Tropomodulin protects α-catenin-dependent junctional-actin networks under stress during epithelial morphogenesis

α-catenin is central to recruitment of actin networks to the cadherin-catenin complex [1, 2], but how such networks are subsequently stabilized against stress applied during morphogenesis is poorly understood. To identify proteins that functionally interact with α-catenin in this process, we performed enhancer screening using a weak allele of the C. elegans α-catenin, hmp-1, thereby identifying UNC-94/tropomodulin. Tropomodulins (Tmods) cap the minus ends of F-actin in sarcomeres [3]. They also regulate lamellipodia [4], can promote actin nucleation [5], and are required for normal cardiovascular development [6, 7] and neuronal growth-cone morphology [8]. Tmods regulate the morphology of cultured epithelial cells [9], but their role in epithelia in vivo remains unexplored. We find that UNC-94 is enriched within a HMP-1-dependent junctional-actin network at epidermal adherens junctions subject to stress during morphogenesis. Loss of UNC-94 leads to discontinuity of this network, and high-speed filming of hmp-1(fe4);unc-94(RNAi) embryos reveals large junctional displacements that depend on the Rho pathway. In vitro, UNC-94 acts in combination with HMP-1, leading to longer actin bundles than with HMP-1 alone. Our data suggest that Tmods protect actin filaments recruited by α-catenin from minus-end subunit loss, enabling them to withstand the stresses of morphogenesis.