Cytokines and their receptor complexes.

The 3-dimensional structures that resulted from X-ray diffraction and nuclear magnetic resonance analyses of a variety of cytokines are reviewed. These proteins form distinct, well-defined superfamilies based on the structures of their monomers, even in the absence of significant sequence homology. However, in several cases, where the active form of a cytokine is a dimer or higher oligomer, these similar monomers adopt very different modes of dimerization. Two examples of complexes of cytokines with receptor molecules are described and the implications for signal transduction are discussed.