Nanomolar, Noncovalent Antagonism of Hedgehog Cholesterolysis: Exception to the "Irreversibility Rule" for Protein Autoprocessing Inhibition

Paracatalytic induction: Subverting specificity in hedgehog protein autoprocessing with small molecules

Paracatalytic inducers are antagonists that shift the specificity of biological catalysts, resulting in non-native transformations. In this Chapter we describe methods to discover paracatalytic inducers of Hedgehog (Hh) protein autoprocessing. Native autoprocessing uses cholesterol as a substrate nucleophile to assist in cleaving an internal peptide bond within a precursor form of Hh. This unusual reaction is brought about by HhC, an enzymatic domain that resides within the C-terminal region of Hh precursor proteins. Recently, we reported paracatalytic inducers as a novel class of Hh autoprocessing antagonists. These small molecules bind HhC and tilt the substrate specificity away from cholesterol in favor of solvent water. The resulting cholesterol-independent autoproteolysis of the Hh precursor generates a non-native Hh side product with substantially reduced biological signaling activity. Protocols are provided for in vitro FRET-based and in-cell bioluminescence assays to discover and characterize paracatalytic inducers of Drosophila and human hedgehog protein autoprocessing, respectively.

Hedgehog Autoprocessing: From Structural Mechanisms to Drug Discovery

Hedgehog (Hh) signaling plays pivotal roles in embryonic development. In adults, Hh signaling is mostly turned off but its abnormal activation is involved in many types of cancer. Hh signaling is initiated by the Hh ligand, generated from the Hh precursor by a specialized autocatalytic process called Hh autoprocessing. The Hh precursor consists of an N-terminal signaling domain (HhN) and a C-terminal autoprocessing domain (HhC). During Hh autoprocessing, the precursor is cleaved between N- and C-terminal domain followed by the covalent ligation of cholesterol to the last residue of HhN, which subsequently leads to the generation of Hh ligand for Hh signaling. Hh autoprocessing is at the origin of canonical Hh signaling and precedes all downstream signaling events. Mutations in the catalytic residues in HhC can lead to congenital defects such as holoprosencephaly (HPE). The aim of this review is to provide an in-depth summary of the progresses and challenges towards an atomic level understanding of the structural mechanisms of Hh autoprocessing. We also discuss drug discovery efforts to inhibit Hh autoprocessing as a new direction in cancer therapy.