Mechanism of Cis-Inhibition of PolyQ Fibrillation by PolyP: PPII Oligomers and the Hydrophobic Effect

Abstract

PolyQ peptides teeter between polyproline II (PPII) and beta-sheet conformations. In tandem polyQ-polyP peptides, the polyP segment tips the balance toward PPII, increasing the threshold number of Gln residues needed for fibrillation. To investigate the mechanism of cis-inhibition by flanking polyP segments on polyQ fibrillation, we examined short polyQ, polyP, and tandem polyQ-polyP peptides. These polyQ peptides have only three glutamines and cannot form beta-sheet fibrils. We demonstrate that polyQ-polyP peptides form small, soluble oligomers at high concentrations (as shown by size exclusion chromatography and diffusion coefficient measurements) with PPII structure (as shown by circular dichroism spectroscopy and (3)J(HN-C alpha) constants of Gln residues from constant time correlation spectroscopy NMR). Nuclear Overhauser effect spectroscopy and molecular modeling suggest that self-association of these peptides occurs as a result of both hydrophobic and steric effects. Pro side chains present three methylenes to solvent, favoring self-association of polyP through the hydrophobic effect. Gln side chains, with two methylene groups, can adopt a conformation similar to that of Pro side chains, also permitting self-association through the hydrophobic effect. Furthermore, steric clashes between Gln and Pro side chains to the C-terminal side of the polyQ segment favor adoption of the PPII-like structure in the polyQ segment. The conformational adaptability of the polyQ segment permits the cis-inhibitory effect of polyP segments on fibrillation by the polyQ segments in proteins such as huntingtin.

Control of the structural landscape and neuronal proteotoxicity of mutant Huntingtin by domains flanking the polyQ tract

“…Several models may explain these findings. The PRD could form a PolyProline-II (PPII) helix that propagates into the polyQ region to promote a more helical conformation in the polyQ rather than a β-sheet structure to disfavor aggregation (Binette et al, 2016; Darnell et al, 2007, 2009). A dynamic PPII-helical PRD within the fibril could also interfere with fibril packing interactions (Bugg et al, 2012), perhaps by adopting a 'bottle brush' like architecture protruding from the amyloidogenic core of the fibril (Isas et al, 2015).…”

The Interaction of Polyglutamine Peptides with Lipid Membranes Is Regulated by Flanking Sequences Associated with Huntingtin

“…The poly(P) domain has also been shown to facilitate the ability of Nt17 to associate specifically with the ER and Golgi (22). Flanking poly(P) sequences induce a PPII-like helical structure on the adjacent poly(Q) domain, which can increase the length of the poly(Q) domain that is needed to induce fibril formation (14,48). Our studies further support the importance of poly(P) in htt aggregation but in the context of lipid membranes.…”

Proteins Containing Expanded Polyglutamine Tracts and Neurodegenerative Disease

“…106 At least in vitro, the ability of C-terminal polyP domains to slow aggregation appears to be related to its influence on the transient conformational sampling of the polyQ tract. 106, 181, 194 The polyP domain typically has a PPII-like helical structure that has been observed to propagate into the polyQ tract in crystal structures. 114 However, this propagation of PPII-like helical structure into the polyQ tract is not observed in the fibril structure 171 , suggesting that if this propagation does occur in monomers in solution that it is lost during the aggregation process.…”