Localized surface activity of torso, a receptor tyrosine kinase, specifies terminal body pattern in Drosophila

The subdivision of the Drosophila body into distinct terminal and central domains depends on the torso (tor) protein, a putative receptor tyrosine kinase that is active at both ends of the early embryo. We show that the tor protein is uniformly expressed along the surface membrane of early embryos despite its localized activity at both poles. Further, we present evidence that polarized activity of this protein depends on other terminal gene functions, one of which may be a localized extracellular ligand generated during oogenesis. Finally, using the temperature-sensitive gain-of-function mutation torRL3, we show that different levels of active tor protein can specify distinct portions of the terminal pattern. Thus, we argue (1) that for functions as a ubiquitous surface receptor that is activated by a spatially restricted ligand, and (2) that localized activity of the tor kinase may generate one or more gradients of intracellular signals that control body pattern.