Pratt, J.; McCann, K.; Kuo, J.; Parker, R.

Tau aggregates are the defining feature of multiple neurodegenerative diseases and contribute to the pathology of disease. However, the molecules affecting tau aggregation in cells are unclear. We previously determined that polyserine-rich domain containing proteins enrich in tau aggregates, assemble into puncta that can serve as sites of tau aggregation, and exacerbate tau aggregation in cells and mice. Herein, we show that polyserine domains are sufficient to define assemblies as sites of tau aggregation, in part, through localization of tau seeds. Purified polyserine self-assembles and directly interacts with monomeric and fibrillar tau. Moreover, polyserine-tau assemblies recruit RNA, leading to faster rates of tau fibrillization in vitro. Using polyserine variants, we found that enrichment in tau aggregates and stimulation of tau aggregation are separable functions of polyserine domains, with polyserine self-assembly stimulating tau aggregation. Together, our results show that polyserine self-assembles and directly interacts with tau to form preferred sites of tau aggregation.