Jd. Wright et al., IDENTIFICATION OF SITES ON EPIDERMAL GROWTH-FACTOR RECEPTORS WHICH ARE PHOSPHORYLATED BY PP60(SRC) IN-VITRO, Biochimica et biophysica acta. Molecular cell research, 1312(2), 1996, pp. 85-93
The Epidermal Growth Factor Receptor (EGF-R) becomes constitutively ty
rosine phosphorylated on two novel sites in v-Src transformed cells, a
nd these phosphorylations are associated with enhanced signaling activ
ity [1]. To determine whether Src could directly phosphorylate these s
ites, we have examined the ability of the Src kinase to phosphorylate
both wild-type and kinase-defective EGF-Rs in vitro. Although purified
Src could phosphorylate EGF-Rs, the pattern of phosphorylation sites
was not identical to what was previously found in vivo [1]: Src in vit
ro directly phosphorylated EGF-Rs on one autophosphorylation site (Tyr
1173) which was not a site of Src-induced in vivo phosphorylation, su
ggesting the in vivo inaccessibility of this site. One Src-specific in
vitro phosphorylation site (Tyr 703) appeared to correspond to one of
the in vivo Src-induced sites (sPY2), but the other Src-specific in v
ivo site (sPY1) was not significantly phosphorylated in vitro, raising
the possibility of a Src-induced tyrosine kinase cascade. The ability
of Src to phosphorylate the EGF-R is consistent with the suggestion t
hat the receptor can function as a kinase substrate independent of its
intrinsic enzymatic activity, as implied by recent studies on signali
ng by kinase-defective EGF-Rs.