Exploring the Influence of Molecular Chaperones on the Aggregation of Transthyretin

Molecular chaperones assist in maintaining protein homeostasis by limiting protein aggregation, including populations of protein aggregates that may be associated with the onset of debilitating and/or often fatal diseases. Chaperones are known to limit protein aggregate populations through disaggregase activity, in which chaperone complexes work together to disassemble multi-subunit aggregate species into smaller populations, or holdase activity, in which chaperones bind to proteins or smaller aggregates to prevent further aggregation. In this dissertation we assess if molecular chaperones can limit the aggregation of human transthyretin (TTR) expressed within a yeast system and explore the relationship between chaperone concentration and activity. We find evidence that may suggest that the primary disaggregase in yeast, Hsp104, is not contributing disaggregase activity to limit the size of higher order TTR complexes. Interestingly, an abundance of a Hsp104 variant associated with potentiated disaggregase activity, Hsp104-A503S, increases the size of higher order TTR complexes. We offer evidence for the importance of the Hsp40, and co-chaperone to Hsp104, Sis1 in limiting the size of higher order TTR complexes and propose a model in which Hsp104-A503S limits Sis1 activity, thereby impairing the effect of Sis1 on higher order TTR complexes. Swapping the expression of Sis1 for its human homolog, DnajB1, results in a substantial increase in smaller TTR complex populations. A conclusion regarding whether Sis1 or DnaJB1 limit the size of higher order TTR complexes through disaggregase or holdase activity cannot be determined from the data in this work. Therefore, this dissertation concludes by discussing potential models in which chaperones facilitate either activity. Future research into understanding the chaperone activity responsible for limiting TTR aggregation could lead to a deeper understanding of the chaperone network responsible for mitigating human protein aggregation and may contribute to future translational research in which the aggregation of disease-related protein is more easily prevented in humans.