The members of the class I cytokine receptor family are involved in a wide range of cellular processes and of high pharmaceutical importance, however, even though the transmembrane receptors have been studied for decades, it has not been fully elucidated yet, how these receptors induce their intracellular response. The overall goal of this thesis was to improve the understanding of class I cytokine receptor activation and regulation at an atomic level. Two members of the class I cytokine receptor family, the human growth hormone receptor (hGHR), and the human erythropoietin receptor (hEPOR) have been investigated. The two receptors form homodimers and bind their cognate hormone in a 2:1 complex. It has been proposed that prior to hormone binding dimerization occurs mainly between the transmembrane domains (TMD) of the two receptor chains.
A new purification method, utilizing the unique characteristics of membrane spanning helices, was designed and hGHR TMD and hEPOR TMD produced in sufficient amounts for spectroscopic investigations. The isolated hGHR TMD was revealed to associate in dimeric complexes in detergent micelles and first presumptions about the dimer interface could be made.
Further, the minimal determinants for specificity between membrane spanning helices were investigated with small artificial low complexity peptides, prior found to activate the EPOR in cells. The placement of single methyl group in the so called transmembrane aptamers (traptamers) determined the stabilizing effect the traptamers on the hEPOR TMD dimeric complex in detergent micelles. To gain a better understanding of hGHR regulation a point mutation in the hGHR intracellular domain (ICD), which has recently been linked to lung cancer, was characterized. The mutation was found to decrease binding of suppressor of cytokine signaling 2 (SOCS2) to the GHR by changing the structural characteristics of the SOC2 binding site.
It can be concluded that the work of this thesis paved the way for a new way of thinking about interactions between membrane spanning helices and underlined the importance of negative regulation of GHR signaling.