Radiation Inactivation Suggests That Human Multidrug Resistance-Associated Protein 1 Occurs as a Dimer in the Human Erythrocyte Membrane

Molecular masses of functional units of two components of 2,4-dinitrophenyl-S-glutathione (DNP-SG) transport across the erythrocyte membrane determined by radiation inactivation were 437 ± 69 kDa for the high-affinity component and 466 ± 67 kDa for the low-affinity component. These results confirm that the multidrug resistance-associated protein (MRP) 1 is responsible for the high-affinity DNP-SG transport across the erythrocyte membrane and suggest that MRP1 exists in the membrane as a dimer. The molecular size of the low-affinity transporter is similar if not identical to that of MRP1. Moreover, while the molecular mass of the DNP-SG-ATPase activity of the erythrocyte membrane corresponds also to that of MRP (375 ± 36 kDa), the molecular mass of the functional unit of dinitrophenol-stimulated ATPase is significantly lower (232 ± 26 kDa), which suggests that thisactivity is linked to a different protein, perhapsaminophospholipid translocase.