Recent studies identified cyclase-associated proteins (CAPs) as important regulators of actin dynamics that control assembly and disassembly of actin filaments (F-actin). While these studies significantly advanced our knowledge of their molecular functions, the physiological relevance of CAPs largely remained elusive. Gene targeting in mice implicated CAP2 in heart physiology and skeletal muscle development. Heart defects in CAP2 mutant mice were associated with altered activity of serum response factor (SRF), a transcription factor involved in multiple biological processes including heart function, but also skeletal muscle development. By exploiting mouse embryonic fibroblasts (MEFs) from CAP2 mutant mice, we aimed at deciphering the CAP2-dependent mechanism relevant for SRF activity. Reporter assays and mRNA quantification by qPCR revealed reduced SRF-dependent gene expression in mutant MEFs. Reduced SRF activity in CAP2 mutant MEFs was associated with altered actin turnover, a shift in the actin equilibrium towards monomeric actin (G-actin) as well as and reduced nuclear levels of myocardin-related transcription factor A (MRTF-A), a transcriptional SRF coactivator that is shuttled out of the nucleus and, hence, inhibited upon G-actin binding. Moreover, pharmacological actin manipulation with jasplakinolide restored MRTF-A distribution in mutant MEFs. Our data are in line with a model in which CAP2 controls the MRTF-SRF pathway in an actin-dependent manner. While MRTF-A localization and SRF activity was impaired under basal conditions, serum stimulation induced nuclear MRTF-A translocation and SRF activity in mutant MEFs similar to controls. In summary, our data revealed that in MEFs CAP2 controls basal MRTF-A localization and SRF activity, while it was dispensable for serum-induced nuclear MRTF-A translocation and SRF stimulation.

最近的研究发现环化酶相关蛋白（CAP）是肌动蛋白动力学的重要调节因子，可控制肌动蛋白丝（F-肌动蛋白）的组装和分解。虽然这些研究极大地提高了我们对其分子功能的认识，但 CAP 的生理相关性在很大程度上仍然难以捉摸。小鼠基因靶向表明 CAP2 与心脏生理学和骨骼肌发育有关。CAP2突变小鼠的心脏缺陷与血清反应因子（SRF）活性的改变有关，SRF是一种参与多种生物过程的转录因子，包括心脏功能和骨骼肌发育。通过利用 CAP2 突变小鼠的小鼠胚胎成纤维细胞 (MEF)，我们的目的是破译与 SRF 活性相关的 CAP2 依赖性机制。报告基因检测和 qPCR mRNA 定量显示突变 MEF 中 SRF 依赖性基因表达减少。CAP2突变体MEF中SRF活性的降低与肌动蛋白更新的改变、肌动蛋白平衡向单体肌动蛋白（G-肌动蛋白）的转变以及心肌素相关转录因子A（MRTF-A）的核水平降低有关，MRTF-A是一种转录因子。 SRF 共激活剂从细胞核中穿出，因此在 G 肌动蛋白结合时受到抑制。此外，用 Jasplakinolide 进行药理肌动蛋白操作可恢复突变 MEF 中的 MRTF-A 分布。我们的数据符合 CAP2 以肌动蛋白依赖性方式控制 MRTF-SRF 通路的模型。虽然 MRTF-A 定位和 SRF 活性在基础条件下受损，但血清刺激诱导突变 MEF 中的核 MRTF-A 易位和 SRF 活性与对照相似。总之，我们的数据显示，在 MEF 中，CAP2 控制基础 MRTF-A 定位和 SRF 活性，而对于血清诱导的核 MRTF-A 易位和 SRF 刺激来说，CAP2 是可有可无的。