Mapping the role of the sDscam domains in cis & trans interactions

In insects, tens of thousands of Down syndrome cell adhesion molecules (Dscam1) interact in a strict homophilic fashion providing a unique cell surface identity to neurons critical for proper neural circuit development. In vertebrates, approximately 60 clustered protocadherins (cPcdhs) engage in a network of homophilic interactions to support an analogous function. In Chelicerata subphylum, which includes both spiders and scorpions, a different group of about 100 proteins called Shortened DSCAM (sDscam) engages in strict homophilic interactions and are a likely candidates to fulfill the same role in neuronal development. The sDscams are unique because on the one hand they are evolutionarily related to the insect Dscam; however, from a functional perspective, the scorpion sDscam behaves similarly to the evolutionarily unrelated vertebrate cPcdhs. To date, it remains unknown how sDscam achieves the strict homophilic interactions among its many similar isoforms. Through my research, I intend to express different fragments of the cell-cell recognition region from four different sDscam isoforms and using biophysical and x-ray crystallography methods I will characterize their recognition mechanisms. Preliminary cross-linking experiments suggest that the three N-terminal Ig domains (Ig1 – Ig3) are required for homophilic interactions. However, structural modeling, suggests that sDscam homophilic interactions is mediated by the N-terminal first Ig domain alone. My results will elucidate the mechanism of self-recognition of sDscam and would provide insight into the evolution of processes in neuronal patterning.