PRLR has no intrinsic kinase activity but associates with Janus kinase 2 (JAK2) (Lebrun et al. 1994, 1995, Campbell et al. 1994, Rui et al. 1994). PRLR to JAK2 binding has been described as constitutive but a recent computational model suggests that roughly half of dimerized Growth Hormone receptors are bound with JAK2 (Barua et al. 2009), a model that may apply to other receptors that promote JAK2 trans-activation. The box 1 region of PRLR is a membrane proximal proline-rich region in the intracellular domain, conserved in all members of the growth hormone rereptor family. This region is critical for JAK2 association; deletion of box 1 virtually abolishes PRLR signaling (Edery et al. 1994). Alanine substitutions of individual residues within box 1 of rat PRLR have shown that the most C-terminal proline (P269 in the Uniprot canonical sequence, 250 in the mature peptide) is critical for association with and subsequent activation of JAK2 (Pezet et al. 1997). It is not known whether the interaction of JAK2 with PRLR is direct or involves an adaptor protein.
When the receptor is activated by ligand binding JAK2 (receptor pre-bound or recruited after ligand binding) becomes activated and phosphorylates the dimerized receptor preferentially at Y611 (position 587 in the mature peptide), a consensus tyrosine phosphorylation site. This is followed by the phosphorylation, dimerization and nuclear translocation of STAT5. There are nine other tyrosines in the cytoplasmic domain, some of which may undergo phosphorylation and may participate in signal transduction.