Is an intact cytoskeleton required for red cell urea and water transport?

Abstract

In order to determine the membrane protein(s) responsible for urea and water transport across the human red cell membrane, we planned to reconstitute purified membrane proteins into phosphatidylcholine vesicles. In preparatory experiments, we reconstituted a mixture of all of the red cell integral membrane proteins into phosphatidylcholine vesicles, but found that p-chloromercuribenzenesulfonate (pCMBS), which normally inhibits osmotic water permeability by approximately 90%, has no effect on this preparation. The preparation was also unable to transport urea at the high rates found in red cells, though glucose transport was normal. White ghosts, washed free of hemoglobin and resealed, also did not preserve normal urea and pCMBS-inhibitable water transport. One-step ghosts, prepared in Hepes buffer in a single-step procedure, without washing, retained normal urea and pCMBS-inhibitable water transport. Perturbation of the cytoskeleton in one-step ghosts, by removal of tropomyosin, or by severing the ankyrin link which binds band 3 to spectrin, caused the loss of urea and pCMBS-inhibitable water transport. These experiments suggest that an unperturbed cytoskeleton may be required for normal urea and pCMBS-inhibitable water transport. They also show that the pCMBS inhibition of water transport is dissociable from the water transport process and suggest a linkage between the pCMBS water transport inhibition site and the urea transport protein.