Evidence For the Presence of A Cdc2‐Like Protein Kinase In the Dinoflagellate Crypthecodinium Cohnii

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Abstract
The unusual nature of mitosis and ancestral organization of the dinoflagellate nucleus prompted the question of whether the cdc2‐like histone H1 kinase, a presumed ubiquitous cell cycle regulator in eukaryotes, is present in these primitive organisms. Western blotting of Crypthecodinium cohnii protein extracts using antibody against the Pro‐Ser‐Thr‐Ala‐Ile‐Arg‐Glu (=PSTAIRE) amino acid sequence motif, conserved in all cdc2 homologues known, revealed one prominent band corresponding to a protein with an apparent relative molecular weight ≈ 34,000, identical in mobility to that from HeLa cells and Physarum polycephalum, higher and lower eukaryotic controls, respectively. Incubation of C. cohnii cell lysates with p13suc1‐sepharose beads, which preferentially, though not exclusively, bind p34cdc2, resulted in precipitation of a 34‐kDa protein which was reactive with anti‐PSTAIRE antibody, selectively competed for by the PSTAIRE peptide and able to phosphorylate histone H1 in vitro. We conclude that the dinoflagellate C. cohnii contains a protein very similar to the cdc2 gene product from fission yeast and its homologues in all eukaryotes studied thus far.