Marino Marco, Svergun Dmitri I, Kreplak Laurent, Konarev Peter V, Maco Bohumil, Labeit Dietmar, Mayans Olga
Division of Structural Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056, Basel, Switzerland.
J Muscle Res Cell Motil. 2005;26(6-8):355-65. doi: 10.1007/s10974-005-9017-6.
The cellular function of the giant protein titin in striated muscle is a major focus of scientific attention. Particularly, its role in passive mechanics has been extensively investigated. In strong contrast, the structural details of this filament are very poorly understood. To date, only a handful of atomic models from single domain components have become available and data on poly-constructs are limited to scarce SAXS analyses. In this study, we examine the molecular parameters of poly-Ig tandems from I-band titin relevant to muscle elasticity. We revisit conservation patterns in domain and linker sequences of I-band modules and interpret these in the light of available atomic structures of Ig domains from muscle proteins. The emphasis is placed on features expected to affect inter-domain arrangements. We examine the overall conformation of a 6Ig fragment, I65-I70, from the skeletal I-band of soleus titin using SAXS and electron microscopy approaches. The possible effect of highly conserved glutamate groups at the linkers as well as the ionic strength of the medium on the overall molecular parameters of this sample is investigated. Our findings indicate that poly-Ig tandems from I-band titin tend to adopt extended arrangements with low or moderate intrinsic flexibility, independently of the specific features of linkers or component Ig domains across constitutively- and differentially-expressed tandems. Linkers do not appear to operate as free hinges so that lateral association of Ig domains must occur infrequently in samples in solution, even that inter-domain sequences of 4-5 residues length would well accommodate such geometry. It can be expected that this principle is generally applicable to all Ig-tandems from I-band titin.
横纹肌中巨大蛋白肌联蛋白的细胞功能是科学关注的主要焦点。特别是，其在被动力学中的作用已得到广泛研究。与此形成强烈对比的是，这种细丝的结构细节却知之甚少。迄今为止，仅有少数来自单结构域组件的原子模型，并且关于多聚体构建体的数据仅限于稀少的小角X射线散射（SAXS）分析。在本研究中，我们研究了与肌肉弹性相关的I带肌联蛋白的多聚免疫球蛋白（Ig）串联体的分子参数。我们重新审视了I带模块的结构域和连接子序列中的保守模式，并根据肌肉蛋白Ig结构域的现有原子结构对这些模式进行了解释。重点放在预期会影响结构域间排列的特征上。我们使用SAXS和电子显微镜方法研究了来自比目鱼肌I带的6Ig片段I65 - I70的整体构象。研究了连接子处高度保守的谷氨酸基团以及介质的离子强度对该样品整体分子参数的可能影响。我们的研究结果表明，I带肌联蛋白的多聚Ig串联体倾向于采用具有低或中等固有柔韧性的伸展排列，这与组成性和差异性表达的串联体中连接子或组成Ig结构域的特定特征无关。连接子似乎并非作为自由铰链起作用，因此在溶液中的样品中，Ig结构域的侧向缔合必定很少发生，即使是4 - 5个残基长度的结构域间序列也能很好地适应这种几何结构。可以预期，这一原则普遍适用于I带肌联蛋白的所有Ig串联体。
