Availability, distribution and kinetics of the transferrin receptors of rabbit reticulocytes.

The binding of transferrin by reticulocytes was studied at 3 degrees C and 37 degrees C. It was demonstrated that 50% of the total number of receptors are in an exposed position on the outer side of the plasma membrane and 50% of them are in a masked position. All of the receptors can take either of the positions. The transition between these positions is temperature dependent and can be observed as the progressive phase of the transferrin binding. It is suggested that the transferrin receptors are in dimers, one of the subunits being in the exposed position the other in the masked position. The subunits change position simultaneously. This movement of the receptors is independent of the presence of transferrin. In the light of these results a new model for transferrin-cell cycle is proposed.