Analysis of a novel cDNA encoding a C219-reactive peptide isolated from methotrexate-selected multidrug-resistant human leukemic cells

We have isolated and sequenced a novel cDNA clone (D320), from methotrexate-selected multidrug-resistant (MDR) human leukemic CCRF-CEM cells, which encodes a peptide reactive with monoclonal antibody (mAb) C219. Despite having been cloned using this ostensibly P-glycoprotein-specific mAb, clone D320 has no significant homology with either the MDR-encoding gene or, at the amino acid (aa) level, with its product, P-glycoprotein. A putative C219-binding site has been identified in D320, which differs at two positions from the 6-aa C219 epitope previously described in P-glycoprotein.