Mechanistic Insight into the Suppression of Polyglutamine Aggregation by SRCP1

Abstract

Protein aggregation is a hallmark of the polyglutamine diseases. One potential treatment for these diseases is suppression of polyglutamine aggregation. Previous work identified the cellular slime mold Dictyostelium discoideum as being naturally resistant to polyglutamine aggregation. Further work identified serine-rich chaperone protein 1 (SRCP1) as a protein that is both necessary in Dictyostelium and sufficient in human cells to suppress polyglutamine aggregation. Therefore, understanding how SRCP1 suppresses aggregation may be useful for developing therapeutics for the polyglutamine diseases. Here we utilized a de novo protein modeling approach to generate predictions of SRCP1’s structure. Using our best-fit model, we generated mutants that were predicted to alter the stability of SRCP1 and tested these mutants’ stability in cells. Using these data, we identified top models of SRCP1’s structure that are consistent with the C-terminal region of SRCP1 forming a β-hairpin with a highly dynamic N-terminal region. We next generated a series of peptides that mimic the predicted β-hairpin and validated that they inhibit aggregation of a polyglutamine-expanded mutant huntingtin exon 1 fragment in vitro. To further assess mechanistic details of how SRCP1 inhibits polyglutamine aggregation, we utilized biochemical assays to determine that SRCP1 inhibits secondary nucleation in a manner dependent upon the regions flanking the polyglutamine tract. Finally, to determine if SRCP1 more could generally suppress protein aggregation, we confirmed that it was sufficient to inhibit aggregation of polyglutamine-expanded ataxin-3. Together these studies provide details into the structural and mechanistic basis of the inhibition of protein aggregation by SRCP1.

Aptamer Reduces Aggregation of Mutant Huntingtin and Rescues Proteostasis Network in Non-Neuronal and Neuronal Cells

“…The effect of this aptamer on aggregation of mutant huntingtin in HEK293 cells was investigated. First, HEK293 cells were co-transfected with pEGFP- Q23 (containing EGFP fused to codons 8–57 inclusive of exon 1 of huntingtin, numbered according to 23Q, Figure S1A) − and pSUPER- pH1-aptamer . The shorter length of huntingtin stretch in pEGFP- Q23 has been shown to be more suitable for transient transfection studies than longer truncations .…”