Disease relevance of smo

• Hence, mutations within the ptc gene that result in the failure of Ptc to inhibit Smo lead to constitutive activity of the Hh signalling pathway and to cancer, such as basal cell carcinoma [1].
• We demonstrate that, as in Drosophila, vertebrate smoothened is essential for hedgehog signaling, and functions upstream of protein kinase A. Further analysis of neural tube defects revealed the absence of lateral floor plate and secondary motoneurons, but the presence of medial floor plate and primary motoneurons in smoothened mutant embryos [2].

High impact information on smo

• Current models view Patched and Smoothened as a preformed receptor complex that is activated by Hedgehog binding [3].
• Here we present evidence that Patched destabilizes Smoothened in the absence of Hedgehog [3].
• In contrast, Hedgehog causes phosphorylation, stabilization, and accumulation of Smoothened at the cell surface [3].
• The smo gene encodes an integral membrane protein with characteristics of G protein-coupled receptors and shows homology to the Drosophila Frizzled protein [4].
• The earliest defect in smo mutant embryos is loss of expression of the Hedgehog-responsive gene wingless between 1 and 2 hr after gastrulation [4].

Biological context of smo

• These results suggest that Ptc normally binds Hh without any help from Smo and hence favor a mechanism of signal transduction in which Hh binds specifically to Ptc and induces a conformational change leading to the release of latent Smo activity [5].
• Conversely, Smo becomes constitutively active if acidic residues replace those phosphorylation sites [6].
• Here we show that the latter response is mediated principally by regulatory elements other than Ci binding sites and not by altered Smo phosphorylation [6].
• Conversely, trapping of activated Smo mutants in the ER prevents Hh target gene activation [7].
• All Hh responses require the seven transmembrane domain protein Smoothened, which itself becomes hyperphosphorylated during Hh signalling [8].

Anatomical context of smo

• Stabilization of the integral membrane protein Smoothened (Smo) at the plasma membrane is a crucial step in Hh signalling but the molecular events immediately downstream of Smo remain to be elucidated [9].
• Hh causes Smo to move from internal membranes to the cell surface [7].
• Using Drosophila salivary gland cells in vivo and in vitro as a new assay for Hh signal transduction, we investigated the regulation of Hh-triggered Smo stabilization and relocalization [7].
• This role is indicated by the ability of altered Smo proteins to produce changes in transcription of Hh-responsive genes in vivo and in cultured cells [10].
• We show that inactivation of smoothened by retroviral insertions in zebrafish results in defects that are characteristic of hedgehog deficiencies, including abnormalities in body size, the central nervous system, adaxial mesoderm, cartilage and pectoral fins [2].

Associations of smo with chemical compounds

• Using immunopurification and mass spectrometry, we identify here 26 serine/threonine residues within the Smo C-terminal cytoplasmic tail that are phosphorylated in Hh-stimulated cells [10].
• Given our present understanding of the Hh signaling pathway, the absence of Hh and Smo raises many questions about the evolution and the function of the PTC, PTR, and Hh-r proteins in C. elegans [11].
• Mutations in the sterol-sensing domain of Patched suggest a role for vesicular trafficking in Smoothened regulation [12].
• Since Hh has been found in lipid rafts in Drosophila, we hypothesized that Patched and Smoothened might also be found in these cholesterol-rich microdomains [13].
• We also provide experiments showing that human Smo, when expressed in Schneider cells, is able to bind the alkaloid cyclopamine, suggesting that it is expressed in a native conformational state [14].

Physical interactions of smo

• We also show that Cos2 binding on Smo is necessary for the Hh-dependent dissociation of Ci from this complex [9].
• In vivo evidence that Patched and Smoothened constitute distinct binding and transducing components of a Hedgehog receptor complex [5].
• It is not known how the Hedgehog signal is relayed from Smoothened to the regulatory complex nor how responses to different levels of Hedgehog are implemented [15].

Regulatory relationships of smo

• We propose that the association of the Cos2 protein complex with Smo at the plasma membrane controls the stability of the complex and allows Ci activation, eliciting its nuclear translocation [9].
• High signaling is initiated when cooperating Smoothened cytoplasmic tails activate Costal and Fused, driving the regulatory complex to its high state [15].
• Altered localization of Drosophila Smoothened protein activates Hedgehog signal transduction [7].
• We found that Ser572 phosphorylation attenuates the Cos2-Smo interaction and promotes Cos2 instability [16].

Other interactions of smo

• Here we show that, consequent on the stabilization of Smo, Cos2 and Fu are destabilized [9].
• Smoothened regulates activator and repressor functions of Hedgehog signaling via two distinct mechanisms [17].
• The contributions of protein kinase A and smoothened phosphorylation to hedgehog signal transduction in Drosophila melanogaster [6].
• In both cases, Hh signaling proceeds through the activation of the seven-transmembrane protein Smoothened (Smo), which is thought to convert the Gli family of transcription factors from transcriptional repressors to transcriptional activators [17].
• Smo acts downstream of or in parallel to Patched, an antagonist of the Hh signal [4].

Analytical, diagnostic and therapeutic context of smo

• Sequence analysis of the smoothened transcription unit reveals a single open reading frame encoding a protein with seven putative transmembrane domains [18].
• Immunocytochemistry data and fractionation studies also show that Patched seems to be required for transport of Smoothened to the membrane [13].
• Immunoprecipitation studies show that Patched specifically interacts with caveolin-1, whereas Smoothened does not [13].
• Patched (Ptc), the receptor for the morphogen Hedgehog (Hh), is active in the absence of ligand and blocks the expression of target genes by inhibiting Smoothened (Smo), an essential transducer of the Hh signal [19].

References