Growth cones at the distal tips of growing nerve axons contain bundles of actin 
filaments distributed throughout the lamellipodium and that project into 
filopodia. The regulation of actin bundling by specific actin binding proteins 
is likely to play an important role in many growth cone behaviors. Although the 
actin binding protein, fascin, has been localized in growth cones, little 
information is available on its functional significance. We used the large 
growth cones of the snail Helisoma to determine whether fascin was involved in 
temporal changes in actin filaments during growth cone morphogenesis. Fascin 
localized to radially oriented actin bundles in lamellipodia (ribs) and 
filopodia. Using a fascin antibody and a GFP fascin construct, we found that 
fascin incorporated into actin bundles from the beginning of growth cone 
formation at the cut end of axons. Fascin associated with most of the actin 
bundle except the proximal 6--12% adjacent to the central domain, which is the 
region associated with actin disassembly. Later, during growth cone 
morphogenesis when actin ribs shortened, the proximal fascin-free zone of 
bundles increased, but fascin was retained in the distal, filopodial portion of 
bundles. Treatment with tumor promoter 12-O-tetradecanoylphorbol-13-acetate 
(TPA), which phosphorylates fascin and decreases its affinity for actin, 
resulted in loss of all actin bundles from growth cones. Our findings suggest 
that fascin may be particularly important for the linear structure and dynamics 
of filopodia and for lamellipodial rib dynamics by regulating filament 
organization in bundles.