values of ATPase activity in the ABCB5 vesicles were 1.8 mM and 65 nmol/min/mg protein, respectively. ABCB5 ATPase activity was 1.25-fold higher in the presence of 100 mu M docetaxel than it was in the absence of docetaxel. These results indicates that the full-length ABCB5 protein has ATPase activity that is sensitive to docetaxel. (C) 2012 Elsevier Inc. All rights reserved.