Structure of an engineered intein reveals thiazoline ring and provides mechanistic insight

Hedgehog proteins create a dynamic cholesterol interface

During formation of the Hedgehog (Hh) signaling proteins, cooperative activities of the Hedgehog INTein (Hint) fold and Sterol Recognition Region (SRR) couple autoproteolysis to cholesterol ligation. The cholesteroylated Hh morphogens play essential roles in embryogenesis, tissue regeneration, and tumorigenesis. Despite the centrality of cholesterol in Hh function, the full structure of the Hint-SRR ("Hog") domain that attaches cholesterol to the last residue of the active Hh morphogen remains enigmatic. In this work, we combine molecular dynamics simulations, photoaffinity crosslinking, and mutagenesis assays to model cholesterolysis intermediates in the human Sonic Hedgehog (hSHH) protein. Our results provide evidence for a hydrophobic Hint-SRR interface that forms a dynamic, non-covalent cholesterol-Hog complex. Using these models, we suggest a unified mechanism by which Hh proteins can recruit, sequester, and orient cholesterol, and offer a molecular basis for the effects of disease-causing hSHH mutations.