The 70 kDa ribosomal protein S6 kinase (p70 S6K) is a serine/threonine kinase. There are two highly homologous members in S6K family, which are named as S6K1 and S6K2, respectively. Both S6Ks belong to the AGC family of serine/threonine kinases and they could be activated in response to many stimulus, like growth factors, insulin and cytokines. In turn, S6Ks phosphorylate its substrates including the ribosomal protein S6, elF4B, and SKAR. It was found that the activity of S6Ks could be controlled by mammalian target of rapamycin (mTOR) [2]. And then, direct phosphorylation of S6K1 by mTOR was confirmed [3; 4]. Therefore, the activity of S6K presented sensitive to rapamycin [8]. However, the sensitivity of S6Ks to rapamycin was found always changing. S6Ks became insensitive to rapamycin when the site T412 was phosphorylated [9]. Until recently, raptor has been proved to be responsible for T412 phosphorylation [6]. Besides, S6K−/− animals showed short-lived and were much smaller than wild type animals. A study suggested that this phenomenon was due to the control of S6K on cell growth rather than proliferation [5]. To date, S6Ks are also related to many serious diseases. For example, S6K1 showed overexpression in breast cancer patients [1]. With the understanding of the importance of S6K in human diseases, the demand for S6K-specific inhibitors increased rapidly. Fortunately, a novel inhibitor specific for S6K1, PF-4708671, was developed to facilitate the study of the functions of S6K1, at the same time, and S6K2 [7].
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