The sterol-sensing domain of Patched protein seems to control Smoothened activity through Patched vesicular trafficking

The Hedgehog (Hh) family of signaling molecules function as organizers in m any morphogenetic processes. Hh signaling requires cholesterol in both sign al-generating and -receiving cells, and it requires the tumor suppressor Pa tched (Ptc) in receiving cells in which it plays a negative role. Ptc both blocks the Hh pathway and limits the spread of Hh, Sequence analysis sugges ts that it has 12 transmembrane segments, 5 of which are homologous to a co nserved region that has been identified in several proteins involved in cho lesterol homeostasis and has been designated the Sterol-sensing domain (SSD ), In the present study, we show that a Ptc mutant with a single amino acid substitution in the SSD induces target gene activation in a ligand-indepen dent manner, This mutant Ptc(SSD) protein shows dominant-negative activity in blocking Hh signaling by preventing the downregulation of Smoothened (Sm o), a positive effector of the Hh pathway. Despite its dominant-negative ac tivity, the mutant Ptc protein functioned like the wild-type protein in seq uestering and internalizing Hh, In addition, we show that Ptc(SSD) preferen tially accumulates in endosomes of the endocytic compartment All these resu lts suggest a role of the SSD of Ptc in mediating the vesicular trafficking of Ptc to regulate Smo activity.