Ee;CDKF;1 protein is a leafy spurge (Euphorbia esula) CDK-activating kinase that is involved in a phosphorylation cascade linked to early stages of cell cycle progression. Yeast two-hybrid screening performed using Ee;CDKF;1 as a bait indicated that one of the interacting proteins was Ee;CDKF;1. Protein-protein interaction of Ee;CDKF;1 was further confirmed by yeast two-hybrid interaction and in vitro pull-down assays. Gel exclusion chromatography and/or native PAGE showed that GST-CDKF;1, MBP-CDKF;1, GST-CDKF;1 devoid of GST, and endogenous Ee;CDKF;1 were capable of forming homo protein complexes which are in dimer, trimer, and/or higher molecular-mass complex in its native state. In addition, Ee;CDKF;1 complexes were autophosphorylated and able to phosphorylate CDK. Moreover, mutant forms of Ee;CDKF;1 (106G/A, 166K/A), which lost autophosphorylation capability completely, were unable to form homo protein complexes in their native state. The result thus demonstrated that autophosphorylation of Ee;CDKF;1 is crucial for both kinase activity and complex formation.
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