Spectrin tetramers of the membranes of enucleated mammalian erythrocytes play a critical role in red blood cell survival in circulation. One of the spectrins, αI, emerged in mammals with enucleated red cells after duplication of the ancestral α-spectrin gene common to all animals. The neofunctionalized αI-spectrin has moderate affinity for βI-spectrin, whereas αII-spectrin, expressed in nonerythroid cells, retains ancestral characteristics and has a 10-fold higher affinity for βI-spectrin. It has been hypothesized that this adaptation allows for rapid make and break of tetramers to accommodate membrane deformation. We have tested this hypothesis by generating mice with high-affinity spectrin tetramers formed by exchanging the site of tetramer formation in αI-spectrin (segments R0 and R1) for that of αII-spectrin. Erythrocytes with αIIβI presented normal hematologic parameters yet showed increased thermostability, and their membranes were significantly less deformable; under low shear forces, they displayed tumbling behavior rather than tank treading. The membrane skeleton is more stable with αIIβI and shows significantly less remodeling under deformation than red cell membranes of wild-type mice. These data demonstrate that spectrin tetramers undergo remodeling in intact erythrocytes and that this is required for the normal deformability of the erythrocyte membrane. We conclude that αI-spectrin represents evolutionary optimization of tetramer formation: neither higher-affinity tetramers (as shown here) nor lower affinity (as seen in hemolytic disease) can support the membrane properties required for effective tissue oxygenation in circulation.

去核哺乳动物红细胞膜的 Spectrin 四聚体在红细胞在循环中的存活中起关键作用。在所有动物共有的祖先α-血影蛋白基因复制后，其中一种血影蛋白α I 出现在具有去核红细胞的哺乳动物中。新功能化的α I- spectrin对β I-spectrin具有中等亲和力，而在非红细胞中表达的α II-spectrin 保留了祖先特征并且对β具有高 10 倍的亲和力I-血影蛋白。已经假设这种适应允许四聚体的快速形成和断裂以适应膜变形。我们通过将α I-Spectrin（R0 和 R1 段）中的四聚体形成位点与α II-Spectrin的四聚体形成位点交换来生成具有高亲和力 Spectrin 四聚体的小鼠来验证这一假设。具有α II β I的红细胞表现出正常的血液学参数，但显示出更高的热稳定性，并且它们的膜明显不易变形；在低剪切力下，它们表现出翻滚行为而不是坦克踩踏。α II β使膜骨架更稳定与野生型小鼠的红细胞膜相比，I 和显示在变形下的重塑显着减少。这些数据表明，血影蛋白四聚体在完整的红细胞中发生重塑，这是红细胞膜正常变形所必需的。我们得出结论，α I-spectrin 代表了四聚体形成的进化优化：亲和力较高的四聚体（如图所示）和较低亲和力（如溶血性疾病中所见）都不能支持循环中有效组织氧合所需的膜特性。