Mouse transporter protein, a membrane protein that regulates cellular multidrug resistance, is localized to lysosomes

Mouse transporter protein (MTP), a small, highly conserved mammalian intrac ellular membrane protein with four putative transmembrane domains, has been implicated in the transport of nucleosides and/or related molecules across intracellular membranes. The production of recombinant MTP in Saccharomyce s cerevisiae alters sensitivity of yeast cells to a heterogeneous group of compounds (e.g., antimetabolites, antibiotics, anthracyclines, ionophores, and steroid hormones) by changing the subcellular compartmentalization of t hese drugs, suggesting that MTP functions similarly in higher organisms. Th e present study was undertaken to define the intracellular location of MTP in mammalian cells, Native MTP was not detected by indirect immunofluoresce nce in cell types that expressed MTP mRNA; therefore, a hemagglutinin (RA) epitope-tagged version of MTP was produced in cultured BHK21 cells by trans ient transfection, and its distribution within cells was determined by conf ocal microscopy using antibodies directed against the HA epitope and variou s organellar proteins. Antibodies directed against HA-MTP colocalized with antibodies against late endosomal and lysosomal proteins but not with antib odies against either Golgi or early endosomal proteins, Analysis of subcell ular fractions from rat liver by immunoblotting with antibodies directed ag ainst MTP demonstrated the presence of a MTP-like protein in Golgi- and lys osome-enriched membranes but not in mitochondria, These results indicate th at MTP resides in late endosomes and lysosomes, a finding that is consisten t with the proposed role for MTP in the movement of a variety of small mole cules across endosomal and lysosomal membranes. MTP shares a number of char acteristics with other lysosome-associated proteins. We, therefore, propose that it be redesignated murine lysosome-associated protein transmembrane 4 .