Actin dynamics at pointed ends regulates thin filament length in striated muscle.
Regulation of actin dynamics at filament ends determines the organization and turnover of actin cytoskeletal structures. In striated muscle, it is believed that tight capping of the fast-growing (barbed) ends by CapZ and of the slow-growing (pointed) ends by tropomodulin (Tmod) stabilizes the uniform lengths of actin (thin) filaments in myofibrils. Here we demonstrate for the first time that both CapZ and Tmod are dynamic on the basis of the rapid incorporation of microinjected rhodamine-labelled actin (rho-actin) at both barbed and pointed ends and from the photobleaching of green fluorescent protein (GFP)-labelled Tmod. Unexpectedly, the inhibition of actin dynamics at pointed ends by GFP-Tmod overexpression results in shorter thin filaments, whereas the inhibition of actin dynamics at barbed ends by cytochalasin D has no effect on length. These data demonstrate that the actin filaments in myofibrils are relatively dynamic despite the presence of capping proteins, and that regulated actin assembly at pointed ends determines the length of thin filaments.
Littlefield
R
R
Department of Cell Biology, The Scripps Research Institute, 10596 North Torrey Pines Road, La Jolla, California 92037, USA.
Almenar-Queralt
A
A
Fowler
V M
VM
D008841:Actin Cytoskeleton / Q000187:drug effects / Q000502:physiology*; D000199:Actins / Q000502:physiology*; D000209:Acute-Phase Proteins / Q000378:metabolism; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002478:Cells, Cultured; D002645:Chickens; D015638:Cytochalasin D / Q000494:pharmacology; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000378:metabolism; D008840:Microfilament Proteins; D018482:Muscle, Skeletal / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism; D051348:Tropomodulin
D000199:Actins; D000209:Acute-Phase Proteins; D002352:Carrier Proteins; D008164:Luminescent Proteins; D008840:Microfilament Proteins; D051348:Tropomodulin; C010781:acute-phase protein rho; D049452:Green Fluorescent Proteins; D015638:Cytochalasin D