The long-term goal of the proposed research is to understand the novel pathway of cell communication in the developing Drosophila eye revealed by the fat facets (faf) gene. faf plays a key role in cell interactions that prevent particular cells present in the earliest stages of eye development from inappropriately becoming photoreceptors, and thus disturbing the symmetry that proper retinal function requires. A combination of molecular and genetic approaches will be used to answer four specific questions. 1. Where is FAF protein expressed and subcellularly localized? Anti-FAF antibodies and epitope-tagged FAF proteins will be generated and used to examine FAF protein expression in developing eyes with both light and electron microscopy. 2. When during eye development is faf function required and in which cells? Heterologous promoters will be used to express faf in portions of the developing eye at different times during development, and the effects on retinal morphogenesis assessed. 3. What are the functional domains of FAF protein? Evolutionarily conserved domains of FAF protein will be identified by isolating the Drosophila virilis faf gene and determining the DNA sequence of its coding region. Also, DNA sequence changes in many mutant faf alleles will be identified. FAF protein domains thus identified will be tested for function in transgenic flies. 4. What are the components of the cell communication pathway in which faf functions? A genetic screen in Drosophila will be conducted to find novel mutations that enhance or suppress the eye defects in faf mutant flies. A screen in yeast for direct protein-protein interactions between FAF protein domains and other eye proteins will be performed. In vertebrates and Drosophila, the use of homologous proteins for cell- cell interactions is commonplace. Thus, although the anatomy of the Drosophila and vertebrate eyes are quite different, the use of Drosophila as a model system will allow the discovery of cell communication mechanisms likely to be common to both.