O'Brien Brett M, Moulick Roumita, Jiménez-Avalos Gabriel, Rajasekaran Nandakumar, Kaiser Christian M, Woodson Sarah A
Chemical Biology Interface Program, Johns Hopkins University, Baltimore, MD 21218 USA.
T. C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218 USA.
bioRxiv. 2024 Jun 3:2024.05.31.596809. doi: 10.1101/2024.05.31.596809.
In Huntington's Disease (HD) and related disorders, expansion of CAG trinucleotide repeats produces a toxic gain of function in affected neurons. Expanded (exp) mRNA forms aggregates that sequester essential RNA binding proteins, dysregulating mRNA processing and translation. The physical basis of RNA aggregation has been difficult to disentangle owing to the heterogeneous structure of the CAG repeats. Here, we probe the folding and unfolding pathways of exp mRNA using single-molecule force spectroscopy. Whereas normal mRNAs unfold reversibly and cooperatively, exp mRNAs with 20 or 40 CAG repeats slip and unravel non-cooperatively at low tension. Slippage of CAG base pairs is punctuated by concerted rearrangement of adjacent CCG trinucleotides, trapping partially folded structures that readily base pair with another RNA strand. We suggest that the conformational entropy of the CAG repeats, combined with stable CCG base pairs, creates a stick-slip behavior that explains the aggregation propensity of exp mRNA.
在亨廷顿舞蹈症（HD）及相关疾病中，CAG三核苷酸重复序列的扩增在受影响的神经元中产生了毒性功能获得。扩增的（exp）mRNA形成聚集体，隔离必需的RNA结合蛋白，从而失调mRNA加工和翻译。由于CAG重复序列的异质结构，RNA聚集的物理基础一直难以厘清。在此，我们使用单分子力谱探究exp mRNA的折叠和去折叠途径。正常mRNA可逆且协同地去折叠，而具有20或40个CAG重复序列的exp mRNA在低张力下会滑动并以非协同方式解开。CAG碱基对的滑动被相邻CCG三核苷酸的协同重排打断，捕获易于与另一条RNA链碱基配对的部分折叠结构。我们认为，CAG重复序列的构象熵与稳定的CCG碱基对相结合，产生了一种粘滑行为，这解释了exp mRNA的聚集倾向。
