Wette Stefan G, Smith Heather K, Lamb Graham D, Murphy Robyn M
Department of Biochemistry and Genetics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria, Australia.
Department of Exercise Sciences, The University of Auckland, Auckland, New Zealand.
Am J Physiol Cell Physiol. 2017 Sep 1;313(3):C327-C339. doi: 10.1152/ajpcell.00077.2017. Epub 2017 Jun 14.
Muscle ankyrin repeat proteins (MARPs) are a family of titin-associated, stress-response molecules and putative transducers of stretch-induced signaling in skeletal muscle. In cardiac muscle, cardiac ankyrin repeat protein (CARP) and diabetes-related ankyrin repeat protein (DARP) reportedly redistribute from binding sites on titin to the nucleus following a prolonged stretch. However, it is unclear whether ankyrin repeat domain protein 2 (Ankrd 2) shows comparable stretch-induced redistribution to the nucleus. We measured the following in rested human skeletal muscle: ) the absolute amount of MARPs and ) the distribution of Ankrd 2 and DARP in both single fibers and whole muscle preparations. In absolute amounts, Ankrd 2 is the most abundant MARP in human skeletal muscle, there being 3.1 µmol/kg, much greater than DARP and CARP (0.11 and ~0.02 µmol/kg, respectively). All DARP was found to be tightly bound at cytoskeletal (or possibly nuclear) sites. In contrast, ~70% of the total Ankrd 2 is freely diffusible in the cytosol [including virtually all of the phosphorylated (p)Ankrd 2-Ser99 form], ~15% is bound to non-nuclear membranes, and ~15% is bound at cytoskeletal sites, likely at the N2A region of titin. These data are not consistent with the proposal that Ankrd 2, per se, or pAnkrd 2-Ser99 mediates stretch-induced signaling in skeletal muscle, dissociating from titin and translocating to the nucleus, because the majority of these forms of Ankrd 2 are already free in the cytosol. It will be necessary to show that the titin-associated Ankrd 2 is modified by stretch in some as-yet-unidentified way, distinct from the diffusible pool, if it is to act as a stretch-sensitive signaling molecule.
肌肉锚蛋白重复序列蛋白（MARPs）是一类与肌联蛋白相关的应激反应分子，被认为是骨骼肌中拉伸诱导信号的转导分子。在心肌中，据报道，长时间拉伸后，心肌锚蛋白重复序列蛋白（CARP）和糖尿病相关锚蛋白重复序列蛋白（DARP）会从肌联蛋白上的结合位点重新分布到细胞核。然而，尚不清楚锚蛋白重复结构域蛋白2（Ankrd 2）是否显示出类似的拉伸诱导的向细胞核的重新分布。我们在静息的人体骨骼肌中测量了以下指标：（1）MARPs的绝对含量；（2）Ankrd 2和DARP在单根肌纤维和整块肌肉标本中的分布。就绝对含量而言，Ankrd 2是人体骨骼肌中含量最丰富的MARP，约为3.1 μmol/kg，远高于DARP和CARP（分别约为0.11 μmol/kg和0.02 μmol/kg）。发现所有的DARP都紧密结合在细胞骨架（或可能是细胞核）位点。相比之下，Ankrd 2总量的约70%在细胞质中可自由扩散[包括几乎所有磷酸化的（p）Ankrd 2-Ser99形式]，约15%与非核膜结合，约15%结合在细胞骨架位点，可能在肌联蛋白的N2A区域。这些数据与Ankrd 2本身或pAnkrd 2-Ser99介导骨骼肌拉伸诱导信号、从肌联蛋白解离并转运到细胞核的观点不一致，因为这些形式的Ankrd 2大部分已经在细胞质中是游离的。如果Ankrd 2要作为一种拉伸敏感的信号分子发挥作用，就必须证明与肌联蛋白相关的Ankrd 2以某种尚未确定的方式被拉伸修饰，与可扩散池不同。
