Mass Spectrometry / Cell Cycle / Cell line / Animals / Threonine / Phosphorylation / Molecular cloning / Protein expression and purification / Xenopus laevis / Protein Expression / Protein Kinase / Biochemistry and cell biology / Phosphorylation / Molecular cloning / Protein expression and purification / Xenopus laevis / Protein Expression / Protein Kinase / Biochemistry and cell biology