The Role of Thick Veins and Saxophone in BMP Signaling. D.G. Stathakis , S. Park , K. Arora. Department of Developmental and Cell Biology, University of California, Irvine, CA., 92697-2275.

   A striking feature of many cell signaling pathways is the utilization of multiple structurally similar ligands to convey distinct types of information. In Drosophila, two BMP ligands, Decapentaplegic (Dpp)and Screw (Scw), are required for specification of cells in the dorsal region of the embryo. Dpp is obligately required in all dorsal cells, while Scw is required to potentiate Dpp activity. We and others have shown that the Dpp and Scw act primarily through independent type I receptors. Thick veins (Tkv) mediates the Dpp signal, while Saxophone (Sax) is required for the response to Scw. Simultaneous activation of both Tkv and Sax results in a strong synergistic response that appears to be essential for the establishment of the complete range of dorsal cell fates in the embryo. Our results indicate that Sax signaling has no phenotypic consequences in the absence of Tkv, although Tkv can signal independently of Sax. Taken together, these data suggest that the synergy between these two receptors allow the receiving cells to integrate signals from two separate pathways. We are using molecular genetic and biochemical approaches to examine the interactions between Tkv and Sax in order to elucidate the mechanistic basis for the differential response to Dpp and Scw signaling. The results from these experiments will be presented.