Biological information is often transmitted by phosphorylation cascades. However, the biological relevance of specific phosphorylation events is often difficult to determine. An invaluable tool to study the effect of kinases and/or phosphatases is the use of phospho- and dephospho-mimetic substitutions in the respective target proteins. Here, we present a generally applicable procedure of how to design, set-up, and carry out phosphorylation modulation experiments and subsequent monitoring of protein activities, taking -cyclin-dependent kinases (CDKs) as a case study. CDKs are key regulators of cell cycle progression in all eukaryotic cells. Consequently, CDKs are controlled at many levels and phosphorylation of CDKs -themselves is used to regulate their kinase activity. We describe in detail complementation experiments of a mutant in CDKA;1, the major cell cycle kinase in Arabidopsis, with phosphorylation-site variants of CDKA;1. CDKA;1 versions were generated either by mimicking a phosphorylated amino acid by replacing the respective residue with a negatively charged amino acid, e.g., aspartate or glutamate, or by mutating it to a non-phoshorylatable amino acid, such as alanine, valine, or phenylalanine. The genetic complementation studies were accompanied by the isolation of these kinase variants from plant extract and subsequent kinase assays to determine changes in their activity levels. This work allowed us to judge the importance of -posttranslational regulation of CDKA;1 in plants and has shown that the molecular mechanistics of CDK function are apparently conserved across the kingdoms. However, the regulatory wiring of CDKs is -strikingly different between plants, animals, and yeast.