NUCLEAR CASEIN-KINASE-II TYPE ACTIVITY IN PEA ROOT-TIP CELLS DURING GERMINATION

A protein kinase activity having the same characteristics as that of c asein kinase II was detected in pea nuclei isolated from meristematic root tips at different times of germination. Like casein kinase II, th is enzyme was a Ca2+-independent kinase able to phosphorylate casein a nd the decapeptide RRREEETEEE as exogenous substrates using [P-32]GTP as phosphoryl donor. This activity was inhibited in the presence of 5 mu g/ml of the specific casein kinase II inhibitor heparin and enhance d in the presence of 5 mM spermine but, unlike other CKII-type kinases , it was not stimulated by 10 mu g/ml polylysine and 100 mM NaCl. More over, the activity of this kinase decreases considerably between 16 an d 72 h of germination, corresponding to two different physiological st ates of the germinating embryo. The presence of nuclear calcium-depend ent protein kinase(s) other than CKII was also revealed. This was able to phosphorylate casein as exogenous substrate and two endogenous pro teins of 101 and 47.5 kDa, using GTP as phosphoryl donor, and was part icularly active in nuclei isolated after 72 h of germination. Active g el analysis enabled the identification of a protein of 44 kDa, probabl y corresponding to the catalytic subunit of the nuclear CKII-type kina se, only in nuclei purified after 16 h of germination. The physiologic al role of the nuclear CKII-type kinase(s) during germination is discu ssed.