SMALL ANKYRINS IN EXCITABLE CELLS

肌浆网（SR）嵌入在肌浆中，在那里它包围着每个肌原纤维的收缩结构。如果没有一种机制来稳定膜并将其定期连接到收缩装置，SR可能会在收缩周期中受损，导致其储存的Ca 2+的不受控制的释放，并对肌肉结构和功能产生深远的影响。 当然，在健康的肌肉中，这种情况不会发生，部分原因是肌原纤维的SR和Z盘之间存在规则的连接。 在这里，我们提出了一个蛋白质，可能形成这些连接的特点。 我们在横纹肌细胞中发现了结构蛋白锚蛋白的小的、选择性剪接形式。 小锚蛋白（缩写为sAnk 1）富集在SR中，特别是在附近肌原纤维的M和Z盘侧翼的位点。 哺乳动物红细胞中的锚蛋白通过将其连接到基于血影蛋白的细胞骨架来稳定细胞膜。 我们假设sAnk 1通过将SR附着在横纹肌细胞的细胞骨架（收缩装置）上来稳定SR。 我们提出了4个具体的目标来测试这个想法及其发展和功能的后果。(i)sAnk 1有一个独特的，高度疏水性的氨基末端序列，很可能锚它的膜和靶向它专门的SR。我们将进行生物化学和细胞转染实验，以确定如何sAnk 1协会优先与SR。（ii）sAnk 1可能集中在SR附近的M和Z磁盘作为一个结果的特异性结合蛋白存在于这些结构。 我们将使用酵母双杂交筛选来鉴定这些配体，并在共聚焦和超微结构水平上进行免疫标记以将它们定位在肌节中。(iii)如果sAnk 1将SR与收缩装置连接起来，那么它及其配体应该在发育中的肌肉中SR的组织和分化中发挥重要作用。 我们将使用生物化学，免疫学和结构方法来确定sAnk 1及其配体在发育过程中的表达和组织方式。(iv)如果sAnk 1是重要的肌浆组织，抑制其活性应破坏SR与收缩装置的关系，改变Ca 2+稳态。 我们将使用转基因技术来操纵sAnk 1及其配体在肌肉细胞中的水平，和生理和超微结构技术，以评估这些操作的SR的组织和功能的影响。因此，我们的研究应有助于阐明负责SR在未成熟和成年横纹肌的形成和组织的机制。 这些机制可能在兴奋-收缩偶联以及在维持未成熟、成年和衰老骨骼肌SR的结构和功能方面具有重要作用。

The sarcoplasmic reticulum (SR) is embedded in the myoplasm, where it surrounds the contractile structures of each myofibril. Without a mechanism to stabilize the membrane and to link it at regular intervals to the contractile apparatus, the SR might be damaged during the contractile cycle, causing the uncontrolled release of its stores of Ca2+ and profound affects on muscle structure and function. In healthy muscle, of course, this does not occur, due in part to the presence of regular links between the SR and the Z disks of myofibrils. Here we propose to characterize a protein that may form these connections. We have discovered small, alternatively spliced forms of the structural protein, ankyrin, in striated muscle cells. The small ankyrins (abbreviated sAnk1) are enriched in the SR specifically at sites flanking the M and Z disks of nearby myofibrils. Ankyrin in mammalian erythrocytes stabilizes the cell membrane by linking it to the spectrin-based cytoskeleton. We hypothesize that sAnk1 stabilizes the SR by attaching it to the cytoskeleton of striated muscle cells -- the contractile apparatus. We propose 4 specific aims to test this idea and its developmental and functional consequences. (i) sAnk1 has an unique, highly hydrophobic amino terminal sequence that is likely to anchor it to membranes and target it specifically to the SR. We will perform biochemical and cellular transfection experiments to determine how sAnk1 associates preferentially with the SR. (ii) sAnk1 may concentrate in the SR near M and Z disks as a result of specific binding to proteins present in those structures. We will use the yeast two-hybrid screen to identify these ligands, and immunolabeling at the confocal and ultrastructural levels to localize them in the sarcomere. (iii) If sAnk1 links the SR to the contractile apparatus, it and its ligands should play an important role in the organization and differentiation of the SR in developing muscle. We will use biochemical, immunological and structural approaches to determine how sAnk1 and its ligands are expressed and organized during development. (iv) If sAnk1 is important for sarcoplasmic organization, inhibiting its activity should disrupt the relationship of the SR with the contractile apparatus and alter Ca2+ homeostasis. We will use transgenic techniques to manipulate the levels of sAnk1 and its ligands in muscle cells, and physiological and ultrastructural techniques to assess the effects of these manipulations on the organization and function of the SR. Our studies should therefore help to elucidate the mechanisms responsible for the formation and organization of the SR in immature and adult striated muscle. These mechanisms may have an important role in excitation-contraction coupling, and in maintaining the structure and function of the SR in immature, adult, and aging skeletal muscle.