F-actin networks are crucial regulators of tissue and cell mechanics. Among the actin-stabilising proteins are the tropomyosins (Tpms), which control interaction of actin-binding proteins with actin filaments. Tpm isoforms contribute to the creation of specialised actin networks with different actin-binding proteins. Whereas the roles of different Tpms in vitro and in muscle-specific biomechanical behaviour are well studied, the in vivo functions of Tpms in non-muscle cells are poorly understood. Now, Velia Fowler and colleagues (Cheng et al., 2018) investigate the function of Tpm isoforms in tissue mechanics in the eye lens, which requires specific biomechanical properties for the purpose of focusing light. The authors find that the isoform Tpm3.5 is associated in small puncta with F-actin-rich lens fibre cell membranes. A partial loss of Tpm3.5 results in softer lenses and compromises the ability of lenses to resume original shape after compression. Furthermore, they observe that the actin-interactor tropomodulin-1 (Tmod1) dissociates from the membrane in cells with lower levels of Tpm3.5, which leads to a reorganisation of the actin networks. Consequently, the mechanical properties of the lens appears to be changed, with a decreased ability to support mechanical load. This study is the first to establish a role for a Tpm in load-bearing non-muscle tissue in vivo.
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