An in vivo analysis of critical functional domains of hmp-1/[alpha]-catenin during Caenorhabditis elegans epidermal morphogenesis

Author / Creator

Shao, Xiangqiang, author

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Summary

Maintaining tissue integrity during epidermal morphogenesis in developing embryos is dependent on stable intercellular adhesions that are formed through the conserved cadherin-catenin complex (CCC)...

Maintaining tissue integrity during epidermal morphogenesis in developing embryos is dependent on stable intercellular adhesions that are formed through the conserved cadherin-catenin complex (CCC). A key mediator of this complex is [small alpha]-catenin, which connects the CCC at the membrane to the F-actin cytoskeleton. Regulated binding of [small alpha]-catenin to both [small beta]-catenin and F-actin is crucial for the proper function of [small alpha]-catenin, which acts as a mechanotransducer at adherens junctions during embryonic development. However, the mechanisms underlying such regulation remains unclear. To better understand the role of [small alpha]-catenin at adherens junctions, we have utilized Caenorhabditis elegans as a model system, which possesses a single classical cadherin, HMR-1, a single [small beta]-catenin, HMP-2, and a single [small alpha]-catenin, HMP-1, that participate in cell adhesion. Here, we first test the requirements for key elements within [small alpha]- and [small alpha]-catenin in maintaining their strong association during morphogenetic movements. A crystal structure shows that the HMP-1/HMP-2 complex forms a five-helix structure that is somewhat different from the mammalian [small alpha]-catenin/[small beta]-catenin complex. We show that the first helix of HMP-1 is necessary for binding HMP-2 avidly in vitro and for efficient recruitment of HMP-1 to adherens junctions in vivo. Conserved residues in HMP-2 flank its binding interface with HMP-1. We show that phosphomimetic substitutions decrease HMP-2 binding to HMP-1, suggesting that phosphorylation of these residues may be important for regulating CCC formation in C. elegans. The adhesion modulation domain (AMD) of HMP-1 is responsible for regulating its F-actin-binding activity. Deleting the AMD of HMP-1 relieves its intramolecular auto-inhibition in vitro and leads to excess actin recruitment to adherens junctions in vivo. We also show that the N terminus of the HMP-1 actin-binding domain is important for its full actin-binding ability. Phosphorylation of HMP-1 S509, which is within the AMD, may regulate F-actin binding and hence organization of junctional proximal actin networks in epidermal cells during C. elegans embryonic development. Our data demonstrate the importance of C. elegans as a simple model system to study the molecular basis of [small alpha]-catenin function and provide novel insights into how [small alpha]-catenin modulates cadherin-dependent cell-cell adhesion during morphogenesis in metazoans.

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