International Immunology, Vol. 14, No. 11, pp. 1263-1271,
November 2002
© 2002 Japanese Society for Immunology
Serine phosphorylation of Stat5 proteins in lymphocytes stimulated with IL-2
Laboratories of 1 Molecular Immunology and 2 Biophysical Chemistry, National Heart, Lung and Blood Institute, NIH, Bethesda, MD 20892-1674, USA 3 Laboratory of Stem Cell Biology/Neurotrophic Factors, US-FDA/CBER, 1401 Rockville Pike/Suite 200N, Rockville, MD 20825-1448, USA 4 Howard Hughes Medical Institute and Department of Medicine, University of California San Francisco, San Francisco, CA 94143-0724, USA 5 Present address: Department of Structure Biology, SmithKline Beecham Pharmaceuticals, King of Prussia, PA 19406, USA 6 Present address: Departments of Biochemistry and Cell Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
Correspondence to: W. J. Leonard; E-mail: wjl{at}helix.nih.gov
Transmission editor: T. Hirano
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 significance 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-deficient 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.
Keywords: cytokine, mass spectrometry, retroviral transduction, signal transduction, T lymphocyte