Tyrosine phosphorylation regulates cytokine-induced dimerization of STAT proteins. Serine phosphorylation has also been found to occur in a number of STAT proteins, including Stat1, Sat3, Stat4, Stat5a, Stat5b and Stat6, and was shown to be important for maximal transcriptional activation mediated by Stat1, Stat3 and Stat4, but not for Stat5a or Stat5b. As these latter proteins were studied in transiently transfected COS-7 cells stimulated with prolactin, we sought to further investigate the

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... ni®cance of their serine phosphorylation in a more physiologically based system in response to IL-2. Both Stat5a and Stat5b were rapidly phosphorylated on serine in response to IL-2 and the phosphorylation site in Stat5a was mapped to Ser780, which is not conserved in Stat5b. In vitro studies with reporter constructs, and experiments in which wild-type and mutant Stat5a retroviruses were used to transduce Stat5a-de®cient splenocytes revealed that the serine mutant constructs were not diminished in their ability to mediate IL-2 signaling and if anything exhibited augmented proliferative capability. Thus, in contrast to the apparent importance of serine phosphorylation for transcriptional activation by Stat1, Stat3 and Stat4 in response to IFN, IL-6 and IL-12 respectively, serine phosphorylation of Stat5a does not enhance Stat5a-mediated signaling in response to IL-2.