Figure 4.

Length regulation and the two-segment model of thin filaments. Schematics at the top depict the two-segment model: in muscles with short distal thin filament segments (fast twitch), the length of nebulin determines the length of the thin filament by interacting with and stabilizing every actin monomer in the proximal segment. In muscles with long distal thin filament segments (slow twitch) and in cardiac muscles, the thin filament length is primarily mediated by the interplay between Tmod and Lmod at the pointed end and the role of nebulin and nebulette is only secondary. Similarly, in invertebrate (Drosophila) muscles, thin filament length is thought to be regulated by the interplay between Tmod and SALS, and the nebulin repeat protein Lasp is only secondary. Note that Lasp is likely to also be present in the A-band, however, in the flight muscle, it is only detected in the Z-disc. The molecular models at the bottom demonstrate that nebulin (colored in orange) is present in the grooves between the two strands of the F-actin helix in the A-band region of thin filaments [196]; PDB: 7QIM.