Functional differences between Stat3α and Stat3β

Stat3β is a short form of Stat3 that differs from the longer form (Stat3α) by the replacement of the C-terminal 55 amino acid residues of Stat3α by 7 residues specific to Stat3β. In COS cells transfected with Stat3 expression plasmids. Both Stat3α and Stat3β were activated for DNA binding and transcription by the same set of growth factors and cytokines and both, when activated, formed homodimers and heterodimers with Stat1. Only Stat3β was active in the absence of added cytokine or growth factor. Activation of each form, including constitutive activation of Stat3β, was correlated with the phosphorylation of tyrosine 705. Activated Stat3β in transfected COS cells was more stable and had greater DNA-binding activity than activated Stat3α. However, relative to DNA-binding activity, Stat3α showed greater transcriptional activity than Stat3β. A mutant of Statα lacking its highly acidic C-terminal 48 amino acids had properties indistinguishable from Stat3β. We conclude that Stat3α and Stat3β have significantly different properties due to the presence or absence of the acidic C-terminal tail of Stat3α rather than the C-terminal sequence peculiar to Stat3β. In addition to its effect on transcription, we speculate that the acidic tail may destabilize the active dimeric form of Stat3α, resulting in lower DNA-binding activity of the Y705-phosphorylated form compared to Stat3β and in more rapid dephosphorylation.