Investigating crosstalk among PTMs provides novel insight into the structural basis underlying the differential effects of Nt17 PTMs on mutant Httex1 aggregation

Post-translational modifications (PTMs) within the first 17 amino acids (Nt17) of the Huntingtin protein (Htt) have been shown to inhibit the aggregation and attenuate the toxicity of mutant Htt proteins in vitro and in various models of Huntingtons disease. Our groups previous studies suggested that the Nt17 PTM code is a combinatorial code that involves a complex interplay between different PTMs. Here, we expand on these studies by investigating the effect of methionine 8 oxidation (oxM8) and crosstalk between this PTM and either lysine 6 acetylation (AcK6) or threonine 3 phosphorylation (pT3) on the aggregation of mutant Httex1. We show that M8 oxidation delays but does not inhibit the aggregation and has no effect on the final morphologies of mutant Httex1 aggregates. This delay in aggregation kinetics could be attributed to the transient accumulation of oligomeric aggregates, which disappear upon the formation of Httex1 oxM8 fibrils. Interestingly, the presence of both oxM8 and AcK6 resulted in dramatic inhibition of Httex1 fibrillization, whereas the presence of oxM8 did not influence the aggregation inhibitory effect of pT3. To gain insight into the structural basis underlying these proteins aggregation properties, we investigated the impact of each PTM and the combination of these PTMs on the conformational properties of the Nt17 peptide by circular dichroism spectroscopy and molecular dynamics simulation. These studies show that M8 oxidation decreases the helicity of the Nt17 in the presence or absence of PTMs and provides novel insight into the structural basis underlying the effects of different PTMs on mutant Httex1 aggregation. PTMs that lower the mutant Httex1 aggregation rate (oxM8, AcK6/oxM8, pT3, pT3/oxM8, and phosphorylation at Serine 13) result in stabilization and increased population of a short N-terminal helix (first eight residues) in Nt17 or decreased abundance of other helical forms, including long helix and short C-terminal helix. PTMs that did not alter the aggregation of mutant Httex1 exhibit a similar distribution of helical conformation as the unmodified peptides. These results show that the relative abundance of N- vs. C-terminal helical conformations and long helices, rather than the overall helicity of Nt17, better explains the effect of different Nt17 PTMs on mutant Httex1; thus, explaining the lack of correlation between the effect of PTMs on the overall helicity of Nt17 and mutant Httex1 aggregation in vitro. Taken together, our results provide novel structural insight into the differential effects of single PTMs and crosstalk between different PTMs in regulating mutant Httex1 aggregation. TOC Figure O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=133 SRC="FIGDIR/small/432155v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@15dbf84org.highwire.dtl.DTLVardef@1c8a50corg.highwire.dtl.DTLVardef@1cbe0c6org.highwire.dtl.DTLVardef@120e0a_HPS_FORMAT_FIGEXP M_FIG C_FIG