Molecular mechanisms of TOM1-mediated protein trafficking in health and disease

Abstract

TOM1 is an adaptor protein involved in the alternative ESCRT-0 pathway. TOM1 functions by recognizing the ubiquitin moieties on internalized receptors and sorting them towards the degradative arm of the lysosomal system. Two mechanisms of TOM1 aberrant function were uncovered: one mechanism due to bacterial infection and hijacking, the other due to a naturally occurring mutation. Both mechanisms highlight the importance of elucidating protein trafficking pathways and mechanisms in immune responses to help identify new targets for drug therapies and immunomodulating treatments. The first mechanism explored is brought about by the invasion of host cells by Shigella flexneri. S. flexneri injects the virulence factor IpgD, which converts phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] to phosphatidylinositol 5-phosphate [PtdIns(5)P]. The increase in PtdIns(5)P is thought to activate Na+/H+ exchange channels that pump H+ out of maturing endosomes. Rather than the lumen of the endosome acidifying, the endosome halts maturation and receptors destined for lysosomal degradation are able to signal continuously. The acidic microenvironment produced at the surface of these signaling endosomes also attracts TOM1 and TOLLIP by their affinities for PtdIns(5)P. At physiological conditions, TOM1 and TOLLIP show a low affinity for PtdIns(5)P, allowing the complex to perform cargo trafficking functions at the surface of early endosomes through their ability to bind ubiquitin moieties. TOM1's sequence includes a DXXLL motif that enhances the binding site of TOM1 VHS domain for ubiquitin, allowing for recognition while in complex with TOLLIP. In these conditions, the epidermal growth factor receptor (EGFR) is efficiently degraded and the cell's ability to detect infection and induce apoptosis is intact. At low pH, however, the TOM1-TOLLIP complex can still form, but its affinity for PtdIns(5)P significantly increases and TOM1's ability to interact with ubiquitin is inhibited. Not only is the EGFR able to continue signaling, but the PI3Kinase/AKT pathway also gets activated, forcing the cell to survive despite infection. The second mechanism explored originates from a naturally occurring mutation in the TOM1 gene that produces an aspartic acid in position 307 instead of the wild-type glycine. Patients with this mutation experienced autoimmune and immune deficiency symptoms. Since the TOM1-TOLLIP complex is known to traffic interleukin 1 receptor type 1, the interaction between TOM1 and TOLLIP was targeted for a potential mechanism of aberrant immune signaling. The mutation in TOM1 caused a change in local structure of the protein, although no secondary structure nor stability and solubility of the protein was altered. The overall affinity of TOM1 for TOLLIP remains unchanged, but the mutation did affect TOM1's ability to commit TOLLIP to cargo trafficking, as the mutant TOM1 protein could not inhibit TOLLIP's binding to PtdIns(3)P as the wild-type TOM1 protein could. Patient cells also displayed a poor autophagic response and the inability to clear autophagosomes, suggesting that TOM1's role in lysosome/autophagosome fusion was also altered by the mutation.