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.

Oxysterols (OHCs) are hydroxylated cholesterol metabolites that play ubiquitous roles in health and disease. Due to the non-covalent nature of their interactions and unique partitioning in membranes, the analysis of live-cell, proteome-wide interactions of OHCs remains an unmet challenge. In this Resource, we present a structurally precise chemoproteomics probe for the osteogenic molecule 20(S)-hydroxycholesterol (20(S)-OHC) and provide a map of its proteome-wide targets in the membranes of living cells. Our target catalogue consolidates diverse OHC ontologies and demonstrates that OHC-interacting proteins cluster with specific processes in immune response and cancer. Competition experiments reveal that 20(S)-OHC is a chemo-, regio-, and stereoselective ligand for the protein Tmem97 (σ2 receptor), enabling molecular reconstruction of the Tmem97:20(S)-OHC binding site. Our results demonstrate that multiplexed, quantitative analysis of cellular target engagement can expose new dimensions of OHC activity and identify actionable targets for molecular therapy.

We utilize ab initio quantum mechanics calculations to evaluate a range of plausible mechanistic pathways for the unexpected formation of a [6−4−4] ring system from an enone− olefin photocycloaddition in the synthesis of (−)-scabrolide A, previously reported by our group. We present a mechanistic analysis that is consistent with all current experimental observations, including the photoexcitation, the C−C bond formation, and the associated chemo-and diastereoselectivity.

Dialkyldiazirine photoaffinity probes are unparalleled tools for the study of small molecule-protein interactions. Here we summarize the basic principles of structure, bonding, and photoreactivity of dialkyldiazirines, current methods for their synthesis, and their practical application in photoaffinity labeling experiments. We demonstrate the unique utility of dialkyldiazirine probes in the context of our recent photoaffinity crosslinking-mass spectrometry analysis to reveal a hidden cholesterol binding site in the Hedgehog morphogen proteins.