Department of Biomedical Engineering, Yale University, 55 Prospect St, New Haven, CT 06511, USA.
Department of Biomedical Engineering, Yale University, 55 Prospect St, New Haven, CT 06511, USA; Department of Cellular and Molecular Physiology, Yale School of Medicine, 333 Cedar St, New Haven, CT 06510, USA.
J Mol Cell Cardiol. 2021 Jun;155:50-57. doi: 10.1016/j.yjmcc.2021.02.012. Epub 2021 Feb 26.
The cardiac thin filament is regulated in a Ca-dependent manner through conformational changes of troponin and tropomyosin (Tm). It has been generally understood that under conditions of low Ca the inhibitory peptide domain (IP) of troponin I (TnI) binds to actin and holds Tm over the myosin binding sites on actin to prevent crossbridge formation. More recently, evidence that the C-terminal mobile domain (MD) of TnI also binds actin has made for a more complex scenario. This study uses a computational model to investigate the consequences of assuming that TnI regulates Tm movement via two actin-binding domains rather than one. First, a 16-state model of the cardiac thin filament regulatory unit was created with TnI-IP as the sole regulatory domain. Expansion of this to include TnI-MD formed a 24-state model. Comparison of these models showed that assumption of a second actin-binding site allows the individual domains to have a lower affinity for actin than would be required for IP acting alone. Indeed, setting actin affinities of the IP and MD to 25% of that assumed for the IP in the single-site model was sufficient to achieve precisely the same degree of Ca regulation. We also tested the 24-state model's ability to represent steady-state experimental data in the case of disruption of either the IP or MD. We were able to capture qualitative changes in several properties that matched what was seen in the experimental data. Lastly, simulations were run to examine the effect of disruption of the IP or MD on twitch dynamics. Our results suggest that both domains are required to keep diastolic cross-bridge activity to a minimum and accelerate myofilament relaxation. Overall, our analyses support a paradigm in which two domains of TnI bind with moderate affinity to actin, working in tandem to complete Ca-dependent regulation of the thin filament.
肌钙蛋白和原肌球蛋白（Tm）的构象变化以 Ca 依赖性方式调节心肌细肌丝。通常认为，在低 Ca 条件下，肌钙蛋白 I（TnI）的抑制肽结构域（IP）与肌动蛋白结合，并将 Tm 置于肌动蛋白上的肌球蛋白结合位点之上，以防止横桥形成。最近，有证据表明 TnI 的 C 端可动结构域（MD）也与肌动蛋白结合，这使得情况变得更加复杂。本研究使用计算模型来研究假设 TnI 通过两个肌动蛋白结合结构域而不是一个来调节 Tm 运动的后果。首先，创建了一个具有 TnI-IP 作为唯一调节结构域的心脏细肌丝调节单元的 16 状态模型。将其扩展为包括 TnI-MD 形成了一个 24 状态模型。对这些模型的比较表明，假设存在第二个肌动蛋白结合位点可以使各个结构域对肌动蛋白的亲和力低于单独使用 IP 所需的亲和力。事实上，将 IP 和 MD 的肌动蛋白亲和力设定为单一位点模型中假设的 IP 亲和力的 25%，足以实现完全相同的 Ca 调节程度。我们还测试了 24 状态模型在破坏 IP 或 MD 的情况下代表稳态实验数据的能力。我们能够捕捉到与实验数据中观察到的相匹配的几个性质的定性变化。最后，进行了模拟以检查破坏 IP 或 MD 对抽搐动力学的影响。我们的结果表明，两个结构域都需要将舒张状态下的横桥活性保持在最低水平，并加速肌丝的松弛。总体而言，我们的分析支持这样一种观点，即 TnI 的两个结构域以中等亲和力与肌动蛋白结合，协同作用完成 Ca 依赖性的细肌丝调节。
