Extracellular signal-regulated kinase (ERK) interacts with signal transducer and activator of transcription (STAT) 5a

Serine phosphorylation of signal transducers and activators of transcriptio n (STAT) 1 and 3 modulates their DNA-binding capacity and/or transcriptiona l activity. Earlier we suggested that STAT5a functional capacity could be i nfluenced by the mitogen-activated protein kinase (MAPK) pathway. In the pr esent study, we have analyzed the interactions between STAT5a and the MAPKs , extracellular signal-regulated kinases ERK1 and ERK2. GH treatment of Chi nese hamster ovary cells stably transfected with the GH receptor (CHOA cell s) led to rapid and transient activation of both STAT5a and ERK1 and ERK2. Pretreatment of cells with colchicine, which inhibits tubulin polymerizatio n, did not inhibit STAT5a translocation to the nucleus and ERK1/2 activatio n. In vitro precipitation with a glutathione-S-transferase-fusion protein c ontaining the C-terminal transactivation domain of STAT5a showed ON-regulat ed association of ERK1/2 with the fusion protein, while this was not seen w hen serine 780 in STAT5a was changed to alanine. In vitro phosphorylation o f the glutathione-S-transferase-fusion proteins using active ERK only worke d when the fusion protein contained wild-type STAT5a sequence. The same exp eriment, performed with full-length wild-type STAT5a and the corresponding S780A mutant, showed that serine 780 is the only substrate in full-length S TAT5a for active ERK. In coimmunoprecipitation experiments, larger amounts of STAT5a-ERK1/2 complexes were detected in cytosol from untreated CHOA cel ls than in cytosol from GH-treated cells, suggesting the presence of prefor med STAT5a-ERK1/2 complexes in unstimulated cells. Transfection experiments with COS cells showed that kinase-inactive ERK1 decreased GH stimulation o f STATE-regulated reporter gene expression. These observations show, for th e first time, direct physical interaction between ERK1/2 and STAT5a and als o clearly identify serine 780 as a target for ERK. Furthermore, it is also established that serine phosphorylation of STAT5a transactivation domain, v ia the MAPK pathway, is a means of modifying OR-induced transcriptional act ivation.