Dispatched and Scube Mediate the Efficient Secretion of the Cholesterol-Modified Hedgehog Ligand

The Hedgehog signaling pathway plays critical roles in metazoan development and in cancer. How the Hedgehog ligand is secreted and spreads to distant cells is unclear, given its covalent modification with a hydrophobic cholesterol molecule, which makes it stick to membranes. We demonstrate that Hedgehog ligand secretion from vertebrate cells is accomplished via two distinct and synergistic cholesterol-dependent binding events, one mediated by the membrane protein Dispatched and the other by a Scube family member, secreted proteins essential for Hedgehog signaling in zebrafish. Cholesterol modification is sufficient for a heterologous protein to interact with Scube, and to be secreted in a Scube-dependent manner. Dispatched and Scube recognize different structural aspects of cholesterol, similar to how Niemann-Pick disease proteins 1 and 2 interact with cholesterol, suggesting a hand-off mechanism for transferring Hedgehog from Dispatched to Scube. Thus, Dispatched and Scube cooperate to dramatically enhance secretion and solubility of the cholesterol-modified Hedgehog ligand.