Characterization of the Stat5 protease

Immature myeloid cells have been shown to transduce signals through a carbo xyl-terminally truncated isoform of Stat5. This functionally distinct signa l transducer and activator of transcription isoform is generated through a unique protein-processing event. Evaluation of numerous cell lines has dete rmined that there is a direct correlation between the expression of truncat ed Stat5 and protease activity. Moreover, protease activity is found only i n the myeloid and not in lymphoid progenitors. To further characterize the protease small quantities have been purified to near homogeneity. Studies o n this purified material indicate that the protease has an apparent molecul ar mass of similar to 25 kDa and is active over a wide range of pH values. The protease will also cleave both activated (i.e. tyrosine-phosphorylated) and inactivate Stat5. Although this activity is sensitive to phenylmethyls ulfonyl fluoride, it is notably not sensitive to several other serine prote ase inhibitors. Additional studies have led to the identification of the un ique site where the protease cleaves Stat5. Mutagenesis of this site render s Stat5 resistant to cleavage. Consistent with the model that Stat5 cleavag e is important for early myeloid development, introduction of a "non-cleava ble" isoform of Stat5 into FDC-P1 cells (a myeloid progenitor line) leads t o significant phenotypic changes.