CELL ADHESION-DEPENDENT DIFFERENCES IN ENDOGENOUS PROTEIN-PHOSPHORYLATION ON THE SURFACE OF VARIOUS CELL-LINES

Endogenous phosphorylation of intact cells was studied with 4 mouse, hamster and human cell lines [SV-40 transformed mouse lung embryonic fibroblast STU-51A-232 B cell, Rous sarcoma virus transformed STU-D1756 cell, hamster SV-40 induced tumor fibroblast K-42A2 cell, and human cervical carcinoma HeLa cell] using [.gamma.-32P]ATP and [.gamma.-32P]GTP as exogenous substrates. With all 4 cell lines distinct differences in the phosphoprotein patterns could be demonstrated for cells grown in suspension culture compared to cells grown in monolayers. In cells grown in suspension 2 major, apparently ubiquitous phosphoproteins with MW of 135,000 (128,000 in HeLa cells) and 105,000, representing up to 60% of total phosphorylation, were phosphorylated. These phosphoproteins and the kinase(s) were located on the surface of the suspension cells. Evidence showed that phosphorylation was apparently not a true endogenous reaction, that rather it occurred by cell-cell collision, showing exponentially increasing 32P incorporation with increasing cell population density. Phosphorylation of pp135 and pp105 was established with ATP as well as with GTP and was not dependent on cyclic nucleotides cAMP, cGMP and cCMP. The substrate-attached cells of all 4 cell lines have protein kinases on the cell surface. The lack of pp135 and pp105 phosphorylation may be due to the fact that these phosphoproteins are not expressed at all on the surface of substrate-attached cells or that these phosphoproteins are already fully phosphorylated.