Serine/threonine kinase receptors: mediators of transforming growth factor beta family signals.

TGF beta represents the largest and most versatile cytokine family known in metazoans. The recent identification of transmembrane serine/threonine kinases as TGF beta family receptors represents a major milestone towards understanding how these factors elicit their varied responses. Genetic and biochemical evidence suggests a general model for the mechanism of activation of these receptors. In this model, the ligand acts as an adaptor, bringing a primary receptor kinase in contact with a second kinase, which becomes phosphorylated and thereby competent to propagate the signal to downstream components. Such a kinase cascade on the membrane defines a new variation in signal transduction. Although many details of this mechanism remain to be clarified, its combinatorial capacity may explain the multifunctional nature of its ligands. Several key genes have been identified whose regulation by these signals mediates cellular responses such as cell cycle arrest. This field is advancing at a fast pace, and the identity of components that carry the receptor signals to these genes should soon be unveiled. It is expected that these advances, as they unfold, will in turn clarify the role of miscreant TGF beta signalling in human diseases, cancer included.