Structural variability and dynamics in the ectodomain of an ancestral-type classical cadherin revealed by AFM imaging

Abstract

Type III cadherin represents the ancestral form of classical cadherin in bilaterian metazoans. Drosophila possesses type III and type IVa cadherins, known as DN- and DE-cadherins, respectively. Mature DN- and DE-cadherins have 15 and 7 extracellular cadherin domain (EC) repeats, respectively, with DN-cadherin EC6–11 homologous to DE-cadherin EC1–6. These EC repeats contain predicted complete or partial Ca2+-free inter-EC linkers that potentially contribute to adhesion. Comparative structure-function studies of DN- and DE-cadherins may help us understand the ancestral and derived states of classical cadherin-mediated adhesion mechanisms. Here, using bead aggregation assays, we found that DN-cadherin EC1–11 and DE-cadherin EC1–6 exhibit Ca2+-dependent adhesive properties. Using high-speed atomic force microscopy (HS-AFM) imaging in solution, we showed that both DN- and DE-cadherin ectodomains share a common morphological framework consisting of a strand-like and a globule-like portion. Furthermore, the DN-cadherin EC repeats were highly variable, flexible in morphology, and with at least three bendable sites, one of which is located in EC6–11 and can act as a flexible hinge. Our findings provide insights into diversification of classical cadherin-mediated adhesion mechanisms. (180 words or less)