After the discoveries of nerve growth factor (NGF) and epidermal growth factor (EGF), it became clear that polypeptide growth factors play an important role in the control of cell growth and differentiation. Many growth factors have been characterized and shown to stimulate pleiotropic responses by binding to and activating cell-surface receptors with protein tyrosine kinase activity (Ullrich and Schlessinger 1990). All receptor tyrosine kinases exhibit a conserved molecular architecture containing a ligand-binding domain attached to a cytoplasmic region via a single transmembrane domain. The various cytoplasmic portions of receptor tyrosine kinases contain, in addition to the conserved catalytic domain, distinct regulatory sequences with tyrosine autophosphorylation sites and serine/threonine phosphorylation sites for different kinases. It was concluded that growth-factor-induced tyrosine phosphorylation is indispensable for signal transduction pathways essential for mitogenesis and transformation, because it was shown that tyrosine kinase activity is required for stimulation of both early and delayed responses such...
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Schlessinger et al. (1992) studied this question.