Detection of the Ligand Activity of the c‐erbB‐2 Protein in Calf Serum

We established NIH3T3 derivatives in which wild‐type c‐erbB‐2 or activated c‐erbB‐2 having a point mutation in the sequence coding for the transmembrane domain was expressed. These cell lines were termed RC and A4, respectively. A4 cells but not RC or NIH3T3 cells grew even in the presence of a low concentration (0.05%) of calf serum (CS), although the rate of their proliferation was low. In media containing 0.1% CS, both A4 cells and RC cells but not NIH3T3 cells could proliferate. Furthermore, RC cells induced foci formation when cultured in media containing 0.5% and 5% CS, while growth of the parental NIH3T3 cells was contact‐inhibited under these conditions. These data suggest the presence of a factor(s) which activates protein‐tyrosine kinase activity of the c‐erbB‐2 protein. In fact, the c‐erbB‐2 protein prepared from RC cells showed CS‐dependent protein‐tyrosine kinase activity when assayed in membrane fractions.