Edelmann H M, Kühne C, Petritsch C, Ballou L M
Research Institute of Molecular Pathology, Vienna, Austria.
J Biol Chem. 1996 Jan 12;271(2):963-71. doi: 10.1074/jbc.271.2.963.
We show here using synchronized Swiss mouse 3T3 fibroblasts that p70 S6 kinase (p70S6k) and mitogen-activated protein kinases (p42mapk/p44mapk) are not only activated at the G0/G1 boundary, but also in cells progressing from M into G1. p70S6k activity increases 20-fold in G1 cells released from G0. Throughout G1, S, and G2 it decreases constantly, so that during M phase low kinase activity is measured. The kinase is reactivated 10-fold when cells released from a nocodazole-induced metaphase block enter G1 of the next cell cycle. p42mapk/p44mapk in G0 cells are activated transiently early in G1 and are reactivated late in mitosis after nocodazole release. p70S6k activity is dependent on permanent signaling from growth factors at all stages of the cell cycle. Immunofluorescence studies showed that p70S6k and its isoform p85S6k become concentrated in localized spots in the nucleus at certain stages in the cell cycle. Cell cycle-dependent changes in p70S6k activity are associated with alterations in the phosphorylation state of the protein. However, examination of the regulation of a p70S6k mutant in which the four carboxyl-terminal phosphorylation sites are changed to acidic amino acids suggests that a mechanism independent of these phosphorylation sites controls the activity of the enzyme during the cell cycle.
我们在此利用同步化的瑞士小鼠3T3成纤维细胞表明，p70 S6激酶（p70S6k）和丝裂原活化蛋白激酶（p42mapk/p44mapk）不仅在G0/G1边界被激活，而且在从M期进入G1期的细胞中也被激活。从G0释放的G1期细胞中，p70S6k活性增加20倍。在整个G1、S和G2期，其活性持续下降，因此在M期测得的激酶活性较低。当从诺考达唑诱导的中期阻滞中释放的细胞进入下一个细胞周期的G1期时，该激酶重新激活10倍。G0期细胞中的p42mapk/p44mapk在G1早期短暂激活，并在诺考达唑释放后的有丝分裂后期重新激活。在细胞周期的所有阶段，p70S6k活性都依赖于生长因子的持续信号传导。免疫荧光研究表明，p70S6k及其同工型p85S6k在细胞周期的某些阶段会在细胞核中的局部斑点中聚集。p70S6k活性的细胞周期依赖性变化与该蛋白磷酸化状态的改变有关。然而，对一种p70S6k突变体的调控研究表明，该突变体的四个羧基末端磷酸化位点被替换为酸性氨基酸，这表明一种独立于这些磷酸化位点的机制在细胞周期中控制着该酶的活性。
