Structural motifs in the extracellular domain of the prolactin receptor govern fold and functionality

The prolactin receptor (PRLR) is an archetype cytokine receptor. It is a single-pass transmembrane receptor with limited complexity that is devoid of enzyme activity. Intracellular signaling involves various receptor-associated kinases including Jak2, Erk1/2, Src and Akt. As the PRLR is emerging as a relevant target in Oncology the understanding of the molecular basis of its activation is crucial. In the frame of an inter-disciplinary consortium involving biophysicists, structural biologists and cell biologists, we have successfully combined complementary approaches such as optical and nuclear magnetic resonance spectroscopic analyses, X-ray crystallography, surface plasmon resonance and cell-based assays to start elucidate the structural features of ligand-receptor interaction. However, the features of the PRLR extracellular domain (ECD) that participate in the transmission of the hormonal message across the cell membrane and/or in selective activation of intracellular signaling cascades remained uncharacterized. In two recently published studies, we identified residues 146 and 170 as two key residues of the PRLR-ECD that control critical receptor properties including basal signaling activity, ligand sensitivity, species specificity, folding, stability and receptor turnover. These two residues are in close proximity of each other in the membrane proximal domain of the PRLR-ECD and participate in a network of interactions with other residues, in particular within a specific residue quartet. Strikingly, these residues are involved in, or close to, the receptor dimerization interface, suggesting that their mechanism of action may involve structural reorientation of the receptor chains that are necessary to (selectively) disseminate the signal from the ECD to the intracellular domain. The identification of such residues in this and other cytokine receptors should affect future structure-directed drug development strategies aimed at providing pathway-selective treatment strategies.