Models describing the proposed function of group I PAKs in myoblast fusion. (A) Group I PAKs act directly downstream of the small GTPase Rac, and in parallel with the Scar and WASP complexes, to promote myoblast fusion. DPak3 and DPak1 are partially redundant, with DPak3 playing a more significant role, in the fusion process. PAKs do not appear to affect actin polymerization or depolymerization, but may regulate actin bundling/cross-linking proteins during myoblast fusion. (B) Group I PAKs regulate podosome invasion during myoblast fusion. In wild-type embryos, PAKs organize the Arp2/3-nucleated, branched actin filaments within the PLS into a densely packed structure (devoid of ribosomes indicated by black dots), which, in turn, efficiently invades the apposing founder cell (FC)/myotube with multiple finger-like protrusions and ultimately leads to fusion pore formation. PAKs may do so by activating actin bundling/cross-linking proteins (A), allowing the formation of highly stiff actin bundles that exert large protrusive forces against the cell membrane. In dpak mutant embryos, the actin filaments are disorganized and dispersed (the actin-enriched area decorated with ribosomes), resulting in a failure in PLS invasion and fusion pore formation.