C. elegans srGAP is an α-catenin M domain-binding protein that strengthens cadherin-dependent adhesion during morphogenesis.

The cadherin-catenin complex (CCC) is central to embryonic development and tissue repair, yet how CCC binding partners function alongside core CCC components remains poorly understood. Here, we establish a previously unappreciated role for an evolutionarily conserved protein, the slit-robo GTPase-activating protein SRGP-1/srGAP, in cadherin-dependent morphogenetic processes in the Caenorhabditis elegans embryo. SRGP-1 binds to the M domain of the core CCC component, HMP-1/α-catenin, via its C terminus. The SRGP-1 C terminus is sufficient to target it to adherens junctions, but only during later embryonic morphogenesis, when junctional tension is known to increase. Surprisingly, mutations that disrupt stabilizing salt bridges in the M domain block this recruitment. Loss of SRGP-1 leads to an increase in mobility and decrease of junctional HMP-1. In sensitized genetic backgrounds with weakened adherens junctions, loss of SRGP-1 leads to late embryonic failure. Rescue of these phenotypes requires the C terminus of SRGP-1 but also other domains of the protein. Taken together, these data establish a role for an srGAP in stabilizing and organizing the CCC during epithelial morphogenesis by binding to a partially closed conformation of α-catenin at junctions.