ABSTRACT
This chapter argues that the phosphatase responsible for dephosphorylating the kinase is a type 2A enzyme. As stated in the preceding chapter, the S6 kinase is directly regulated by a phosphorylation-dephosphorylation mechanism in response to mitogens or oncogenes. The activity of this enzyme, therefore, could be differentially modulated through different kinases and phosphatases whose activity in turn may be controlled by other molecules. The biphasic nature of S6 kinase activation may only be a hint of the complexity of the situation. The latter group could include such enzymes as c-raf, which appears to be activated by increased tyrosine phosphorylation, and the microtubule-associated protein 2 kinase, which is phosphorylated on tyrosyl and threonyl residues and which can partially reactivate an S6 kinase II from Xenopus eggs.
