Figure 7.
Different types of Z-discs from fast mouse psoas myofibril and α-actinin organization in the thinner form
(A–C) XY slice view (A), equator-filtered cross-section view (B), and a schematic diagram showing the pattern (C) of a Z-disc in the thinner form within a tomogram. The thickness of the Z-disc was measured between where α-actinins bind to the antiparallel thin filaments. The length of a single thin filament within the Z-disc was also measured. The organization of the filaments in this Z-disc is rhomboidal, likely resulting from a tilted orientation of the Z-disc on the grid and a slight stretching of the myofibril. We used this Z-disc for our analysis because most of the α-actinin densities could be unambiguously assigned. Scale bar, 50 nm.
(D–F) XY slice view (D), cross-section view (E), and a schematic diagram of a tomogram showing a Z-disc in the thick form with a square pattern (F). This is the predicted organization of a Z-disc from skeletal muscle. However, it is difficult to assign individual α-actinin densities in these tomograms hampering a non-biased annotation. Scale bar, 50 nm.
(G) Slice through the same tomogram as (A) depicting the thin-form Z-disc and two I-bands. Scale bar, 100 nm.
(H) Cryo-ET based 3D model of the Z-disc showing antiparallel thin filaments from two adjacent sarcomeres (green and magenta) and the α-actinins (blue) cross-linking the filaments.
(I) Tilted view of the model shown in (H).
(J) Slice of an example unit in the Z-disc depicting one thin filament with its neighboring antiparallel thin filaments and the α-actinins connecting them. A doublet of α-actinins is highlighted by a red arrow head. Scale bar, 20 nm.
(K) Cryo-ET-based 3D model of the same region in (J). Actin filament model is derived from the thin filament reconstruction in Figure 6A, excluding tropomyosin. The α-actinin model is derived from the crystal structure of an α-actinin dimer (PDB: 4D1E).
(L) Projection image (7-nm thickness) of the 3D reconstruction of α-actinin obtained from averaging the sub-volumes as picked in (H) and (I), depicting four domains (marked by red arrow heads) which correspond to the four spectrin-like repeats (SRs) in the rod region. Scale bar, 20 nm.