Genetic and pharmacological blockade of constitutively activated CED-10/Rac. (A) In the smg-1(cc546ts); reIs6[Plin-26::ced-10(Q61L)::NMDS3ʹUTR] background, different mutations reduced levels of lethality. Two independent strain constructions with the max-2(nv162) mutation in the known Rac effector Pak suppressed lethality. Not shown is a synthetic lethal phenotype of smg-1(cc546ts); reIs6 in combination with disruption of the other Pak ortholog, pak-1(ok448). We constructed a strain with pak-1(ok448) as a heterozygote but could not homozygose the pak-1 mutant chromosome. Unlike MAX-2/Pak, PAK-1/Pak has been implicated as an effector of both CED-10/Rac and CDC-42/Cdc42, as well as a GTPase- and kinase-independent component of the Pak-Pix-Git1 complex that regulates the cytoskeleton (Hoefen and Berk 2006; Lucanic et al. 2006; Peters et al. 2013). Thus, disruption of PAK-1, unlike disruption of MAX-2, is expected to impact multiple signaling systems, perhaps explaining the synthetic lethality observed when PAK-1 is deleted in the CA-CED-10/Rac transgenic background at any temperature. A putative null mutation in PES-7/IQGAP, pes-7(gk123), a putative Rac effector identified in mammalian studies (Watanabe et al. 2015), weakly suppressed lethality, while mutations in other Rac effectors PKN-1/PKN (pkn-1(ok1673)) (Lu and Settleman 1999) and UNC-115/AbLIM (unc-115(e2225)) (Struckhoff and Lundquist 2003) failed to suppress. (B) Rescue of toxicity of constitutively active CED-10/Rac at higher concentrations of the Rac inhibitor, EHT 1864. Classes of phenotypes were binned as in Figure 1. (C) DIC image of an embryo conditionally expressing GFP at 23°C. The lumen of the pharynx is out of the plane of focus, facing left. (D) DIC image of a ruptured embryo conditionally expressing CA-Rac/CED-10 at 23°C. The lumen of the intact pharynx is in focus and facing left (white arrowhead), indicating that development persists even when epithelial morphogenesis is disrupted. (E) DIC image of an embryo conditionally expressing constitutively activated Rac/CED-10 at 23°C and grown on 1% DMSO. The lumen of the intact pharynx is in focus and facing downward (white arrowhead). (F) DIC image of an embryo conditionally expressing constitutively activated Rac/CED-10 at 23°C and grown on 30 µM EHT 1864 in 1% DMSO.