Tropomodulin assembles early in myofibrillogenesis in chick skeletal muscle: evidence that thin filaments rearrange to form striated myofibrils

Actin filament lengths in muscle and nonmuscle cells are believed to depend on the regulated activity of capping proteins at both the fast growing (ba rbed) and slow growing, (pointed) filament ends. In striated muscle, the po inted end capping protein, tropomodulin, has been shown to maintain the len gths of thin filaments in mature myofibrils, To determine whether tropomodu lin might also be involved in thin filament assembly, we investigated the a ssembly of tropomodulin into myofibrils during differentiation of primary c ultures of chick skeletal muscle cells. Our results show that tropomodulin is expressed early in differentiation and is associated with the earliest p remyofibrils which contain overlapping and misaligned actin filaments. In a ddition, tropomodulin can be found in actin filament bundles at the distal tips of growing myotubes, where sarcomeric cr-actinin is not always detecte d, suggesting that tropomodulin caps actin filament pointed ends even befor e the filaments are cross-linked into Z bodies by a-actinin, Tropomodulin s taining exhibits an irregular punctate pattern along the length of premyofi brils that demonstrate a smooth phalloidin staining pattern for F-actin, St rikingly, the tropomodulin dots often appear to be located between the clos ely spaced, dot-like Z bodies that are stained for a-actinin, Thus, in the earliest premyofibrils, the pointed ends of the thin filaments are clustere d and partially aligned with respect to the Z bodies (the location of the b arbed filament ends). At later stages of differentiation, the tropomodulin dots become aligned into regular periodic striations concurrently with the appearance of striated phalloidin staining for F-actin and alignment of Z b odies into Z lines. Tropomodulin, together with the barbed end capping prot ein, CapZ, may function from the earliest stages of myofibrillogenesis to r estrict the lengths of newly assembled thin filaments by capping their ends ; thus, transitions from nonstriated to striated myofibrils in skeletal mus cle are likely due principally to filament rearrangements rather than to fi lament polymerization or depolymerization. Rearrangements of actin filament s capped at their pointed and barbed ends may be a general mechanism by whi ch cells restructure their actin cytoskeletal networks during cell growth a nd differentiation.