A moving cell needs to make new actin filaments at its ever-advancing leading edge. How does the cell's actin-polymerizing machinery keep up with this shifting demand? Jung et al. (page 1479) come up with a tantalizing clue in this issue . They report that the Dictyostelium protein CARMIL (Capping protein, Arp2/3, Myosin I Linker) links certain type I myosins to the actin-nucleating Arp2/3 complex. Type I myosins are nonprocessive motors, but the high concentration of actin at the leading edge, or a higher-order complex containing multiple myosins, may allow these motors to transport Arp2/3 forwards. Additionally, CARMIL appears to be a novel activator of Arp2/3's actin-nucleating activity.
Jung et al. believe that CARMIL is the first cytoskeletal protein besides actin known to interact with capping protein. Given its high concentration in the cell, CARMIL could be important in sequestering...
