Cytosol counterparts of plasma membrane proteins and their incorporation into the plasmalemma of normal and transformed cells

Abstract Isolated plasma membranes from BHK and pyBHK cells incorporated radioactivity when incubated in corresponding cytosol fractions which had been labelled in vivo by growing the cells in 14 C-labelled protein hydrolysate or in vitro by lactoperoxidase-catalysed iodination. The incorporation increased linearly with time and reached saturation at high cytosol : membrane protein ratios in the incubation mixture. Polyacrylamide gel electrophoresis of treated membranes showed that for plasmalemma from both BHK and pyBHK cells four major membrane proteins were clearly labelled, suggesting that they had been exchanged for counterparts in the cytosol. The same proteins were labelled in each type of membrane. These proteins were also discernible in polypeptide profiles of labelled cytosol and showed higher specific radioactivities in the cytosol than in the treated membranes. Inasmuch as these proteins were not detached from the membrane by sonication, high ionic strength or chelation, they appear to be integral rather than peripheral. The data thus indicate that integral proteins native to the plasma membrane are present in the cytosol and can incorporate into pre-existing plasmalemma.