Structural Biology of Shared Cytokine Receptors

Recent structural information for complexes of cytokine receptor ectodomains bound to their ligands has significantly expanded our understanding of the macromolecular topology and ligand recognition mechanisms used by our three principal shared cytokine signaling receptors—gp130, γc, and βc. The gp130 family receptors intricately coordinate three structurally unique cytokine-binding sites on their four-helix bundle cytokine ligands to assemble multimeric signaling complexes. These organizing principles serve as topological blueprints for the entire gp130 family of cytokines. Novel structures of γc and βc complexes show us new twists, such as the use of a nonstandard sushi-type α receptors for IL-2 and IL-15 in assembling quaternary γc signaling complexes and an antiparallel interlocked dimer in the GM-CSF signaling complex with βc. Unlike gp130, which appears to recognize vastly different cytokine surfaces in chemically unique fashions for each ligand, the γc-dependent cytokines appear to seek out some semblance of a knobs-in-holes shape recognition code in order to engage γc in related fashions. We discuss the structural similarities and differences between these three shared cytokine receptors, as well as the implications for transmembrane signaling.