In vitro and in vivo reconstitution of the cadherin–catenin–actin complex from Caenorhabditis elegans

The ternary complex of cadherin, β-catenin, and α-catenin regulates actin-dependent cell–cell adhesion. α-Catenin can bind β-catenin and F-actin, but in mammals α-catenin either binds β-catenin as a monomer or F-actin as a homodimer. It is not known if this conformational regulation of α-catenin is evolutionarily conserved. The Caenorhabditis elegans α-catenin homolog HMP-1 is essential for actin-dependent epidermal enclosure and embryo elongation. Here we show that HMP-1 is a monomer with a functional C-terminal F-actin binding domain. However, neither full-length HMP-1 nor a ternary complex of HMP-1–HMP-2(β-catenin)–HMR-1(cadherin) bind F-actin in vitro, suggesting that HMP-1 is auto-inhibited. Truncation of either the F-actin or HMP-2 binding domain of HMP-1 disrupts C. elegans development, indicating that HMP-1 must be able to bind F-actin and HMP-2 to function in vivo. Our study defines evolutionarily conserved properties of α-catenin and suggests that multiple mechanisms regulate α-catenin binding to F-actin.