Phosphorylation

Receptors and signal transduction

The first stage in an idealised signal transduction chain is binding of the ligand (hormone or other effector molecule) to its receptor. In animal cells phosphorylation on tyrosine residues has been shown to be an important consequence of ligand binding in many transmembrane receptors (for example for insulin, platelet-derived growth factor, epidermal growth factor); other receptor types, operating via the G protein/phospholipase C or adenylate cyclase routes, have their receptor activity modulated by phosphorylation (for example the β2-adrenergic receptor). A useful summary of the roles of phosphorylation in these and other animal systems is given by Hardie (this volume).

In plants, receptors for growth regulators have been characterised to varying degrees (Napier & Venis, 1990); however, there is no evidence for a role for phosphorylation in either their regulation or their transduction mechanisms. The only evidence for a transmembrane receptor protein kinase comes from maize, in which a cDNA clone encoding a putative serine/threonine kinase structurally related to receptor tyrosine kinases has been described (Walker & Zhang, 1990). The extracellular domain of this protein is similar to that of Brassica S-locus glycoproteins involved in the self-incompatibility mechanism of this genus (see section on glycosylation, below). The structural similarity to tyrosine kinases like the epidermal growth factor receptor suggests that the extracellular domain of the maize protein interacts with a ligand and that this activates the kinase domain on the cytoplasmic side.