Patched represses the Hedgehog signalling pathway by promoting modification of the Smoothened protein

Hedgehog (Hh) signalling plays a central role in many developmental process es in both vertebrates and invertebrates [1], The multipass membrane-spanni ng proteins Patched (Ptc) [2-4] and Smoothened (Smo) [5-7] have been propos ed to act as subunits of a putative Hh receptor complex. According to this view, Smo functions as the transducing subunit, the activity of which is bl ocked by a direct interaction with the ligand-binding subunit, Ptc [8], Act ivation of the intracellular signalling pathway occurs when Hh binds to Ptc [8-11], an event assumed to release smo from Ptc-mediated inhibition. Evid ence for a physical interaction between smo and Ptc is so far limited to st udies of the vertebrate versions of these proteins when overexpressed in ti ssue culture systems [8,12], To test this model, we have overexpressed the Drosophila Smo protein in vivo and found that increasing the levels of smo protein per se was not sufficient for activation of the pathway, Immunohist ochemical staining of wildtype and transgenic embryos revealed distinct pat terns of Smo distribution, depending on which region of the protein was det ected by the antibody. Our findings suggest that Smo is modified to yield a non-functional form and this modification is promoted by Ptc in a nonstoic hiometric manner.