Cryo-electron tomography of cardiac myofibrils reveals a contraction-induced lattice twist in the Z-discs.

The Z-disc forms a boundary between sarcomeres, which constitute structural and functional units of striated muscle tissue. Actin filaments from adjacent sarcomeres are cross-bridged by -actinin in the Z-disc, allowing transmission of tension across the myofibril. Despite decades of studies, the 3D structure of Z-disc has been elusive due to the limited resolution of conventional electron microscopy. Here, we observed porcine cardiac myofibrils using cryo-electron tomography and reconstructed the 3D structures of the actinactinin cross-bridging complexes within the Z-discs in relaxed and activated states. We found that the -actinin showed a contraction-induced swing motion along with a global twist in the actin lattice. Our observation suggests that the elasticity and the integrity of the Z-disc during the muscle contraction cycle are maintained by the structural flexibility within the actin-actinin complex.