The product of the ABC half-transporter gene ABCG2 (BCRP/MXR/ABCP) is expressed in the plasma membrane

The products of the ABC gene family can be generally classified as either f ull-transporters of half-transporters. Full-transporters are expressed in t he plasma membrane, whereas half-transporters are usually found in intracel lular membranes. Recently, an ABC half-transporter, the ABCG2 gene product Breast Cancer/Mitoxantrone Resistance Protein (BCRP/ MXR), has been shown t o cause mitoxantrone and topotecan resistance. The purpose of this study wa s to determine the expression and the intracellular localization of this pr otein in various drug-resistant cell lines. BCRP/MXR expression was determi ned by Western blot and immunohistochemistry. This protein is highly overex pressed in several drug-resistant cell lines and localizes predominantly to the plasma membrane, instead of to intracellular membranes as seen with al l Other known half-transporters. Therefore, BCRP/MXR is unique among the AB C half-transporters by being localized to the plasma membrane. (C) 2000 Aca demic Press.