Characterization of transformation-sensitive membrane-proteins in chick embryo fibroblasts transformed with avian sarcoma virus

When chick embryo fibroblasts (CEF) were transformed with avian sarcoma virus (ASV), marked increases of membrane proteins with molecular weights of 90,000 daltons (P90) and 79,000 daltons (P79) and a decrease of a 50,000 dalton protein (P50) were detected. The mechanism of accumulation of the transformation-sensitive protein P90 was studied. On two-dimensional gel electrophoresis, P90 had the same isoelectric point as P91, a membrane component of nontransformed cells. From the similarity of tryptic peptide maps of ¹²⁵I-labeled P90 and ¹²⁵I-labeled-P91, and the findings that ¹⁴C]glucosamine was incorporated into P91 but not into P90 and that P90 accumulated when glycosylation was blocked with tunicamycin, it was concluded that P90 and P91 have very similar amino acid sequences, but that P91 has an oligosaccharide side chain not present in P90. However, no evidence of the interconversions of P90 and P91 was obtained in chase experiments after specific labeling of one of the two proteins. It was concluded that accumulation of P90 resulted from reduced glycosylation of growing polypeptides of P90, the synthesis of which was enhanced in transformed cells. The enhanced synthesis of P90 in transformed cells seemed to be controlled by the concentration of glucose in the medium, since large excess of glucose diminished the amount of P90 accumulated in transformed cells, and the accumulation of P90 was also observed in nontransformed cells by deprivation of glucose in the culture medium as shown by Shiu et al. (Glucose Depletion Accounts for the Induction of Two Transformation-Sensitive Membrane Proteins in Rous Sarcoma Virus-Transformed Chick Embryo Fibroblasts. Proc. Nat. Acad. Sci. USA74, 3840–3844, 1977).