Signal Transduction
How do cells communicate with each other during development to determine their positions within the organism and to take on the appropriate fates? The Drosophila eye is a useful model system in which to answer such questions. The Hedgehog, Epidermal growth factor, Notch, and Wingless pathways play important roles in setting up the ordered pattern of photoreceptor differentiation. Mutations that disrupt this pattern are easy to identify and have led us to uncover some unexpected signaling mechanisms.

Lipid modification of signaling proteins
Signaling proteins must convey information between cells by traveling in the extracellular space. Surprisingly, we discovered that both the Hedgehog and Spitz signaling molecules are modified with fatty acids by the same enzyme, Rasp, and that these modifications are essential for their function (Lee et al., Dev. Biol. 2001; Lee and Treisman, Curr. Biol. 2001; Miura et al., Dev. Cell 2006). How can hydrophobic modifications enhance the function of diffusible molecules? For Spitz, the answer seems to be that the fatty acid restricts its diffusion, allowing enough Spitz to build up close to the source to activate the receptor on nearby cells (Miura et al., Dev. Cell 2006; Steinhauer et al., in preparation). We are still investigating how lipid-modified proteins can be released to act over a longer range. We are also looking at whether this mechanism is used to regulate the activity of other classes of signaling proteins.

Endocytosis and receptor activation
Receptor activation triggers a complex cascade of events that ultimately result in a cellular response. These events may be compartmentalized within the cell. The epidermal growth factor receptor (EGFR) is endocytosed and eventually degraded when it binds its ligand, Spitz. We have found mutations that interrupt this endocytic process and also reduce EGFR signaling, suggesting that signaling may occur on endosomes rather than at the plasma membrane (Miura et al., Development 2008; Legent et al., in preparation). One consequence of endocytosis is proteolytic cleavage of the receptor; we are testing whether the cleaved cytoplasmic domain can function in parallel to the canonical downstream signaling pathway.

Protein degradation and signaling
Regulation of protein stability through ubiquitylation and proteasomal degradation is a common mechanism in signaling pathways. We are looking at potential novel targets for ubiquitin ligase complexes in the Wingless, Hedgehog and EGFR pathways (Lee et al., 2002; Legent et al., 2012; Suisse et al., in preparation).