Peterson R T, Desai B N, Hardwick J S, Schreiber S L
Howard Hughes Medical Institute, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA.
Proc Natl Acad Sci U S A. 1999 Apr 13;96(8):4438-42. doi: 10.1073/pnas.96.8.4438.
The FKBP12-rapamycin-associated protein (FRAP; also called RAFT1/mTOR) regulates translation initiation and entry into the cell cycle. Depriving cells of amino acids or treating them with the small molecule rapamycin inhibits FRAP and results in rapid dephosphorylation and inactivation of the translational regulators 4E-BP1(eukaryotic initiation factor 4E-binding protein 1) and p70(s6k) (the 70-kDa S6 kinase). Data published recently have led to the view that FRAP acts as a traditional mitogen-activated kinase, directly phosphorylating 4E-BP1 and p70(s6k) in response to mitogenic stimuli. We present evidence that FRAP controls 4E-BP1 and p70(s6k) phosphorylation indirectly by restraining a phosphatase. A calyculin A-sensitive phosphatase is required for the rapamycin- or amino acid deprivation-induced dephosphorylation of p70(s6k), and treatment of Jurkat I cells with rapamycin increases the activity of the protein phosphatase 2A (PP2A) toward 4E-BP1. PP2A is shown to associate with p70(s6k) but not with a mutated p70(s6k) that is resistant to rapamycin- and amino acid deprivation-mediated dephosphorylation. FRAP also is shown to phosphorylate PP2A in vitro, consistent with a model in which phosphorylation of PP2A by FRAP prevents the dephosphorylation of 4E-BP1 and p70(s6k), whereas amino acid deprivation or rapamycin treatment inhibits FRAP's ability to restrain the phosphatase.
FKBP12-雷帕霉素相关蛋白（FRAP；也称为RAFT1/mTOR）调节翻译起始和进入细胞周期。剥夺细胞中的氨基酸或用小分子雷帕霉素处理细胞会抑制FRAP，并导致翻译调节因子4E-BP1（真核起始因子4E结合蛋白1）和p70(s6k)（70 kDa的S6激酶）迅速去磷酸化并失活。最近发表的数据导致了这样一种观点，即FRAP作为一种传统的丝裂原活化激酶，在有丝分裂刺激下直接磷酸化4E-BP1和p70(s6k)。我们提供的证据表明，FRAP通过抑制一种磷酸酶间接控制4E-BP1和p70(s6k)的磷酸化。雷帕霉素或氨基酸剥夺诱导的p70(s6k)去磷酸化需要一种对花萼海绵诱癌素A敏感的磷酸酶，用雷帕霉素处理Jurkat I细胞会增加蛋白磷酸酶2A（PP2A）对4E-BP1的活性。已证明PP2A与p70(s6k)相关，但与对雷帕霉素和氨基酸剥夺介导的去磷酸化有抗性的突变型p70(s6k)不相关。还证明FRAP在体外使PP2A磷酸化，这与一种模型一致，即FRAP对PP2A的磷酸化可防止4E-BP1和p70(s6k)的去磷酸化，而氨基酸剥夺或雷帕霉素处理会抑制FRAP抑制磷酸酶的能力。
