Development of a constitutively active mutant form of the prolactin receptor, a member of the cytokine receptor family.

The extracellular domain of the PRL receptor (PRL-R) is composed of two subdomains of approximately 100 amino acids, S1 and S2. To explore the functional significance of these subdomains in PRL binding and signal transduction, deletion mutants of S1 or/and S2 subdomains were constructed. We report here the inability of each of these mutant receptor forms to bind PRL after expression in COS-7 cells. We also studied the abilities of these different mutant receptors to respond to hormonal stimulation after transfection of each mutant complementary DNA into CHO-K1 cells along with a chimeric gene containing the promoter of a milk protein gene (beta-lactoglobulin) fused to chloramphenicol acetyltransferase coding sequence. Somewhat unexpectedly, a constitutively (PRL-independent) mutant form of the PRL-R was obtained after deletion of the S2 subdomain. Moreover, we analyzed, in CHO-K1 cells, the biological activity of chimeric receptors constructs in which each subdomain sequence was replaced by an unrelated, but coding, sequence of foreign protein, and we confirmed a specific requirement for the S1 sequence in the constitutive activity. In contrast, the S2 subdomain produced an inhibitory effect on S1 constitutive activity. Cotransfection experiments with the wild-type receptor and the constitutive mutant receptor provided evidence that the wild-type receptor was able to inhibit the constitutive activity of the deleted mutant. Furthermore, in the mouse mammary epithelial cell line HC11, the constitutive PRL-R form was able to induce transcription of the beta-casein gene in the absence of PRL. These results suggest a complex signal transduction process that implicates each extracellular PRL-R subdomain. Possible mechanisms for the constitutive effect are discussed.