Fig. 8. Models of regulation of lamellipodia and filopodia. (A) Parallel pathways model. Lamellipodia and filopodia are regulated by two parallel pathways: from GTPase Rac through Scar/WAVEs to lamellipodia, and from Cdc42 through WASPs to filopodia. (B) Cascade pathways model. Small GTPases signal to Scar/WAVEs and WASPs to initiate lamellipodia, which are subsequently transformed into filopodia by additional signals. (C) Refined cascade model for Drosophila. Our data emphasize SCAR as the main regulator of lamellipodia formation, whereas WASp in Drosophila cells is dispensable for both lamellipodia and filopodia and may play a role in other actin-dependent activities. Additional signals driving reorganization of lamellipodia in filopodia may originate from Cdc42 and act through effectors other than WASP, e.g. IRSp53 (Krugmann et al., 2001) and Drf3 (Peng et al., 2003).