STRUCTURAL DETERMINANTS OF THE RIGIDITY OF THE RED-CELL MEMBRANE

Although the stability and viscoelasticity of the red cell membrane ar e undoubtedly governed by the membrane's underlying protein skeleton, the mechanism by which this network controls elasticity is uncertain. The structural constraints, that impose end-to-end spacing on the spec trin molecules well below that in free solution, may impart rubbery (e ntropic) elasticity to the system. However, other enthalpic and entrop ic contributions due to interactions between spectrin chains or betwee n spectrin and other proteins, the lipid bilayer or the solvent must a lso prevail. To relate structural features to elasticity, explicit mea surements of membrane rigidity are required. The most widely used meas urement is that of the membrane shear elastic modulus by micropipette aspiration. Analysis of genetic variants of membrane structure have sh own that the density of spectrin is directly correlated with membrane rigidity. Although cross-linking of the skeleton increases rigidity, i nterruption of the continuity of the network by dissociating spectrin tetramers into dimers does not reduce rigidity as might be expected. O n the other hand, external ligands that cause new interactions between integral proteins and the skeletal network do increase rigidity. More over, hereditary ovalocytes, which have a deletion of 9 amino acids fr om band 3 at the first point of entry into the membrane, are extremely rigid. This mutation is associated with decreased translational and r otational mobility of the band 3, and may impair flexural freedom of i ts cytoplasmic domain. It thus appears that elasticity may be regulate d not only by the structure of the spectrin network, but also by its i nteractions with and freedom of motion relative to the lipid bilayer.