Identification of individual amino acids in platelet-derived growth factor that contribute to the specificity towards the beta-type receptor.

Platelet-derived growth factor constitutes a family of three isoforms (PDGF-AA, -AB, and -BB) composed of two homologous polypeptide chains (A and B). These isoforms interact with two types of receptors termed alpha or beta. Whereas PDGF-AA binds only to the alpha-receptor, PDGF-BB binds and activates both receptors with high affinity. To map regions that are specific for the beta-receptor, we introduced mutations into PDGF-AA located in previously identified epitopes [1991, Biochemistry 30, 3303-3309]. A single amino acid exchange in domain II of PDGF-AA (Ala67----Arg) was sufficient to bring about a reduced but significant activation of the beta-receptor. In domain I the exchange of residues Pro26----Arg together with Ser28----Asn switched the specificity towards the beta-receptor. These data indicate that parts of the exposed domains are indeed involved in receptor binding. Since these single mutations lead to mutant proteins which are about 100-fold less active than PDGF-BB, it is suggested that other amino acid residues also participate in the binding to the receptors.