Patched acts catalytically to suppress the activity of Smoothened

Abstract

Mutations affecting the transmembrane proteins Patched (Ptc) or Smoothened (Smo) that trigger ligand-independent activity of the Hedgehog (Hh) signalling pathway are associated with human tumours such as basal cell carcinoma (BCC) and medulloblastoma. Despite extensive genetic studies demonstrating the importance of these receptor components in embryonic patterning and cancer, the mechanism by which Ptc regulates Smo is not understood. Here we report that Ptc and Smo are not significantly associated within Hh-responsive cells. Furthermore, we show that free Ptc (unbound by Hh) acts sub-stoichiometrically to suppress Smo activity and thus is critical in specifying the level of pathway activity. Patched is a twelve-transmembrane protein with homology to bacterial proton-driven transmembrane molecular transporters; we demonstrate that the function of Ptc is impaired by alterations of residues that are conserved in and required for function of these bacterial transporters. These results suggest that the Ptc tumour suppressor functions normally as a transmembrane molecular transporter, which acts indirectly to inhibit Smo activity, possibly through changes in distribution or concentration of a small molecule.

Oxysterols stimulate Sonic hedgehog signal transduction and proliferation of medulloblastoma cells

“…Given the potent ability of oxysterols to stimulate Smo activity, one of them may be an endogenous signal that turns on Smo in response to Shh. Ptc, which is related to the intestinal Niemann-Pick C1-like1 cholesterol transporter and the resistance-nodulation-cell division family of pump proteins (9,10), may control Smo activity by restricting its contact with activating sterols (Fig. 4A) (19), suggesting that a constantly active conformation of Smo eliminates the need for a conformational change induced by sterols.…”

“…The mechanism by which Ptc1 inhibits Smo is poorly understood and does not appear to involve binding of Ptc1 to Smo. Smo can be activated or inhibited by various small molecules (8), and it has been hypothesized that Ptc1, which is related to the resistance-nodulation-cell division family of bacterial pumps (9) and to the Niemann-Pick C1-like1 cholesterol transporter (10), modulates Smo activity by regulating the distribution of an endogenous small molecule Smo ligand (8,9). However, the existence or identity of such an endogenous ligand remains unknown.…”

Constitutive activation of the shh–ptc1 pathway by a patched1 mutation identified in BCC

“…Two consequences of shh binding are stimulation of cellular proliferation and upregulation of Gli1 activity (Dahmane et al, 1997;Green et al, 1998;Liu et al, 1998;Kenney and Rowitch, 2000;Barnes et al, 2001;Lowrey et al, 2002). shh-N has been shown to induce Gli1 transcriptional activity (from two-to sixfold over mock) using a Gli1-luciferase reporter system (Murone et al, 1999;Zeng et al, 2001;Taipale et al, 2002). Moreover, expression of oncogenic forms of smo proteins independent of shh-N cellular exposure enhances Gli1-luciferase reporter activity about 6-8-fold over mock-transfected cells (Murone et al, 1999).…”

All Mammalian Hedgehog Proteins Interact with Cell Adhesion Molecule, Down-regulated by Oncogenes (CDO) and Brother of CDO (BOC) in a Conserved Manner

“…Reception of the Hh signal involves Patched (Ptc) (16), a 12-pass integral membrane protein with homology to proton-driven bacterial transporters, and Smoothened (17), a 7-pass integral membrane protein with homology to G-protein coupled receptors. In the absence of Hh, Ptc inhibits the activity of Smoothened (17). Hh relieves this inhibition, resulting in the activation of Smoothened and downstream effectors that converge on Gli family transcription factors to alter expression levels of target genes (17).…”

“…In the absence of Hh, Ptc inhibits the activity of Smoothened (17). Hh relieves this inhibition, resulting in the activation of Smoothened and downstream effectors that converge on Gli family transcription factors to alter expression levels of target genes (17).…”