Abstract
A cellular protein with a molecular mass of approximately 36 kilodaltons is the presumed target protein of the src protein [the transforming protein encoded by Rous sarcoma virus (RSV)]. The cellular location of the phosphorylated 36-kilo-dalton protein (pp36) in chicken embryo fibroblasts transformed by the Schmidt-Ruppin strain of RSV has been investigated. In these studies, two-dimensional electrophoresis was used for detection of the phosphoproteins in total cell extracts and also in fractionated subcellular components. We conclude that pp36 is localized in the plasma membrane, on the basis of the following observations. (i) Fractionation of 32P-labeled cell extracts showed that pp36 is almost exclusively localized in the crude membrane fraction and no appreciable amount was found in nuclear or cytoplasmic fractions. (ii) On further fractionation of the crude membrane fraction, pp36 was localized mostly in the plasma membrane rather than in other membranous fractions. (iii) Isolated plasma membrane by itself phosphorylated the 36-kilodalton protein on incubation with [gamma-32P]ATP.