Dynamic uncapping

The surge of actin growth toward the cell periphery is stopped abruptly by capping protein (CP). In vitro, this protein sticks like glue, but in vivo it comes off the ends of those filaments rapidly, say Miyoshi et al. (page 947). Cycles of severing and annealing may thus be more important than expected in creating a dynamic actin array.

The researchers had previously tracked the lifetimes of actin filaments in the lamellipodia of spreading Xenopus fibroblasts. They sought explanations for the range of lifetimes by looking at actin regulatory proteins such as CP. Surprisingly, the binding half life of CP in vivo was barely more than a second, compared with almost half an hour in vitro. The uncapped filaments appeared to resume their growth.

Although actin turnover is ∼20-fold slower than CP turnover,...