REAL-TIME KINETIC MEASUREMENTS OF THE INTERACTIONS BETWEEN LACTOGENICHORMONES AND PROLACTIN-RECEPTOR EXTRACELLULAR DOMAINS FROM SEVERAL SPECIES SUPPORT THE MODEL OF HORMONE-INDUCED TRANSIENT RECEPTOR DIMERIZATION

Interactions of recombinant soluble prolactin receptors-extracellular domains (PRLR-ECDs) from rabbit, rat, and cow and human growth hormone receptor ECD with immobilized human growth hormone, several prolactin s, and bovine placental lactogen were studied utilizing surface plasmo n resonance. This method enables real-time kinetic measurements of the interactions and calculations of kinetic constants and of the stoichi ometry of interaction, even in cases where only transient interactions occur. In contrast to gel filtration or crystallographic studies, whe re in most cases the interaction of PRLR-ECDs with various lactogenic hormones indicated formation of 1:1 complexes, our surface plasmon res onance experiments indicated in all cases the transient formation of a 2:1 complex. In most of the interactions the 2:1 complex was very uns table and underwent rapid dissociation to a 1:1 complex. This situatio n was particularly characteristic of homologous interactions involving hormone and receptor from the same species and was mainly attributed to increased dissociation constants, We suggest that as in the ease of growth hormone PRLR activation occurs via hormone-induced transient h omodimerization of the receptor, lasting only a few seconds, and that this may be sufficient to initiate the biological signal. Once the sig nal is initiated, the receptor dimer is no longer required. Its rapid dissociation to a 1:1 complex or to its components may even be advanta geous in that it permits activation of additional receptors.