PDF(Pubmed)
Abstract：
Cardiac muscle contraction occurs due to repetitive interactions between myosin thick and actin thin filaments (TF) regulated by Ca2+ levels, active cross-bridges, and cardiac myosin-binding protein C (cMyBP-C). The cardiac TF (cTF) has two nonequivalent strands, each comprised of actin, tropomyosin (Tm), and troponin (Tn). Tn shifts Tm away from myosin-binding sites on actin at elevated Ca2+ levels to allow formation of force-producing actomyosin cross-bridges. The Tn complex is comprised of three distinct polypeptides - Ca2+-binding TnC, inhibitory TnI, and Tm-binding TnT. The molecular mechanism of their collective action is unresolved due to lack of comprehensive structural information on Tn region of cTF. C1 domain of cMyBP-C activates cTF in the absence of Ca2+ to the same extent as rigor myosin. Here we used cryo-EM of native cTFs to show that cTF Tn core adopts multiple structural conformations at high and low Ca2+ levels and that the two strands are structurally distinct. At high Ca2+ levels, cTF is not entirely activated by Ca2+ but exists in either partially or fully activated state. Complete dissociation of TnI C-terminus is required for full activation. In presence of cMyBP-C C1 domain, Tn core adopts a fully activated conformation, even in absence of Ca2+. Our data provide a structural description for the requirement of myosin to fully activate cTFs and explain increased affinity of TnC to Ca2+ in presence of active cross-bridges. We suggest that allosteric coupling between Tn subunits and Tm is required to control actomyosin interactions.
摘要：
心肌收缩是由于肌球蛋白厚和肌动蛋白细丝（TF）之间的重复相互作用而发生的，受Ca2水平调节，有源跨桥，和心肌肌球蛋白结合蛋白C（cMyBP-C）。心脏TF(cTF)有两条不等效的链，每个都由肌动蛋白组成，原肌球蛋白(Tm),和肌钙蛋白(Tn)。Tn在升高的Ca2水平下将Tm从肌动蛋白上的肌球蛋白结合位点移开，以形成产生力的肌动球蛋白交叉桥。Tn复合物由三种不同的多肽组成-Ca2+结合TnC，抑制性TnI,和Tm结合TnT。由于缺乏关于cTFTn区的全面结构信息，它们的集体作用的分子机制尚未解决。cMyBP-C的C1结构域在不存在Ca2+的情况下激活cTF,达到与严格肌球蛋白相同的程度。在这里，我们使用天然cTF的cryo-EM来显示cTFTn核心在高和低Ca2水平上采用多种结构构象，并且两条链在结构上不同。在高Ca2+水平，cTF不完全被Ca2+激活,而是以部分或完全激活状态存在。完全活化需要TnIC末端的完全解离。在存在cMyBP-CC1结构域的情况下，Tn核心采用完全激活的构象，即使没有Ca2+。我们的数据提供了肌球蛋白完全激活cTF的要求的结构描述，并解释了在存在活性交叉桥的情况下TnC对Ca2的亲和力增加。我们建议Tn亚基和Tm之间的变构耦合是控制肌动球蛋白相互作用所必需的。
