Cc. Gregorio et al., REQUIREMENT OF POINTED-END CAPPING BY TROPOMODULIN TO MAINTAIN ACTIN FILAMENT LENGTH IN EMBRYONIC CHICK CARDIAC MYOCYTES, Nature, 377(6544), 1995, pp. 83-86
CONTROL of actin filament length and dynamics is important for cell mo
tility and architecture and is regulated in part by capping proteins t
hat block elongation and depolymerization at both the fast-growing (ba
rbed) and slow-growing (pointed) ends(1-4). Tropomodulin is a capping
protein for the pointed end of the actin filament(5,6); it is associat
ed with the free, pointed ends of the thin filaments in striated muscl
e, where it is thought to bind to both tropomyosin and actin(7,8). In
embryonic chick cardiac myocytes, tropomodulin assembles after the thi
n, as well as the thick, filaments have become organized into periodic
I and A bands(8), suggesting that tropomodulin might be involved in m
aintaining actin filament length. Here we show that microinjection of
an antibody that inhibits tropomodulin's pointed-end-capping activity
in vitro results in a marked elongation of actin filaments from their
pointed ends and a >80% reduction in the percentage of beating cells.
This demonstrates that pointed-end capping by tropomodulin is required
to maintain actin filament length in vivo and that this is essential
for contractile function in embryonic chick cardiac myocytes.