Group I PAKs Functions Downstream of Rac to Regulate Podosome Invasion During Myoblast Fusion in vivo. Rui Duan, Peng Jin, Fengbao Luo, Elizabeth Chen. Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, MD.

   The p21-activated kinases (PAKs) regulate a wide variety of cellular activities including cell proliferation, apoptosis, polarity, migration and shape changes. Here, we have identified a novel function of group I PAKs in cell-cell fusion. We show that the two Drosophila group I PAKs, DPak3 and DPak1, play partially redundant roles in myoblast fusion in vivo. While DPak3 has a major function in myoblast fusion, DPak1 plays a minor role that could only be revealed in the absence of DPak3. During myoblast fusion, DPak3 is enriched at the sites of fusion, colocalizing with the F-actin focus within a podosome-like structure (PLS) in wild-type embryos, whereas DPak1 is recruited to the site of fusion in dpak3 mutant embryos to compensate for the loss of DPak3. In embryos lacking DPak3 or both PAKs, the actin filaments within the PLS are disorganized and dispersed, resulting in defective PLS invasion and failure in fusion pore initiation. We further show that the small GTPase Rac regulates both the activation and localization of DPak3 during myoblast fusion, and that the kinase activity of DPak3 is required for its function in vivo. We propose a model whereby group I PAKs act downstream of Rac to organize the actin filaments into a dense focus within the PLS, which, in turn, effectively invades the adherent founder cell and promotes fusion pore initiation during myoblast fusion.