The Asn-420-linked sugar chain in human epidermal growth factor receptor suppresses ligand-independent spontaneous oligomerization - Possible role ofa specific chain in controllable receptor activation

To elucidate a role(s) of Asn-linked sugar chain(s) in the function of epid ermal growth factor receptor (EGFR), a series of the EGFR mutants were prep ared in which potential glycosylation sites in the domain III were eliminat ed by site-directed mutagenesis, Although the wild-type and mutants of Asn- 328, Asn-337, and Asn-389 underwent autophosphorylation in response to epid ermal growth factor (EGF), the Asn-420 --> Gln mutant was found to be const itutively tyrosine-phosphorylated. This abnormal ligand-independent phospho rylation of the mutant appears to be due to a ligand-independent spontaneou s oligomer formation, as shown by a crosslinking experiment using the purif ied soluble extracellular domain (sEGFR), As revealed by the dissociation o f the Asn-420 --> Gln sEGFR oligomer by simple dilution, it seems likely th at the equilibrium is shifted toward oligomer formation to an unusual degre e. Furthermore, it was also found that the mutation caused a loss of the ab ility to bind EGF, These findings suggest that the sugar chain linked to As n-420 plays a crucial role in EGF binding and prevents spontaneous oligomer ization of the EGFR, which may otherwise lead to uncontrollable receptor ac tivation, and support the view of a specific role of an Asn-linked sugar ch ain in the function of a glycoprotein.