Endogenous ADP-ribosylation of elongation factor 2 in polyoma virus-transformed baby hamster kidney cells

Polyoma virus-transformed baby hamster kidney (pyBHK) cells were cultured in medium containing [32P]orthophosphate and 10% (vol/vol) fetal bovine serum. A 32P-labeled protein with an apparent molecular mass of 97 kDa was immunoprecipitated from cell lysates with antiserum to ADP-ribosylated elongation factor 2 (EF-2). The 32P labeling of the protein was enhanced by culturing cells in medium containing 2% serum instead of 10% serum. The 32P label was completely removed from the protein by treatment with snake venom phosphodiesterase and the digestion product was identified as [32P]AMP, indicating the protein was mono-ADP-ribosylated. HPLC analysis of tryptic peptides of the 32P-labeled 97-kDa protein and purified EF-2, which was ADP-ribosylated in vitro with diphtheria toxin fragment A and [32P]NAD, demonstrated an identical labeled peptide in the two proteins. The data strongly suggest that EF-2 was endogenously ADP-ribosylated in pyBHK cells. Maximum incorporation of radioactivity in EF-2 occurred by 12 hr and remained constant over the subsequent 12 hr. It was estimated that 30-35% of the EF-2 was ADP-ribosylated in cells cultured in medium containing 2% serum. When 32P-labeled cultures were incubated in medium containing unlabeled phosphate, the 32P label was lost from the EF-2 within 30 min.