Role of basic residues within or near the predicted transmembrane helix 2 of the human breast cancer resistance protein in drug transport

The human breast cancer resistance protein (BCRP/ABCG2) me-diates efflux of drugs and xenobiotics out of cells. In this study, we investigated the role of five basic residues within or near trans-membrane (TM) 2 of BCRP in transport activity. Lys452, Lys453, His457, Arg465, and Lys473 were replaced with Ala or Asp. K452A, K453D, H457A, R465A, and K473A were stably expressed in human embryonic kidney (HEK) cells, and their plasmamembrane expression and transport activities were examined. All of the mu-tants were expressed predominantly on the plasma membrane of HEK cells. After normalization to BCRP levels, the activities of K452A and H457A in effluxing mitoxantrone, boron-dipyr-romethene-prazosin, and Hoechst33342 were increased approx-imately 2- to 6-fold compared with those of wild-type BCRP, whereas the activities of K453D and R465A were decreased by 4