Solution structure and hydrodynamics of C. elegans UNC-6: a nematode paralogue of the vertebrate key axon guidance protein Netrin-1

Abstract UNCoordinated-6 (UNC-6) was the first member of the netrin family to be discovered in Caenorhabditis elegans . With homology to human netrin-1, it is a key signaling molecule involved in directing axon migration in nematodes. Similar to netrin-1, UNC-6 interacts with multiple receptors (UNC-5 and UNC-40, specifically) to guide axon migration in development. As a result of the distinct evolutionary path of UNC-6 compared to vertebrate netrins, we decided to employ an integrated approach to study its solution behavior and compare it to the high-resolution structure we previously published on vertebrate netrins. Dynamic light scattering and analytical ultracentrifugation on UNC-6 (with and without its C-domain) solubilized in a low-ionic strength buffer suggested that UNC-6 forms high-order oligomers. An increase in the buffer ionic strength resulted in a more homogeneous preparation of UNC-6, that was used for subsequent solution x-ray scattering experiments. Our biophysical analysis of UNC-6 Δ C solubilized in a high-ionic strength buffer suggested that it maintains a similar head-to-stalk arrangement as netrins −1 and −4. This phenomenon is thought to play a role in the signaling behavior of UNC-6 and its ability to move throughout the extracellular matrix.