※ PLK family introduction

    PLK (Polo-like kinase) belong to the "Other" group. PLKs are key regulators in cell cycle. Four members have been identified in human genomes which are known as PLK1-4. Sequence analysis show that PLKs are defined by two features: an N-terminal ser/thr protein kinase domain and a C-terminal duplicated polo-box region. Kinase catalytic is highly conserved among all PLKs while the two Polo-box regions are much less conserved which functions as a single unit named "PBD". Human PLK1 is expressed during late G2 and M phases and regulate much of the machineries which are required in mitosis. PLK2 is expressed primarily in early G1 and control the entry into S phase. The expression level of PLK3 is constant durning cell cycle and plays a important role in many stress response pathways. In addition, PLK2 and PLK3 play some similar roles with PLK1 in mitotic functions including Golgi fragmentation and cytokinesis. Like Plk1, Plk4 is essential for cell viability and studies show that PLK4 can also function as tumor suppressor protein (1).

Reference
1. Lowery, D.M., Lim, D. and Yaffe, M.B. (2005) Structure and function of Polo-like kinases. Oncogene, 24, 248-259. PMID: 15640840.