Nebulin, a giant actin-binding protein, is the largest member of a family of proteins (including N-RAP, nebulette, lasp-1 and lasp-2) which are assembled in a variety of cytoskeletal structures and expressed in different tissues. For decades nebulin has been thought to act as a molecular ruler, specifying the precise length of actin filaments in skeletal muscle. However, emerging evidence suggests that nebulin should not be viewed as a ruler but as an actin filament stabilizer required for length maintenance. Nebulin has also recently been implicated in an array of regulatory functions independent of its role in actin filament length regulation. In this review, we discuss the current evolutionary, biochemical, and functional data for the nebulin family of proteins -a family whose members, both large and small, function as cytoskeletal scaffolds and stabilizers.

To further understand the functional role of lasp-2, a focal adhesion protein, a survey identifies two novel binding partners: vinculin and paxillin. The data suggest that lasp-2 has an important role in regulating the composition and dynamics of focal adhesions, as well as significant roles in metastasis, with dual functions in migration and invasion.

Tropomodulin1 (Tmod1) is an actin-capping protein that plays an important role in actin filament pointed-end dynamics and length in striated muscle. No mechanisms have been identified to explain how Tmod1's functional properties are regulated. The purpose of this investigation was to explore the functional significance of the phosphorylation of Tmod1 at previously identified Thr54. Rat cardiomyocytes were assessed for phosphorylation of Tmod1 using Pro-Q Diamond staining and (32)P labeling. Green fluorescent protein-tagged phosphorylation-mimic (T54E) and phosphorylation-deficient (T54A) versions of Tmod1 were expressed in cultured cardiomyocytes, and the ability of these mutants to assemble and restrict actin lengths was observed. We report for the first time that Tmod1 is phosphorylated endogenously in cardiomyocytes, and phosphorylation at Thr54 causes a significant reduction in the ability of Tmod1 to assemble to the pointed end compared with that of the wild type (WT; 48 vs. 78%, respectively). In addition, overexpression of Tmod1-T54E restricts actin filament lengths by only ∼3%, whereas Tmod1-WT restricts the lengths significantly by ∼8%. Finally, Tmod1-T54E altered the actin filament-capping activity in polymerization assays. Taken together, our data suggest that pointed-end assembly and Tmod1's thin filament length regulatory function are regulated by its phosphorylation state.