Structural Insights into Ca(2+)-Calmodulin Regulation of Plectin 1a-Integrin β4 Interaction in Hemidesmosomes

The mechanical stability of epithelial cells, which protect organisms from harmful external factors, is maintained by hemidesmosomes via the interaction between plectin 1a (P1a) and integrin α6β4. Binding of calcium-calmodulin (Ca(2+)-CaM) to P1a together with phosphorylation of integrin β4 disrupts this complex, resulting in disassembly of hemidesmosomes. We present structures of the P1a actin binding domain either in complex with the N-ter lobe of Ca(2+)-CaM or with the first pair of integrin β4 fibronectin domains. Ca(2+)-CaM binds to the N-ter isoform-specific tail of P1a in a unique manner, via its N-ter lobe in an extended conformation. Structural, cell biology, and biochemical studies suggest the following model: binding of Ca(2+)-CaM to an intrinsically disordered N-ter segment of plectin converts it to an α helix, which repositions calmodulin to displace integrin β4 by steric repulsion. This model could serve as a blueprint for studies aimed at understanding how Ca(2+)-CaM or EF-hand motifs regulate F-actin-based cytoskeleton.