Orchestration of developmental processes as well as function and homeostasis of tissues depend on fine tuned intercellular communication, necessitating carefully controlled extracellular gradients of a wide range of signalling molecules. Signalling by the Wnt family of secretory glycolipoproteins is renowned for its crucial roles in embryonic development and tissue homeostasis and is subject to a multi-layered system of regulation. Conversely, dysregulation of Wnt signalling is implicated in developmental defects, degenerative diseases, and cancer. Dickkopf (Dkk) proteins are secreted regulators of canonical Wnt/β-catenin signalling, which compete with the Wnt morphogens for their coreceptors LRP5 and -6. A second class of cell surface Dkk receptors, called Kremen (Krm1 and -2), amplify Dkk function by promoting cell surface expression of LRP5/6 in the absence of Dkk and inducing a rapid internalization in its presence.
The goal of this proposal is to elucidate the molecular mechanisms behind the Wnt-regulatory activity of Dkk and Krm. I will use protein produced by transient secretory mammalian expression to reconstitute binary and ternary complexes of LRP5/6, Dkk, and Krm in vitro. A wide range of biophysical methods including analytical ultracentrifugation and surface plasmon resonance will be applied to characterize complex formation with respect to stoichiometry, minimal functional modules, affinity and kinetics. X-ray crystallization and electron microscopy will be used to study the structures of isolated complex components and the structural changes associated with complex formation. Based on biophysical and structural data, functional mutants of the proteins will be created and tested in a cellular setting using e.g. a Wnt-responsive bioluminescence reporter assay or confocal microscopy. By studying the interaction of LRP5/6, Dkk and Krm, I shall provide a new level of insight into the regulation of the Wnt signalling system.
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