The spatial control of Torso RTK activation: a C-terminal fragment of the Trunk protein acts as a signal for Torso receptor in the Drosophila embryo

Regulated activation of receptor tyrosine kinases depends on both the prese nce of the receptors at the cell surface and on the availability of their l igands, In Drosophila, the torso tyrosine kinase receptor is distributed al ong the surface of the embryo but it is only activated at the poles by a di ffusible extracellular ligand generated at each pole that is trapped by the receptor, thereby impeding further diffusion. Although it is known that th is signal depends on the activity of several genes, such as torso-like and trunk, it is still unclear how is generated. The identification of the sign al responsible for the torso receptor activation is an essential step towar ds understanding the mechanism that regulates the local restriction of tors o signalling. Here we report that a fragment containing the carboxy-termina l 108 amino acids of the trunk protein retains trunk activity and is suffic ient to activate torso signalling. We also show that this fragment bypasses the requirements for the other genes involved in the activation of the tor so receptor. These results suggest that a cleaved form of the trunk protein acts as a signal for the torso receptor. We therefore propose that the res tricted activation of the torso receptor is defined by the spatial control of the proteolytic processing of the trunk protein.