Identification and Characterization of Constitutively Active Stat5
We have recently identified several constitutively active STAT5s using polymerase chain reaction (PCR)-driven random mutagenesis followed by retrovirus- mediated expression screening. One such mutant STAT5A1*6 harbors two point mutations; one in the effector domain and the other in the DNA binding domain, and induces factor-independent growth of several IL-3-dependent cell lines. The mutant STAT5A1*6 shows constitutive phosphorylation on its tyrosine residues, translocates into the nucleus, binds the target DNA sequence, and stimulates transcription in the absence of IL-3 stimulation. Further biochemical analyses suggested that a molecular basis for the constitutive activity of the mutant STAT5A is the increased stability of the phosphorylated form of STAT5. In addition to the ability of inducing factor- independent growth of IL-3-dependent cell lines, the mutant STAT5A induces apop- tosis or differentiation under some conditions or in a different cell line, implicating STAT5 as a molecular switch for pleiotropic functions of cytokine receptors. The relationship between active STAT5 and leukemogenesis will also be discussed.
KeywordsRandom Mutagenesis Hematopoietic Factor Active STAT5s Nonreceptor Protein Tyrosine Kinase Mutant STAT5B
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