Cytokine receptors: structure and signal transduction.

In the past 2-3 years, a number of cytokine receptors have been partly characterized and the cDNAs for the ligand binding chains cloned. This has revealed that cytokine receptors are complex. Many are known to be multichain receptors (e.g. IL-2) and since their mechanism of signal transduction is not obvious, it is likely that other proteins yet to be defined take part in the signalling process. The cloning of the receptor ligand binding chain has revealed that (unlike cytokines), there are major families of receptors. Some are members of the Ig supergene family (e.g. IL-1 receptor), others are members of the nerve growth factor receptor family (e.g. TNF), but the majority are members of the haematopoietic growth factor family (e.g. IL-3, GM-CSF). Yet other cytokine receptors do not belong to a family, e.g. IFN-gamma.