The erythrocyte membrane skeleton is a protein network that provides deformability and stability to erythrocytes. Defects in the network lead to dysfunction and diseases. Erythrocyte tropomodulin (E-Tmod) of 41 kDa is an actin-capping protein at the slow-growing end, and together with tropomyosin 5 or 5b, has been proposed to form a ''molecular ruler'' dictating the length of actin protofilament of 37 nm in the network. We have previously created an E-Tmod knockout mouse model by targeted disruption of exon 1, which contains the AUG initiation codon. In this study, we showed that the embryonic lethality of the E-Tmod À/À mice was rescued by breeding with transgenic mice overexpressing E-Tmod in the heart and investigated the biomechanics of erythrocytes and its network topology. Western blot analysis revealed that a cytosolic E-Tmod29 isoform, which lacks the N-terminal F-actin binding domain, remains intact in the E-Tmod À/À erythrocytes, but is highly dimerized by oxidation. Micropipette aspiration indicated a higher elastic shear modulus and ektacytometry showed a lower average integrated elongation index. E-Tmod À/À mice had more microcytic erythrocytes with more compacted network on transmission electron microscopy. These results demonstrated the importance of E-Tmod41, the membrane-bound long isoform, in the network organization and mechanobiology of erythrocytes.
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