Class I cytokine receptors are involved in important biological functions of both physiological and pathological nature in mammals. However, the molecular details of the cross-membrane signal transduction through these receptors remain obscure. One of the major reasons for this is the lack of structural knowledge on their membrane-embedded transmembrane domains (TMDs), which connect the extracellular ligand binding domains to the intracellular signaling platforms. The overall aim of this thesis work was to improve our understanding of the class I cytokine receptor signaling across the membrane bilayer via structural characterizations of TMD representatives.
To enable structural studies of these domains, an organic-extraction based strategy for efficient production of isotope-labeled TMDs with or without short intrinsically disordered regions was developed. This strategy successfully provided ample material of high quality for structural studies with NMR spectroscopy of several class I cytokine receptor TMDs.
Furthermore, the structure of a class I cytokine receptor TMD in DHPC micelles was solved with solution-state NMR spectroscopy. Additionally, since structural studies of intact proteins with a globular domain in combination with a membrane embedded domain and an intrinsically disordered domain is exceptionally challenging, this structure, along with data collected on the adjacent domains in isolation, was utilized to present the first full-length integrative structure of a class I cytokine receptor. This integrative structure opens up for interpreting these receptors in their intact form and offers unique insights on the topology of single-pass transmembrane receptors with intrinsically disordered domains.
Dimerization of the TMDs of class I cytokine receptors has been shown to be important for driving the formation of inactive receptor dimers, and crossmembrane signal transduction is hypothesized to occur through switching between two or more TMD dimer conformations. The structural characteristics of TMD homo- and heterodimers were therefore additionally investigated with NMR spectroscopy. Finally, the structural properties of the TMD and the juxtamembrane region in conjunction were addressed.
In conclusion, the work presented in this thesis provides an important new view into the previous structural blind-spot of class I cytokine receptors, as well as build a foundation for further studies of these challenging domains. Supplemented by a review of the current collection of TMD structures from single-pass transmembrane receptors, the thesis as a whole provides important insights on the structure and function in the membrane as well as highlight the open questions to be addressed in the years to come.