Identification and characterization of a plasma membrane phosphoprotein which is present in Chinese hamster lung...

Studies have been carried out to analyze the phosphoprotein composition of plasma membranes from Chinese hamster lung cells resistant to the action of adriamycin. Gel electrophoretic analysis of [32Pi]-labeled proteins revealed that plasma membranes from resistant cells contain a phosphoprotein of 180,000 molecular weight (P180) which is not detected in drug sensitive cells. Protein P180 can also be identified after phosphorylation of resistant plasma membranes in an in vitro protein kinase system. Pulse-chase experiments indicated that the P180 was metabolically active and underwent cycles of phosphorylation and dephosphorylation in the cell. Additional studies showed that, in the presence of N-ethylmaleimide (NEM), there was a major increase in the uptake of adriamycin in resistant cells. A similar effect was observed with KCN but not with sodium azide. When resistant cells were grown in the presence of [32Pi] and then incubated in the presence of NEM, there was a considerable increase in the phosphorylation of P180. In contrast, many other plasma membrane proteins were dephosphorylated under these incubation conditions. The results suggest the possibility that, as P180 was hyperphosphorylated, the protein was inactivated and this contributed to the ability of resistant cells to accumulate adriamycin.