Selective coupling of STAT factors to the mouse prolactin receptor

Prolactin has been reported to induce distinct sets of signal transducers and activators of transcription (STAT) in a cell‐type‐specific fashion. In the mammary epithelium, although STAT1, STAT3, STAT5A, STAT5B and STAT6 are present in a latent form, only STAT5A and STAT5B are activated. This selective activation of STAT5 by prolactin was also observed in COS‐7 cells cotransfected with the long form of the mouse prolactin receptor (PRL‐R) and expression vectors for STAT1, STAT3, STAT5 and STAT6. Mutated PRL‐Rs and chimeric erythropoietin/gp130 (EPO/gp130) receptors with a tyrosine‐containing motif attached at the carboxy terminus were employed to determine the sites in the PRL‐R required for the specific activation of STAT5. The experiments revealed the importance of two motifs containing Y477 and Y578 in the PRL‐R. When linked to the EPO/gp130 receptor, these sequences were sufficient to specifically induce DNA binding of STAT5 and to activate transcription from the β‐casein gene promoter. By contrast, only weakly they induced DNA binding of STAT6 and STAT3 and did not induce STAT1. A synthetic nonapeptide with phosphorylated Y477 was able to disrupt STAT5 DNA binding in vitro. Our results define structural domains within the carboxy terminus of the PRL‐R which recruit STAT5 for signalling and which are capable of distinguishing STAT5 from other STAT proteins. The activity of STAT5 forms with deletions of the carboxy terminus was induced more strongly than that of their full‐length counterparts 2 min after activation of the PRL‐R. This effect was not dependent on the presence of Y477 and Y578 in the PRL‐R, indicating that facilitated activation of short STAT5 isoforms relies on mechanisms other than increased coupling to specific regions of the PRL‐R.