Liu F, Barral J M, Bauer C C, Ortiz I, Cook R G, Schmid M F, Epstein H F
Department of Neurology, Baylor College of Medicine, Houston, TX 77030, USA.
Cell Struct Funct. 1997 Feb;22(1):155-62. doi: 10.1247/csf.22.155.
Thick filaments are stable assemblies of myosin that are characteristic of specific muscle types from both vertebrates and invertebrates. In general, their structure and assembly require remarkably precise determination of lengths and diameters, structural differentiation and nonequivalence of myosins, a high degree of inelasticity and rigidity, and dynamic regulation of assembly and disassembly in response to both extracellular and intracellular signals. Directed assembly of myosin in which additional proteins function in key roles, therefore, is more likely to be significant than the simple self assembly of myosin into thick filaments. The nematode Caenorhabditis elegans permits a wide spectrum of biochemical, genetic, molecular and structural approaches to be applied to the experimental testing of this hypothesis. Biochemical analysis of C. elegans thick filaments reveals that paramyosin, a homologue of the myosin rod that is the unique product of a single genetic locus, exists as two populations which differ by post-translational modification. The major paramyosin species interacts with the two genetically specified myosin heavy chain isoforms. The minor paramyosin species is organized within the cores of the thick filaments, where it is associated stoichiometrically with three recently identified proteins P20, P28 and P30. These proteins have now been characterized molecularly and contain unique, novel amino acid sequences. Structural analysis of the core shows that seven paramyosin subfilaments are crosslinked by additional internal proteins into a highly rigid tubule. P20, P28 and P30 are proposed to couple the paramyosin subfilaments together into the core tubule during filament assembly. Mutants that affect paramyosin assembly are being characterized for alterations in the core proteins. A fourth protein has been identified recently as the product of the unc-45 gene. Computational analysis of this gene's DNA suggests that the predicted protein may exhibit protein phosphatase and chaperone activities. Genetic analysis shows that three classes of specific unc-45 mutant proteins differentially interact with the two myosins during thick filament assembly. The unc-45 protein is proposed to be a myosin assemblase, a protein catalyst of thick filament assembly.
粗肌丝是肌球蛋白的稳定聚集体，是脊椎动物和无脊椎动物特定肌肉类型的特征。一般来说，它们的结构和组装需要精确确定长度和直径、肌球蛋白的结构分化和不等价性、高度的非弹性和刚性，以及响应细胞外和细胞内信号对组装和解聚的动态调节。因此，在肌球蛋白的定向组装中，其他蛋白质发挥关键作用，这比肌球蛋白简单地自组装成粗肌丝更有可能具有重要意义。线虫秀丽隐杆线虫允许广泛的生化、遗传、分子和结构方法应用于这一假设的实验测试。对秀丽隐杆线虫粗肌丝的生化分析表明，副肌球蛋白是肌球蛋白杆的同源物，是单个基因座的独特产物，以两个群体的形式存在，这两个群体因翻译后修饰而有所不同。主要的副肌球蛋白物种与两种基因指定的肌球蛋白重链异构体相互作用。次要的副肌球蛋白物种在粗肌丝的核心内组织，在那里它与三种最近鉴定的蛋白质P20、P28和P30化学计量相关。这些蛋白质现在已经进行了分子表征，并包含独特的新氨基酸序列。对核心的结构分析表明，七个副肌球蛋白亚丝通过额外的内部蛋白质交联成一个高度刚性的小管。有人提出，P20、P28和P30在细丝组装过程中将副肌球蛋白亚丝连接在一起形成核心小管。正在对影响副肌球蛋白组装的突变体进行核心蛋白改变的表征。最近已鉴定出第四种蛋白质是unc-45基因的产物。对该基因DNA的计算分析表明，预测的蛋白质可能具有蛋白磷酸酶和伴侣活性。遗传分析表明，三类特定unc-45突变蛋白在粗肌丝组装过程中与两种肌球蛋白有不同的相互作用。有人提出unc-45蛋白是一种肌球蛋白组装酶，是粗肌丝组装的蛋白质催化剂。
