NEW EMBO MEMBER'S REVIEW: In and out of Torso RTK signalling

Abstract
The ability of cells to respond to extracellular signals relies on a set of mechanisms that are of widespread use in different developmental contexts and are highly conserved among different organisms. One such mechanism is built upon the presence of receptor tyrosine kinase (RTK) molecules in the cell membrane that can be activated by ligands outside the cell and transduce this signal by a well conserved pathway of intracellular molecules to finally elicit different cell responses in terms of morphology and/or gene activation. The Drosophila Torso pathway has been used as one of the model systems to genetically analyse the activity of the RTK signalling pathways. In particular, different studies in this and other systems have allowed identification of the components of these transducing mechanisms and conclusions to be drawn about their interaction. A general conclusion of these experiments is that tyrosine kinase receptors appear to activate a shared group of intracellular effectors, including the Ras/Raf/MAPK cascade. This conclusion has driven many studies to look for the specificity of the different transduction pathways at the events taking place specifically at both ends of the signalling pathways, namely, those leading to the activation of the receptor molecules and those occurring downstream of the phosphorylation cascade. It is the analysis of these events that can help us to understand the great variety of responses that can be elicited by the different RTK signalling pathways. The conserved intracellular mechanisms acting downstream of the Torso receptor have already been reviewed elsewhere and, thus, here we will address specifically the issue of the mechanisms leading to the Torso receptor activation and those responsible for regulating the expression of the Torso pathway target genes. ### How to locally activate a widespread receptor? Transferring positional information from the ovarian cells to the embryo . Torso is a RTK (Sprenger et al ., 1989) that is distributed …