A disorder-to-order structural transition in the cytosolic tail of Fz4 is responsible for the misfolding of the L501fsX533 Fz4 mutant.

Frizzled 4 belongs to the superfamily of G protein coupled receptors. The unstructured cytosolic tail of the receptor is essential for the activation of different intracellular signalling pathway. Mutations in the fz4 gene are responsible for the Familial exudative vitreoretinopathy (FEVR), among these the autosomal dominant frameshift mutation L501fsX533 that codes for a new carboxy terminal tail. Here we show that the new tail generated by the frameshift mutation acquires a helix-loop-helix conformation. The helices are amphipathic, display affinity for membranes and resemble the structure of Influenza Hemagglutinin fusion peptide at the pH of fusion. Shape and curvature of the mutant tail is proven essential for proper membrane interaction in vitro. The new fold of the tail is necessary for the aggregation and retention in the Endoplasmic Reticulum of the receptor via its interaction with the membrane and is sufficient to induce the misfolding and affect trafficking of a chimeric VSVG protein containing the mutated tail. Shifting the fold of the tail to a more disordered conformation relocates the receptor to the Plasma Membrane. Such disordered to ordered structural transition was never described in mammalian receptors and pinpoints the need to consider structural transition as possible outcome of mutations in unstructured regions of proteins.