Our lab is investigating how cells become polarized to permit cell shape changes and movements during morphogenesis. We focus here on the movement of hypodermal cells during embryogenesis. Around the 400-cell stage in C. elegans development, the hypodermal cells rearrange into six rows and begin migrations. The two center rows undergo dorsal intercalation, while the two lateral rows migrate ventrally. These cell movements are dependent on the actin cytoskeleton and its regulation. Our genetic screens for mutations that specifically affect the earliest movements of the C. elegans hypodermis have identified three genes,
gex-1,
gex-2, and
gex-3, which encode three proteins thought to be essential regulators of actin dynamics. GEX-1, GEX-2, and GEX-3 are the C. elegans homologs of WAVE/Scar, PIR121/SRA-1/P140, and KETTE/NAP1/HEM2, respectively. The homologs of GEX-1, GEX-2, and GEX-3 have been proposed to function in a complex that promotes actin nucleation by activating a major nucleator of actin, the Arp2/3 complex. Loss of any of the three gex genes, or of a probable fourth member of the complex, B0336.6/ABI-1/2, the only ABI-1/2 homolog in C. elegans, gives a complete Gex phenotype: gut on the exterior, due to a complete failure of epidermal cell enclosure. We present our in vivo biochemical analysis of the C. elegans GEX complex. We have depleted the individual GEX proteins using genetic mutations and RNAi to test how each GEX component contributes to the GEX complex. We find that these molecules are found in a complex in vivo in C. elegans, and that GEX-2, GEX-3, as well as ABI-2, serve to stabilize GEX-1 in vivo. As a complementary approach, we are analyzing the phenotypic consequences of removing each of the GEX complex components. Newly available imaging tools allow us to analyze the function of GEX proteins during hypodermal morphogenesis. Our experiments address a key question in the field of actin dynamics: how exactly do GEX-2, GEX-3, and their homologs regulate the function of GEX-1/WAVE/Scar? Our biochemical and phenotypic data show that the simplest models for GEX-1 regulation by GEX-2 and GEX-3 are probably incorrect. Our biochemical and cell biological results together suggest a model for how this complex of proteins regulates hypodermal morphogenesis in C. elegans. *The first two authors contributed equally.