Figure 6.

Structure and organization of thin filaments in the I-band

(A) Sub-volume average of the thin filament in the I-band fitted with a homology model of the complete thin filament including troponin. Homology model is based on PDB: 6KN8.

(B) 3D reconstruction of the thin filament in the I-band excluding troponin. The actin filament is depicted in green and tropomyosin in blue. At this resolution, the four subdomains of actin form a clear “U” shape (marked by the arrow head) and indicates actin barbed end faces toward the Z-disc.

(C) Tropomyosin along thin filaments takes the C-state in the I-band (blue) and the M-state in the A-band (yellow).

(D) XZ views of sarcomeres at the A-band and I-band highlighting the disappearance of the hexagonal pattern of thin filaments. Two hexagonal units of thin filaments and their corresponding positions in the I-band are indicated by green and orange triangles. Displacement of the filaments from A-band to I-band are shown as arrows. Asterisk denotes a filament that moved out of the field of view during the A–I transition.

(E) Slice through a tomogram depicting the Z-disc and two I-bands from two adjacent sarcomeres.

(F) 3D model of thin filaments showing the same region as in (E).

(G) Cross-section views of the positions indicated by dotted lines in (F), showing the pattern of thin filaments during the I-Z-I transition. Filaments are traced in green triangles from the I-band to Z-disc of the top sarcomere and in magenta triangles from the Z-disc to the I-band of the bottom sarcomere. In the Z-disc image, antiparallel filaments in the center are labeled with green and magenta dots.

(H) A schematic diagram shows the hexagonal pattern in the A-band, the irregular pattern in the I-band and the rhomboid pattern in the Z-disc.

Scale bars, 2 nm (A), 50 nm (D–G).

See also Figure S6.