THE TORSO PATHWAY IN DROSOPHILA - A MODEL SYSTEM TO STUDY RECEPTOR TYROSINE KINASE SIGNAL-TRANSDUCTION

In the Drosophila embryo, specification of terminal cell fates that re sult in the formation of both the head (acron) and tail (telson) regio ns is under the control of the torso (tor) receptor tyrosine kinase. T he current knowledge suggests that activation of tor at the egg pole i nitiates a signal transduction pathway that is mediated sequentially b y the guanine nucleotide releasing factor son of sevenless (Sos), the p21(Ras1) GTPase, the serine/threonine kinase D-raf and the tyrosine/t hreonine kinase MAPKK (Dsor1). Subsequently, it is postulated that act ivation, possibly by phosphorylation, of a transcription factor at the egg poles activates the transcription of the terminal gap genes taill ess and huckebein. These gap genes, which encode putative transcriptio n factors, then control the expression of more downstream factors that ultimately result in head and tail differentiation. Also involved in tor signaling is the non-receptor protein tyrosine phosphatase corkscr ew (csw). Here, we review the current model and discuss future researc h directions in this field.