STRUCTURAL BASIS FOR THE VESICOELASTICITY OF THE HUMAN ERYTHROCYTE-MEMBRANE - FOLDED SPECTRIN HYPOTHESIS

We have examined the structure of spectrin, which is a major component of the filamentous network underlying the human erythrocyte membrane, either as solublized dimers and tetramers or as a constituent of intact, as well as fragmented, skeletons. Electron micrographs of negatively stained specimens showed that spectrin molecules freshly liberated from the membrane bilayer assume a condensed structure. These condensed spectrin molecules then undergo gradual extension over a several-minute to hours period into the long, thin, flexible filaments commonly observed in the low-angle platinum-shadowed preparations of spectrin that were isolated by low-ionic-strength extraction. Our data support the hypothesis that the spectrin molecules assume a condensed rather than extended structure at the surface of the membrane in the erythrocyte at rest and suggest that the reversible interconversion of spectrin from a condensed to an extended conformation could be the molecular basis for the remarkable deformability and elasticity long observed in the membranes of intact red cells under mechanical shear stress.