Utilization of γ-³²P-GTP by ectoprotein kinase of 3T3 and SV40 3T3 cells

The ectoprotein membrane kinase(s) of 3T3 and SV40 3T3 cells were found to use GTP as a phosphate donor. Phosphorylation with GTP was similar to that found with ATP, previously used to identify the kinase activity in this system (Mastro and Rozengurt, 1976). Under the same conditions, phosphorylation was greater with ATP than GTP. However, with either nucleotide the phosphorylation was higher in SV40 transformed and growing 3T3 cells than in quiescent ones. Using SDS polyacrylamide gel electrophoresis and radioautography we found that similar substrates were phosphorylated with GTP or ATP.