Cloning of cDNAs encoding multidrug-resistance (MDR) associated, P-glycoprotein related protein (mini-P-glycoprotein) overexpressed in murine MDR cells.

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As reported previously, an approximately 65 kDa cellular protein, designated mini-P-glycoprotein (Pgp_<mini>), was originally identified in multidrug resistant (MDR) murine cell lines. In those studies, the Pgp_<mini> was suggested to share both an antigen epitope and some nucleotide sequences with P-glycoprotein (P-gp), and increased expression of this protein was found to be correlated with the level of MDR,although, in contrast to the P-gp, Pgp_<mini> was neither glycosylated nor phosphorylated. In the present study, we also demonstrated that some human MDR cells overexpress Pgp_<mini> as well as P-gp. Furthermore, we constructed a cDNA library from a highly MDR murine cell line into lambdagt10, and using a modified differential hybridization screening, we have identified cDNA clones which encode Pgp_<mini>. After the subcloning into plasmid vectors, we sequenced them. Sequence homology search of this gene shows it to be most closely related to P-gp, particularly mdrla. To clarify the function, works are currently underway to clone hybridomas producing monoclonal antibodies specific to Pgp_<mini> and to transfect with the cDNA into drug-sensitive cells to express it.