SUMMARY The WAVE regulatory complex (WRC) controls actin cytoskeletal dynamics throughout the cell by stimulating the actin nucleating activity of the Arp2/3 complex at distinct membrane sites. However, the factors that recruit the WRC to specific locations remain poorly understood. Here we have identified a large family of potential WRC ligands, consisting of ~120 diverse membrane proteins including protocadherins, ROBOs, netrin receptors, Neuroligins, GPCRs and channels. Structural, biochemical and cellular studies reveal that a novel sequence motif that defines these ligands binds to a highly conserved interaction surface of the WRC formed by the Sra and Abi subunits. Mutating this binding surface in flies resulted in defects in actin cytoskeletal organization and egg morphology during oogenesis, leading to female sterility. Our findings directly link diverse membrane proteins to the WRC and actin cytoskeleton, and have broad physiological and pathological ramifications in metazoans.

Altogether our results show that Cdc42 functions with Par6 and aPKC to regulate E-Cad endocytosis and define Cip4 and WASp as regulators of the early E-Cad endocytic events in epithelial tissue.

The actin cytoskeleton and associated motor proteins provide the driving forces for establishing the astonishing morphological diversity and dynamics of mammalian cells. Aside from functions in protruding and contracting cell membranes for motility, differentiation or cell division, the actin cytoskeleton provides forces to shape and move intracellular membranes of organelles and vesicles. To establish the many different actin assembly functions required in time and space, actin nucleators are targeted to specific subcellular compartments, thereby restricting the generation of specific actin filament structures to those sites. Recent research has revealed that targeting and activation of actin filament nucleators, elongators and myosin motors are tightly coordinated by conserved protein complexes to orchestrate force generation. In this Cell Science at a Glance article and the accompanying poster, we summarize and discuss the current knowledge on the corresponding protein complexes and their modes of action in actin nucleation, elongation and force generation.

Cip4 integrates membrane trafficking and actin dynamics through WASP and WAVE. First, Cip4 promotes membrane invaginations and triggers the vesicle scission by recruiting Dynamin to the neck of nascent vesicles. Second, Cip4 recruits WASP and WAVE proteins to induce actin polymerization, supporting vesicle scission and providing the force for vesicle movement.

Drosophila myoblast fusion proceeds in two steps. The first one gives rise to small syncytia, the muscle precursor cells, which then recruit further fusion competent myoblasts to reach the final muscle size. We have identified Kette as an essential component for myoblast fusion. In kette mutants, founder cells and fusion-competent myoblasts are determined correctly and overcome the very first fusion. But then, at the precursor cell stage, fusion is interrupted. At the ultrastructural level, fusion is characterised by cellcell recognition, alignment, formation of prefusion complexes, electron dense plaques and membrane breakdown. In kette mutants, electron dense plaques of aberrant length accumulate and fusion is interrupted owing to a complete failure of membrane breakdown. Furthermore, we show that kette interacts genetically with blown fuse (blow) which is known to be required to proceed from prefusion complexes to the formation of the electron dense plaques. Interestingly, a surplus of Kette can replace Blow function during myogenesis. We propose a model in which Dumbfounded/Sticks and stones-dependent cell adhesion is mediated over Rolling Pebbles, Myoblast city, Crk, Blown fuse and Kette, and thus induces membrane fusion.