Deletion of the kinase insert sequence of the platelet-derived growth factor beta-receptor affects receptor kinase activity and signal transduction.

A characteristic feature of the platelet-derived growth factor (PDGF) beta-receptor is the presence of an insert sequence in the protein tyrosine kinase domain. A receptor mutant which lacks the entire insert of 98 amino acids was expressed in CHO cells, and its functional characteristics were compared with those of the wild-type receptor. The mutant receptor bound PDGF-BB with high affinity and mediated internalization and degradation of the ligand with efficiency similar to that of the wild-type receptor but did not transduce a mitogenic signal. It was found to display a decreased autophosphorylation after ligand stimulation and had a decreased ability to phosphorylate exogenous substrates; phosphofructokinase was not phosphorylated at all, whereas a peptide substrate was phosphorylated, albeit at a lower rate compared with phosphorylation by the wild-type receptor. Furthermore, the mutant receptor did not mediate actin reorganization but mediated an increase in c-fos expression. The data indicate that the insert in the kinase domain of the PDGF beta-receptor is important for the substrate specificity or catalytic efficiency of the kinase; the deletion of the insert interferes with the transduction of some, but not all, of the signals that arise after activation of the receptor.