Cytokines of the beta-common (βc) family, namely granulocyte-macrophage colony stimulating factor (GM-CSF), interleukin (IL)-3 and IL-5, are regulators of normal and malignant haematopoiesis and have recently been shown to act outside the haematopoietic system. These βc cytokines activate several signalling pathways, which ultimately help in cell proliferation and differentiation by binding to a signalling unit consisting of a cytokine specific α-subunit and a β-subunit that is shared by all three cytokines, hence the name beta-common . The βc subunit exists as an intertwined homodimer on the cell surface and the prevailing paradigm is that the cytokine and α-subunit first form a binary complex, which then associates with the βc dimer to form a hexameric ternary complex. Certain signalling pathways may require the self-assembly of two hexamers to form a dodecameric complex [2, 3].
Our recent studies on the βc cytokine family suggest surprising alternative scenarios in the receptor assembly pathway, which may be relevant in varying degrees to the assembly pathways of other receptors in the Type I cytokine superfamily. Analytical ultracentrifugation suggests that the cytokine and α-subunit can independently associate with the βc dimer, challenging the prevailing hypothesis about the formation of an initial cytokine:α-subunit binary complex before associating with the βc dimer. We have also observed the various states of assembly in solution using small angle X-ray scattering, analytical ultracentrifugation and transmission electron microscopy. In addition, we have identified an antibody that interferes with higher order complexes formation, a powerful reagent to discriminate between the biological functions of the hexamer versus the dodecamer complexes. This antibody could also form the basis for the development of more potent antibodies that might be useful as therapeutics in diseases such as asthma and some leukaemias, where bc cytokines are frequently involved.