| The morphogenetic Hedgehog (Hh) signal transduction pathway is evolutionarily conserved and required for proper animal growth, maintenance and development. Disruption of the Hh pathway in humans leads to a variety of disease states that range from developmental disorders to cancer in children and adults. Its importance to human health is exemplified by the growing number of cancers shown to have elevated Hh signaling activity. As a means of developing and identifying targets for therapeutic intervention of aberrant Hh signaling, I investigated the fundamental mechanisms underlying Hh signal transduction. To this end, I studied the Hh pathway in the fruit fly Drosophila melanogaster, taking advantage of the rich genetic and biochemical tools available for this model organism. A large multi-protein cytosolic Hh signaling complex (HSC) functions to properly interpret the Hh signal. The Ser/Thr protein kinase Fused (Fu) is an integral member of the HSC. Fu appears necessary to ensure that the HSC remain in an 'off' state in the absence of Hh, but is also required for full HSC activity in the presence of Hh. Here I describe work characterizing the role of Fu in HSC function. I found that Fu is engaged in a number of protein interactions with the other components of the HSC, including with itself. Fu is directly bound to the kinesin-related protein Costal2. Targeted disruption of their interaction leads to a total loss of Hh activation. Furthermore, Fu appears capable of forming self-associated monomers. By promoting self-association in vivo, we are able to rescue a fu loss of function phenotype in Drosophila. Finally we discover that the HSC is in direct contact with Smoothened, the G-protein coupled receptor-like transmembrane protein. By examining the consequences of identifying and disrupting protein complex interactions, we are now able to derive a mechanistic model of Hh signal transduction. |
