INHIBITION OF NUCLEOSIDE DIPHOSPHATE KINASE-ACTIVITY BY IN-VITRO PHOSPHORYLATION BY PROTEIN-KINASE CK2 - DIFFERENTIAL PHOSPHORYLATION OF NDP KINASES IN HELA-CELLS IN CULTURE

Although a number of nucleoside diphosphate kinases (NDPKs) have been reported to act as inhibitors of metastasis or as a transcription fact or in mammals, it is not known whether these functions are linked to t heir enzymatic activity or how this protein is regulated. In this repo rt, we show that in vitro protein kinase CK2 catalyzed phosphorylation of human NDPK A inhibits its enzymatic activity by inhibiting the fir st step of its ping-pong mechanism of catalysis: its autophosphorylati on. Upon in vivo P-32 labeling of HeLa cells, we observed that both hu man NDPKs, A and B, mere autophosphorylated on histidine residues, how ever, only the B isoform appeared to be serine phosphorylated.