Journal Article

Two Patched molecules engage distinct sites on Hedgehog yielding a signaling-competent complex

Li, Xiaochun, Schmiege, Philip, Qi, Xiaofeng, Coutavas, Elias

2018

Overview

The Hedgehog (HH) signaling pathway is important in development, and excessive HH signaling is associated with cancer. Signaling occurs through the G protein–coupled receptor Smoothened. The pathway is repressed by the membrane receptor Patched-1 (PTCH1), and this inhibition is relieved when PTCH1 binds the secreted protein HH. Two recent papers have described structures of HH bound to PTCH1, but surprisingly, each described a different binding epitope on HH. Qi et al. present a cryo–electron microscopy structure that explains this apparent contradiction by showing that a single HH protein uses both of these interfaces to engage two PTCH1 receptors (see the Perspective by Sommer and Lemmon). Functional assays suggest that both interfaces must be bound for efficient signaling. Science , this issue p. eaas8843 ; see also p. 26 The cryo–electron microscopy structure of a key complex involved in regulating a pathway important in development and cancer is elucidated. Aberrant Hedgehog (HH) signaling leads to various types of cancer and birth defects. N-terminally palmitoylated HH initiates signaling by binding its receptor Patched-1 (PTCH1). A recent 1:1 PTCH1-HH complex structure visualized a palmitate-mediated binding site on HH, which was inconsistent with previous studies that implied a distinct, calcium-mediated binding site for PTCH1 and HH co-receptors. Our 3.5-angstrom resolution cryo–electron microscopy structure of native Sonic Hedgehog (SHH-N) in complex with PTCH1 at a physiological calcium concentration reconciles these disparate findings and demonstrates that one SHH-N molecule engages both epitopes to bind two PTCH1 receptors in an asymmetric manner. Functional assays using PTCH1 or SHH-N mutants that disrupt the individual interfaces illustrate that simultaneous engagement of both interfaces is required for efficient signaling in cells.

• Publisher: The American Association for the Advancement of Science
• Subject: Assaying / Asymmetry / Binders / Binding sites / Birth / Birth defects / Blocking / Calcium (extracellular) / Calcium - chemistry / Calcium - physiology / Calcium chloride / Calcium ions / Calcium-binding protein / Cancer / Cholesterol / Congenital defects / Cryoelectron Microscopy / Electron microscopy / Epitopes / Hedgehog protein / Hedgehog Proteins - chemistry / Hedgehog Proteins - genetics / Hedgehog Proteins - ultrastructure / Humans / Insertion / Interfaces / Microscopy / Multiprotein Complexes - chemistry / Multiprotein Complexes - genetics / Multiprotein Complexes - ultrastructure / Mutation / Palmitic acid / Palmitic Acid - chemistry / Patched protein / Patched-1 Receptor - chemistry / Patched-1 Receptor - genetics / Patched-1 Receptor - ultrastructure / Physiology / Protein Domains / Proteins / Receptors / Signal Transduction / Signaling / Structural analysis / Structural Analysis (Linguistics) / Structural Analysis (Science)