Third, Ptch and Smoothened may shuttle oppositely between plasma membrane and endocytic vesicles in response to Hedgehog ligand. They may not interact physically, Ptch working catalytically rather than stoi-chiometrically. In contrast, Incardona et al. suggested that Ptch and Smoothened colocalize in the absence of Hedgehog and both proteins endocytose on ligand binding, after which Smoothened subsequently segregates from the Ptch-Hedgehog complex.
Fourth, the network has a bifunctional transcription regulator, Cubitus interruptus. In the absence of Hedgehog ligand, a truncated transcription repressor is generated that binds target genes and blocks transcription. With Hedgehog ligand, a full-length transcriptional activator binds target genes and up-regulates transcription.
Hedgehog is proficient at both short- and long-range signalling. It is tightly regulated on many levels. Cholesterol-modified Hedgehog, which should be anchored to the cell membrane, can be further modified at the lipid rafts where it multimerizes and becomes soluble and diffusible. Dispatched (OMIM 607502; another 12-pass transmembrane protein) functions to release multimeric Hedgehog, making it available for long-range signalling.
The up-regulation of Ptch expression, resulting in Ptch protein at the cell membrane, sequesters Hedgehog and limits its spread beyond the cells in which it is produced. Thus, a balance is created by the antagonism of Hedgehog and Ptch, whose relative concentrations alternate with regard to each other. Many other factors including Megalin, Rab23, Hip, GAS1, PKA, GSK3, CK1, Slimb, SAP18, and CBP are essential for the Hedgehog signalling network (see review in Ref. ).
Besides Ptch, Megalin can also bind Hedgehog with high affinity. Megalin-mediated endocytosis of multimeric Hedgehog may perhaps regulate its availability to Ptch by competing with Ptch to limit the levels of multimeric Hedgehog. Alternatively, Megalin may deliver multimeric Hedgehog to vesicular pools of Ptch.
Hedgehog binding to Ptch alleviates Ptch-mediated repression of Smo by a conformational change that frees Smo for downstream signalling. Kalderon reviewed four models for the regulation of Smo by Ptch and Hh. The classical model has already been discussed. A second model postulates that Hh activates Smo by causing dissociation of the Ptch/Smo complex. In a third model, Ptch inhibits Smo through a diffusible intermediate and Hh binding to Ptch alters the activity of the intermediate, allowing Smo to become activated. In a fourth model, Ptch acts catalytically via a small molecule to suppress Smo, and on Hh binding to Ptch, Smo is activated by becoming dissociated from Ptch and the small molecule.
Intracellular localization of Ptch was demonstrated by Capdevila et al., who induced a mutation in Shibire [OMIM 602377] that affects endocytosis. Incardona
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.