The transforming protein P68gag-ros of avian sarcoma virus UR2 is a transmembrane protein with the gag portion protruding extracellularly.

We examined the topography of P68gag-ros on the plasma membrane of UR2-transformed chicken embryo fibroblasts. First, radioiodination of intact UR2-transformed cells resulted in the labelling of P68. Second, immunofluorescence experiments showed that anti-p19 antibody, but not an anti-ros serum, stained nonpermeablized UR2-transformed nonproducer cells. Furthermore, protease digestion of intact UR2-transformed cells removed the putative extracellular domain (the p19 portion in P68gag-ros), leaving peptide fragments (p48/p46) which conformed to the size of the ros-encoded sequence in P68. Greater than 60% of P68 molecules were accessible to protease digestion. Based on these results, we conclude that P68gag-ros is a transmembrane protein located primarily on the plasma membrane of UR2-transformed cells. The p19 portion is exposed extracellularly while the kinase domain of ros lies within the cell.