SRCP1 Conveys Resistance to Polyglutamine Aggregation.

The polyglutamine (polyQ) diseases are a group of nine neurodegenerative diseases caused by the expansion of a polyQ tract that results in protein aggregation. Unlike other model organisms, Dictyostelium discoideum is a proteostatic outlier, naturally encoding long polyQ tracts yet resistant to polyQ aggregation. Here we identify serine-rich chaperone protein 1 (SRCP1) as a molecular chaperone that is necessary and sufficient to suppress polyQ aggregation. SRCP1 inhibits aggregation of polyQ-expanded proteins, allowing for their degradation via the proteasome, where SRCP1 is also degraded. SRCP1's C-terminal domain is essential for its activity in cells, and peptides that mimic this domain suppress polyQ aggregation in vitro. Together our results identify a novel type of molecular chaperone and reveal how nature has dealt with the problem of polyQ aggregation.