Aggregation of Huntingtin Exon 1 Proteins at Flat and Curved Membrane Surfaces

Huntingtin exon-1 (HTTex1) aggregation at cellular membranes contributes to the propagation of toxic protein assemblies in Huntington’s disease. We explore the thermodynamic and structural mechanisms linking membrane binding, curvature sensing, and nucleation of the aggregates. Here, we use the OpenAWSEM coarse-grained force field code with an effective membrane potential to quantify the folding and surface aggregation behavior of three HTTex1 constructs on both flat lipid bilayers and spherical vesicles. The computed free energy profiles reveal a strong α-helical NT17-mediated affinity (ΔGbind = −9 kcal/mol) and a curvature-dependent enhancement of this binding, with effective enrichments of protein concentration at the membrane surface of approximately 1000-fold for the NT17 by itself, compared to 18-fold for the polyQ-extended constructs NT17-polyQ and 36-fold for NT17-polyQ-polyP. The free-energy aggregation landscapes demonstrate that membrane proximity also enhances the formation of larger oligomers and promotes early oligomerization through N-terminal anchoring. Analyzing curvature-sensation analyses across vesicle radii shows deeper insertion on highly curved surfaces along with stronger binders, consistent with experimental vesicle-binding assays. Our results establish a mechanistic framework for understanding how membranes can act as two-dimensional platforms that both concentrate HTTex1 and template the formation of aggregation nuclei.

亨廷顿蛋白外显子1（Huntingtin exon-1, HTTex1）在细胞膜上的聚集，会促进亨廷顿病（Huntington’s disease）中毒性蛋白聚集体的传播。本研究探究了连接膜结合、曲率感知与聚集体成核的热力学与结构机制。本研究采用带有有效膜电势的OpenAWSEM粗粒度力场（coarse-grained force field）代码，定量分析了三种HTTex1构建体在平面脂质双层膜（lipid bilayers）与球形囊泡（spherical vesicles）上的折叠与表面聚集行为。计算得到的自由能谱显示，α螺旋（α-helical）结构域NT17介导的强亲和力（结合自由能ΔGbind = −9 kcal/mol），以及结合作用随曲率变化的增强效应：单独的NT17在膜表面的蛋白质浓度富集倍数约为1000倍，而多聚谷氨酰胺（polyQ）延伸的构建体NT17-polyQ仅为18倍，NT17-polyQ-polyP则为36倍。自由能聚集景观图表明，膜邻近效应还可促进更大寡聚体（oligomers）的形成，并通过N端锚定加速早期寡聚化（oligomerization）过程。对不同囊泡半径的曲率感知分析显示，在高曲率表面上蛋白质插入更深，且结合亲和力更强，这与实验开展的囊泡结合测定（vesicle-binding assays）结果一致。本研究结果构建了一套机制性框架，可用于阐释细胞膜如何作为二维平台，既实现HTTex1的富集，又作为模板推动聚集核的形成。