Department of Medicine II, University of Ulm, 89081 Ulm, Germany.
J Cell Sci. 2011 Sep 15;124(Pt 18):3127-36. doi: 10.1242/jcs.084772. Epub 2011 Aug 18.
Assembly, maintenance and renewal of sarcomeres require highly organized and balanced folding, transport, modification and degradation of sarcomeric proteins. However, the molecules that mediate these processes are largely unknown. Here, we isolated the zebrafish mutant flatline (fla), which shows disturbed sarcomere assembly exclusively in heart and fast-twitch skeletal muscle. By positional cloning we identified a nonsense mutation within the SET- and MYND-domain-containing protein 1 gene (smyd1) to be responsible for the fla phenotype. We found SMYD1 expression to be restricted to the heart and fast-twitch skeletal muscle cells. Within these cell types, SMYD1 localizes to both the sarcomeric M-line, where it physically associates with myosin, and the nucleus, where it supposedly represses transcription through its SET and MYND domains. However, although we found transcript levels of thick filament chaperones, such as Hsp90a1 and UNC-45b, to be severely upregulated in fla, its histone methyltransferase activity - mainly responsible for the nuclear function of SMYD1 - is dispensable for sarcomerogenesis. Accordingly, sarcomere assembly in fla mutant embryos can be reconstituted by ectopically expressing histone methyltransferase-deficient SMYD1. By contrast, ectopic expression of myosin-binding-deficient SMYD1 does not rescue fla mutants, implicating an essential role for the SMYD1-myosin interaction in cardiac and fast-twitch skeletal muscle thick filament assembly.
肌节的组装、维持和更新需要肌节蛋白高度组织化和平衡的折叠、运输、修饰和降解。然而，介导这些过程的分子在很大程度上是未知的。在这里，我们分离了斑马鱼突变体 flatline（fla），它仅在心脏和快肌中表现出肌节组装紊乱。通过定位克隆，我们确定了 SET 和 MYND 结构域蛋白 1 基因（smyd1）内的无义突变是导致 fla 表型的原因。我们发现 SMYD1 的表达仅限于心脏和快肌细胞。在这些细胞类型中，SMYD1 定位于肌节的 M 线，在那里它与肌球蛋白物理结合，并且在核内，它通过 SET 和 MYND 结构域据称抑制转录。然而，尽管我们发现厚丝伴侣蛋白（如 Hsp90a1 和 UNC-45b）的转录水平在 fla 中严重上调，但它的组蛋白甲基转移酶活性——主要负责 SMYD1 的核功能——对于肌节发生是可有可无的。因此，通过异位表达缺乏组蛋白甲基转移酶活性的 SMYD1 可以重建 fla 突变体胚胎中的肌节组装。相比之下，异位表达肌球蛋白结合缺陷的 SMYD1 不能拯救 fla 突变体，暗示了 SMYD1-肌球蛋白相互作用在心脏和快肌厚丝组装中的重要作用。
