Cytokines are immunomodulating cell signalling molecules that are very important in the inflammatory processes of the body. They send signals to other inflammatory cells which gives rise to a cascade of reactions including the recruitment and movement of inflammatory cells to the site of action. The signals are initiated when cytokines bind to specific receptors.
Cytokine receptor structure
The cytokine receptors are divided into six main families based on their 3-D structure and are further categorized into many subgroups. The receptors are essentially high-affinity molecular complexes that initiate and propagate signals, beginning the cytokine-mediated communication between cells.
These cell receptor complexes are heterodimeric or heterotrimeric structures that have specific cytokine binding and recognition sites, depending on the category of superfamily they belong to. For example, the cyokine receptor gamma (γ) subunit chain is a common glycoprotein found in many interleukin (IL) receptors including the IL-2, IL-4, IL-7, Il-9, IL-15 and IL-1 receptors.
Similarly, the transmembrane glycoprotein 130 (gp130) is a sub unit found specifically in the IL-6 family of receptors. Many members of the tumor necrosis factor receptor family (TNF) contain a cytoplasmic death domain that is needed for TNF functions such as apoptosis.
There are several mechanisms by which cytokine receptors can act. Membrane receptors have intact cell-signalling domains and can transmit signals to the nucleus of the target cells after the cytokine binds to the receptor.
Alternatively, receptors may act by antagonizing cytokine functions and regulating responses. In some cases, the receptors themselves may act as signalling molecules.