Conformational heterogeneity of P-glycoprotein.

P-glycoprotein (P-gp) acts as an active efflux mechanism for a large number of cytostatics and seems to be involved in the frequent failure of cancer chemotherapy. The molecular events of substrate recognition and transport still are not understood completely. We show here that the percentage of P-gp epitopes available for labeling with UIC2 monoclonal antibody is increased significantly after methanol permeabilization/fixation of cells. At the same time, binding of the MRK16 and 4E3 anti-P-gp antibodies is changed only moderately. Confocal microscopical images of UIC2-PE-labeled cells show that the epitopes becoming available after fixation are situated mainly in the plasma membrane. Thus, only a minority of P-gp molecules are accessible for UIC2 in the cell membrane of live cells, and methanol treatment can expose a large pool of previously plasma membrane-embedded, cryptic UIC2 epitopes. The UIC2-reactive P-gp molecules do not appear to be sequestered spatially, as suggested by the high fluorescence resonance energy transfer efficiency measured between the fluorescently labeled competing UIC2 and MRK16 antibodies, suggestive of P-gp dimerization and oligomerization on live cells.