Isomorphisms in tropomyosin modulate binding, flexibility and thin filament function

活着的有机体会移动。最壮观的例子是在显微镜下有条纹外观的肌肉。我们每天都依靠它们来实现血液循环、姿势、速度和力量。这些基本的生命起源于产生运动的(肌丝)蛋白质的相互作用。其中，原肌球蛋白是应用的重点。*它能做什么？原肌球蛋白开启和关闭肌肉。这是至关重要的，因为监管至关重要。没有它，肌肉就像一辆失控的汽车，没有点火和刹车。当Ca(II)从体内释放出来时，原肌球蛋白刺激细丝的结构重排，然后激活运动肌球蛋白。其结果是大量的ATP水解和收缩。当Ca(II)被重新吸收时，相反的情况发生了，随后发生了松弛。原肌球蛋白之所以能够做到这一点，是因为：(A)它与自身聚合，形成一条灵活的、连续的链；(B)它与其他蛋白质(如肌动蛋白、肌钙蛋白)的相互作用是钙敏感的。“调整”原肌球蛋白。动物运动的多样性是不同类型肌肉的结果--快肌、慢肌和心肌--这些肌肉利用肌丝蛋白的异构体来收缩和放松。蛋白质异构体就像近亲；它们有家族相似性，但有不同的特征(例如，氨基酸序列略有不同)。原肌球蛋白如何促进肌肉多样性一直是个谜。原肌球蛋白的主要亚型是Tpm1.1(以前的α)和Tpm2.2(以前的β)。它们共有39个氨基酸替换。问：他们有什么不同之处？直到最近才得到答复(Lohmeier-Vogel&Heeley 2016)。报道了肌球蛋白ATPase在聚合、肌钙蛋白结合和调节方面的差异。2016年的研究为提出下一个问题奠定了基础。哪些同构决定了原肌球蛋白的同型？同构将在Tpm1.1和2.2之间互换，目的是表征特定同构的作用。(I)部分也与尚未在蛋白质水平上研究的疾病突变(目前为&gt；50)有关。原肌球蛋白在极端条件下对于蛋白质来说，寒冷的问题是僵硬的。在冬季，北大西洋接近冰点，与温血动物(哺乳动物)的情景相比，气温下降了30摄氏度。然而，这是一个天然的海洋栖息地。一个“大”问题是：原肌球蛋白如何避免被“束缚”？最近的工作(Fdge&Heeley 2015)表明在鲑鱼Tpm1.1中存在3种‘弯曲’策略(更少的离子对，更多的甘氨酸和核心位置的甘氨酸和极性残基)。这些“冷同构”对鲑鱼Tpm1.1功能的影响-聚合、肌钙蛋白结合和调节-将通过氨基酸与热稳定的哺乳动物同源物的互换来探索。与第(I)部分一样，第(Ii)部分将提供原肌球蛋白的功能图景(即结合位点)的信息。*****

Living organisms move. The most spectacular examples are muscles that have a striped appearance under the microscope. We rely on them daily for blood circulation, posture, speed & strength. These basics of life arise from the interplay of motion-producing (myofilament) proteins. One of these, tropomyosin, is the focus of the application. ******What does it do? Tropomyosin turns muscle on and off. It is essential because regulation is essential. Without it, a muscle would be like a uncontrollable vehicle, one lacking an ignition and brakes. When Ca(II) is released from internal stores, tropomyosin instigates a structural rearrangement of the thin filament, which then activates the motor myosin. The outcome is substantial ATP hydrolysis and contraction. When Ca(II) is taken back up, the reverse happens and relaxation ensues. Tropomyosin is able to do this because: (a) it polymerises with itself, forming a flexible, continuous strand and (b) its interactions with other proteins (eg actin, troponin) are Ca(II)-sensitive.******Part (I). 'Tuning' tropomyosin. The diversity of animal locomotion is an outcome of different types of muscle - fast, slow and cardiac - that deploy isoforms of the myofilament proteins to contract and relax. Protein isoforms are like cousins; they bear a family resemblance but have distinguishing features (eg slightly different amino acid sequences). How tropomyosin contributes to muscle diversity has long-been an enigma. ******The major isoforms of tropomyosin are Tpm1.1 (formerly α) & Tpm2.2 (formerly β). They share 39 amino acid substitutions. The 'big' Q. What do they do differently? was answered only recently (Lohmeier-Vogel & Heeley 2016). Differences in polymerisation, troponin binding & regulation of myosin ATPase were reported. The 2016 study establishes a foundation to ask the next Q. Which isomorphisms determine tropomyosin isotype? Isomorphisms will be interchanged between Tpm1.1 and 2.2 with the intent of characterising the role of a particular isomorphism. Part (I) is also of relevance to disease mutations (currently > 50) not yet studied at the protein level.*********Part (II). Tropomyosin in extreme conditions The problem of cold for a protein is rigidity. During the winter, the N. Atlantic approaches freezing, a drop of > 30 oC compared to the warm-blooded (mammalian) scenario. Yet this is a natural marine habitat. A 'big' Q. is: How does tropomyosin avoid being ‘straitjacketed'? Recent work (Fudge & Heeley 2015) has shown the existence of 3 'flexing' strategies (fewer ion pairs, more glycine & polar residues in core sites) within salmon Tpm1.1. The effect of these 'cold isomorphisms' on salmon Tpm1.1 function - polymerisation, troponin binding & regulation - will be probed by interchange of amino acids with the thermally-stable mammalian homologue. Part (II), as with Part (I), will provide information on tropomyosin's functional landscape (ie binding sites). *****