Communication between protein tyrosine and protein serine/threonine phosphorylation

In the present article, we have reviewed several promising areas of research, the results of which support the existence of enzymatic cascades linking extracellular growth factor receptor/tyrosine kinases with intracellular serine/threonine kinases. Proceeding either "upstream" from select serine/threonine kinases or "downstream" from the receptors, characterization of both the distal and proximal elements of these cascades is yielding insights into the multiple pathways of signal transduction. It is anticipated that future work will define the intermediate effectors in these cascades and provide us with a clear understanding of the biochemical mechanisms underlying intracellular responses to extracellular signals.