Activities of the Cytoplasmic Domains of Patched-1 Modulate but Are Not Essential for the Regulation of Canonical Hedgehog Signaling*

Abstract

The Hedgehog (Hh) pathway is a highly conserved signaling cascade crucial for cell fate determination during embryogenesis. Response to the Hh ligands is mediated by the receptor Patched-1 (Ptch1), a 12-pass transmembrane glycoprotein. Despite its essential role in Hh signaling and its activity as a tumor suppressor, Ptch1 remains largely uncharacterized. We demonstrate here that Ptch1 binds to itself to form oligomeric structures. Oligomerization is mediated by two distinct, structurally disordered, intracellular domains spanning amino acids 584–734 (“middle loop”) and 1162–1432 (C terminus). However, oligomerization is not required for Ptch1-dependent regulation of the canonical Hh pathway operating through Smo. Expression of a mutant protein that deletes both regions represses the Hh pathway and responds to the addition of Hh ligand independent of its inability to bind other factors such as Smurf2. Additionally, deletion of the cytoplasmic middle loop domain generates a Ptch1 mutant that, despite binding to Hh ligand, constitutively suppresses Hh signaling and increases the length of primary cilia. Constitutive activity because of deletion of this region is reversed by further deletion of specific sequences in the cytoplasmic C-terminal domain. These data reveal an interaction between the cytoplasmic domains of Ptch1 and that these domains modulate Ptch1 activity but are not essential for regulation of the Hh pathway.