Cells need to reliably control their proteome composition to maintain homeostasis and regulate growth. How protein synthesis and degradation interplay to control protein expression levels remains unclear. Here, we combined a tandem fluorescent timer and pulse-chase protein labeling to disentangle how protein synthesis and degradation control protein homeostasis in single live mouse embryonic stem cells. We discovered substantial cell-cycle dependence in protein synthesis rates and stabilization of a large number of proteins around cytokinesis. Protein degradation rates were highly variable between cells, co-varied within individual cells for different proteins, and were positively correlated with synthesis rates. This suggests variability in proteasome activity as an important source of global extrinsic noise in gene expression. Our approach paves the way toward understanding the complex interplay of synthesis and degradation processes in determining protein levels of individual mammalian cells.

细胞需要可靠地控制其蛋白质组组成，以维持体内稳态并调节生长。蛋白质合成和降解如何相互作用以控制蛋白质表达水平仍不清楚。在这里，我们结合了串联荧光计时器和脉冲追踪蛋白质标记，以弄清蛋白质合成和降解如何控制单个活小鼠胚胎干细胞中的蛋白质稳态。我们发现蛋白质合成速率和细胞质分裂周围大量蛋白质的稳定化对细胞周期的依赖性。蛋白降解率在细胞之间是高度可变的，对于不同的蛋白在单个细胞内是共变的，并且与合成率呈正相关。这表明蛋白酶体活性的可变性是基因表达中全局外在噪声的重要来源。