THE DROSOPHILA SMOOTHENED GENE ENCODES A 7-PASS MEMBRANE-PROTEIN, A PUTATIVE RECEPTOR FOR THE HEDGEHOG SIGNAL

smoothened(smo) is a segment polarity gene required for correct patter ning of every segment in Drosophila. The earliest defect in smo mutant embryos is loss of expression of the Hedgehog-responsive gene wingles s between 1 and 2 hr after gastrulation. Since smo mutant embryos cann ot respond to exogenous Hedgehog (Hh) but can respond to exogenous Win gless, the Smo product functions in Hh signaling. Smo acts downstream of or in parallel to Patched, an antagonist of the Hh signal. The smo gene encodes an integral membrane protein with characteristics of G pr otein-coupled receptors and shows homology to the Drosophila Frizzled protein. Based on its predicted physical characteristics and on its po sition in the Hh signaling pathway, we suggest that smo encodes a rece ptor for the Hh signal.