Effects of Cyclic AMP on the Activity of Soluble Protein Kinases in Lemna paucicostata

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Abstract
Three protein kinases which phosphorylate histone were isolated from cellular extract of Lemna plants. They were separated by elution from DEAE-Sephacel column and referred to as PI, PIT and PHI. The PI protein kinase activity was partially inhibited by 10μM cyclic AMP, cyclic GMP or cyclic IMP, while the PII enzyme was activated in the presence of these cyclic nucleotides. The PIII enzyme was cAMPindependent, but slightly inhibited by cyclic CMP and cyclic UMP. The molecular weights of these three protein kinases were 165,000, 85,000 and 145,000, respectively, as estimated from Sephacryl S-300 gel filtration. A single cyclic AMP-binding protein was detected in the PII enzyme fraction by using the photoaffinity cAMP-analogue, 8-N3-cAMP. The protein which specifically bound [3H]-8-N3-cAMP had an apparent molecular weight of 48,000 as determined by SDS-polyacrylamide gel electrophoresis. The phosphorylation of cellular proteins in Lemna was examined by SDS-polyacrylamide gel electrophoresis. Four phosphorylated polypeptides were detected, the phosphorylations of which were stimulated by cAMP. The molecular weights of these four polypeptides were 59,000, 19,000, 16,000 and 14,000, respectively.