Actin dynamics at pointed ends regulates thin filament length in striated muscle

Regulation of actin dynamics at filament ends determines the organization a nd turnover of actin cytoskeletal structures. In striated muscle, it is bel ieved that tight capping of the fast-growing (barbed) ends by Cap Z and of the slow-growing (pointed) ends by tropomodulin (Tmod) stabilizes the unifo rm lengths of actin (thin) filaments in myofibrils. Here we demonstrate for the first time that both Cap Z and Tmod are dynamic on the basis of the ra pid incorporation of microinjected rhodamine-labelled actin (rho-actin) at both barbed and pointed ends and from the photobleaching of green fluoresce nt protein (GFP)-labelled Tmod, Unexpectedly, the inhibition of actin dynam ics at pointed ends by GFP-Tmod overexpression results in shorter thin fila ments, whereas the inhibition of actin dynamics at barbed ends by cytochala sin D has no effect on length. These data demonstrate that the actin filame nts in myofibrils ave relatively dynamic despite the presence of capping pr oteins, and that regulated actin assembly at pointed ends determines the le ngth of thin filaments.