Cryo-electron tomography of cardiac myofibrils reveals a 3D lattice spring within the Z-discs

Abstract

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 α-actinin in the Z-disc, allowing transmission of tension across the myofibril. Despite decades of studies, the 3D structure of Z-disc has remained 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 actin-actinin cross-bridging complexes within the Z-discs in relaxed and activated states. We found that the α-actinin dimers showed contraction-dependent swinging and sliding motions in response to a global twist in the F-actin lattice. Our observation suggests that the actin-actinin complex constitutes a molecular lattice spring, which maintains the integrity of the Z-disc during the muscle contraction cycle.

The molecular basis for sarcomere organization in vertebrate skeletal muscle

“…In addition, a rare ''split-head'' conformation also occurs when the two heads from one myosin binds to two different adjacent thin filaments (10 pairs observed; 0.6% of all heads) (Figures 3D and S4). This conformation has been previously suggested (Offer and Elliott, 1978) and vaguely indicated by 2D projection images (Hirose and Wakabayashi, 1993). Our observation provides direct proof of this conformation in three-dimensions in the rigor state vertebrate sarcomere.…”

“…Our analysis shows that this arrangement was not an in vitro artifact but a feature of native sarcomeres. Interestingly, these doublets were not apparent in the recent reconstruction of the F-actin-α-actinin complex from branched cardiac myofibrils obtained through sub-volume averaging, in which the angles between α-actinin and actin resemble the thick-form Z-disc ( Oda and Yanagisawa, 2020 ). Importantly, as single α-actinins were not distinguishable in these tomograms, this study relied strongly on subtomogram averaging at certain positions, assuming a highly symmetric Z-disc.…”

“…The recently presented structure from resin-embedded sections of the highly atypical Z-disc of the midshipman sonic muscle, structures that are at least 10 times thicker than the Z-discs of representative vertebrate skeletal muscles, revealed a fundamental arrangement of transverse crosslinks with an axial displacement of 19.2 nm, in line with previous work at lower resolution ( Burgoyne et al., 2015 , 2019 ; Perz-Edwards and Reedy, 2011 ). A further study of the Z-disc from isolated branches of cardiac myofibrils using electron cryo-tomography (cryo-ET) determined averaged structures of actin-α-actinin complexes in two states (putatively, the relaxed and activated states) at ~23 Å resolution ( Oda and Yanagisawa, 2020 ). These two structures highlight changes within the Z-disc in response to torque applied by the myosin heads on thin filaments, whereby α-actinin can pivot via linker domains between the central “rod” domain and the N-terminal actin binding domain.…”

Order from disorder in the sarcomere: FATZ forms a fuzzy but tight complex and phase-separated condensates with α-actinin

“…( F ) Surface of the rod-α-actinin-2/FATZ-1 structure showing the sequence conservation of α-actinin interacting residues for FATZ-1 (alignment done using 1505 α-actinins from vertebrates). ( G ) Model of F-actin/α-actinin-2/FATZ-1 (F-actin in light and dark gray) based on a cryo–electron tomography structure of the Z-disk ( 47 ) and our integrative model. See also figs.…”

“…Electron tomography reconstructions of paracrystalline vertebrate Z-disks show a defined orientation for α-actinin-2 rod in cross-linked actin filaments ( 47 , 69 ). This cannot be explained in the context of the built-in flexibility in the neck region of α-actinin, which allows rotation of the rod along its longitudinal axis ( 12 ).…”