CLONING OF HUMAN STAT5B - RECONSTITUTION OF INTERLEUKIN-2-INDUCED STAT5A AND STAT5B DNA-BINDING ACTIVITY IN COS-7 CELLS

We have isolated a second human Stat5 cDNA, Stat5B, and demonstrated t hat the genes encoding both Stat5A and Stat5B are located at chromosom e 17q11.2, Both genes were constitutively transcribed in peripheral bl ood lymphocytes. By using specific antisera, we demonstrated that both Stat5A and Stat5B are activated by interleukin-2 (IL-2) in peripheral blood lymphocytes, natural killer-like YT leukemia cells, and human T cell lymphotropic virus type I-transformed MT-2 T cells. In COS-7 cel ls, which constitutively express the Janus family tyrosine kinase Jak1 , reconstitution of IL-2-induced Stat5A and Stat5B DNA binding activit ies was dependent on the coexpression of Jak3 along with the IL-2 rece ptor beta chain and the common cytokine receptor gamma-chain. This IL- 2-induced Stat5 activation was dependent on the presence of either of two tyrosines (Tyr-392 or Tyr-510) in the IL-2 receptor beta chain, in dicating that either of these two tyrosines can serve as a docking sit e, Moreover, we demonstrated that human Stat5 activation is also depen dent on Tyr-694 in Stat5A and Tyr-699 in Stat5B, indicating that these tyrosines are required for dimerization. The COS-7 reconstitution sys tem described herein provides a valuable assay for further elucidation of the IL-2-activated JAK-STAT pathway.