Trk-fused Gene (TFG) Organizes the Early Secretory Pathway and Regulates Neuronal Function

Roughly one third of the genes in the human genome encode transmembrane or secreted proteins which are responsible for countless functions involved in sustaining life. All of these proteins are translated at the endoplasmic reticulum (ER) where they are then trafficked to their final destinations, either inside or outside the cell. These cargo proteins are packed into coat protein II (COPII) transport carriers at ER exit sites to head towards the Golgi, to be properly processed. The formation of these vesicles is tightly regulated to properly form, traffic, and fuse with one another. While the basics of this process are understood, further mechanistic work is required. The goal of my work was to better understand the regulation of this early secretory pathway by the new modulator of COPII known as the TRK-fused gene (TFG). TFG localizes at the ER exit site and affects protein secretion. Thus, characterization of this protein will help to give a more complete understanding to the complex mechanisms as to how higher order organisms efficiently traffic cargo from the ER to the Golgi and their roles in disease.;In this thesis, I presented evidence that the R106C mutation in the coiled coil region of TFG leads to hereditary spastic paraplegia (HSP), a degenerative motor neuron disease that affects the lower extremities. Furthermore, TFG localizes down axons and is important for the suitable maintenance of ER tubules in neuronal cells, which are necessary for proper cellular function. Furthermore, I investigated TFG in human cell lines and observed that the protein localizes with COPII vesicles. Depletion of TFG disturbs COPII localization and slows the secretion of cargo through the early secretory pathway. Combining different biochemical approaches, I discovered that the N-terminus of TFG forms a cup-shaped ring-like structure and a C-terminus which interacts with one another to form the meshwork. Finally, biochemical data indicates that the R106C mutation perturbs the ring structure of TFG and leads to improper localization of the protein within the cell. This work shows the necessity of properly functioning TFG protein for cellular viability and the effect that its perturbation has on disease.