Transducing the hedgehog signal.

Regardless of the status of Smo/Ptc associations, we now have to consider which, if any, of the newly found responses of Smo to Hh actually determine Smo signaling activity. As there really are no definitive data to guide us here, I will take the opportunity to discuss some possibilities. Although it is logical to expect that Smo activity would increase roughly in proportion to its total concentration, clear exceptions have already been noted in artificial situations where smo RNA is overproduced or PKA activity is inhibited. It is possible that in these situations, Smo protein does not reach the plasma membrane, or perhaps a particular domain of the plasma membrane, and that Smo concentration in a specific subcellular location is, in fact, the key determinant of Smo activity. However, even if this is true, additional regulation might be expected.One pivotal issue is that we do not know the biochemical activity of Smo. Although its seven transmembrane domain structure suggested that it might activate a trimeric G protein, there has been very little evidence supporting this idea prior to a recent study of human Smo in frog melanophores (DeCamp et al. 2000xDeCamp, D.L, Thompson, T.M, deSauvage, F.J, and Lerner, M.R. J. Biol. Chem. 2000; 275: 26322–26327Crossref | PubMed | Scopus (75)See all ReferencesDeCamp et al. 2000). Smo activation might nevertheless involve a conformational change that is characteristic of G protein coupled receptors (GPCRs). This idea comes from a recent study of the locations of amino acid substitutions that constitutively activate vertebrate Smo (Taipale et al. 2000xTaipale, J, Chen, J.K, Cooper, M.K, Wang, B, Mann, R.K, Milenkovic, L, Scott, M.P, and Beachy, P.A. Nature. 2000; 406: 1005–1009Crossref | PubMed | Scopus (847)See all ReferencesTaipale et al. 2000). One could easily imagine a conformational change resulting from binding of Hh to a Ptc–Smo complex or from Hh-induced dissociation of the complex. In the case of GPCRs, this creates a binding site for G proteins (Bockaert and Pin 1999xBockaert, J and Pin, J.P. EMBO J. 1999; 18: 1723–1729Crossref | PubMedSee all ReferencesBockaert and Pin 1999). In the case of Smo, this might create a binding site that links it directly to a cytoplasmic Ci effector complex.What about Smo phosphorylation? Although increased Smo phosphorylation might result in accumulation of active Smo at the plasma membrane (Denef et al. 2000xDenef, N, Neubuser, D, Perez, L, and Cohen, S.M. Cell. 2000; 102: 521–531Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesDenef et al. 2000), such causative links have not been established. In reality, we might expect a more complex relationship that will require tracking the status and impact of each site of Smo phosphorylation. One reason for expecting complex regulation by phosphorylation is that we already know of two kinases, PKA and Fused, involved in Hh signaling; we know that several proteins (Fused, Costal-2, and now Smo) become hyperphosphorylated during Hh signaling (Ingham 1998xIngham, P.W. EMBO J. 1998; 17: 3505–3511Crossref | PubMed | Scopus (358)See all ReferencesIngham 1998), and yet, there is also some evidence that Hh signaling might reduce the phosphorylation of some proteins (Chen et al. 1999xChen, C.-H, von Kessler, D.P, Park, W, Wang, B, Ma, Y, and Beachy, P.A. Cell. 1999; 98: 305–316Abstract | Full Text | Full Text PDF | PubMed | Scopus (201)See all ReferencesChen et al. 1999). For the sake of discussion, I outline below one possible model for how specific kinases and phosphorylations might regulate Hh signaling at the cell surface.The role of PKA demands special attention because Alcedo et al. have shown that Smo protein accumulates dramatically when PKA is inhibited. There is no evidence yet regarding the relevant target for PKA, but Smo contains a cluster of putative PKA sites in its C-terminal cytoplasmic domain. It is therefore tempting to draw a direct analogy to similar sites in Ci and guess, as Alcedo et al. suggest, that PKA targets Smo for degradation by direct phosphorylation. If this occurs, Hh signaling could lead to the accumulation of hyperphosphorylated Smo simply by blocking degradation of PKA-phosphorylated Smo. To complete this model I would suggest that a Ptc-associated protein (perhaps a kinase) normally targets PKA-phosphorylated Smo for degradation (Figure 1DFigure 1D), and that Hh interrupts this process by dissociating Smo from Ptc. Hh/Ptc complexes would be internalized and degraded (Figure 2Figure 2), limiting the movement of Hh across cells (Chen and Struhl 1996xChen, Y and Struhl, G. Cell. 1996; 87: 553–563Abstract | Full Text | Full Text PDF | PubMed | Scopus (626)See all ReferencesChen and Struhl 1996) and prolonging the half-life of activated Smo roughly in proportion to Hh dose. Liberated Smo might bring one or more associated kinases to the Ci complex, leading to phosphorylation of Costal-2 and Fused. These phosphorylations may in turn stimulate release of Ci-155 from cytoplasmic anchorage, association of Ci-155 with a transcriptional coactivator and hence, movement of active Ci-155 into the nucleus. These last ideas are, of course, highly speculative and represent just one of many models that will be prompted by the recent reports delving into Smo and Ptc metabolism.*E-mail: ddk1@columbia.edu