The molecular basis for the elasticity of the human erythrocyte membrane was explored. Skeletons were released from ghosts in Triton X-100 and their dimensions followed by dark-field microscopy and packed volume. The rest size of skeletons was assumed to reflect the balance point between expansion (deformation) driven by electrostatic repulsions among the excess of fixed negative charges on the proteins and contraction (recovery) driven by their elasticity. The size of skeletons decreased with increasing temperature. This finding suggests that entropy drives elasticity. The requisite entropy change could be associated with either the configurational freedom of flexible protein chains or with the solvation of side chains exposed during protein dissociation (hydrophobic effects). To distinguish between these two alternatives, we tested the impact of two weak denaturants, 10% ethanol and 20 nM lithium 3,5-diiodosalicylate. Both agents reversibly promoted the expansion of skeletons, presumably by reducing their elasticity. Since the conformation of random coils and globular proteins should not be significantly altered by these mild treatments, this finding strongly suggests a role for weak interdomain and/or interprotein associations. We conclude that the elasticity of the red cell membrane skeleton may not derive from the configurational entropy of flexible coils. Rather, the elastic energy may arise from reversible dissociations of weak but specific intramolecular and/or intermolecular contacts, presumably within deformed spectrin filaments.
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