from G2 phase. In early G2 phase, Cdk1 is inactivated by phosphorylation of T14 and Y15 residues by Wee1 and Myt1 kinases. The initial activation of cyclin B Cdk1 occurs at the centrosome in prophase. This involves Cdk1 dephosphorylation WZ3146 at T14 and Y15 by Cdc25 phosphatase family and cyclin B phosphorylation at Ser126 128 by MPF and Ser133 by Plk1. Chk1 and Chk2 are transducers of ATR and ATM dependent signaling in response to DNA damage. Chk1 has been detected at the interphase centrosome, and inhibition of Chk1 resulted in premature centrosome separation. Chk2 was also reported to localize to the centrosome and could be phosphorylated at Thr 68 26 and Ser 28 by Plk1, which co localized with Chk2 at the centrosome in early mitosis.
Chk1 is activated by ATR in cells treated with ultraviolet AZD0530 radiation, whereas Chk2 is activated by ATM in cells exposed to ionizing radiation. Activation of ATM ATR initiates the subsequent protein kinase cascade through both p53 dependent and independent pathways. In p53 dependent pathways, p53 is phosphorylated on Ser 15 and Ser 20 and then activates downstream targets genes, such as p21 and 14 3 3, which play an important role in G2 M checkpoint through inhibition of Cdk1 cyclin B. In the p53 independent pathway, Chk1 and Chk2 phosphorylate Cdc25 at Ser 216, which down regulate Cdc25 activity by promoting 14 3 3 protein and nuclear export. Chk1 2 also phosphorylates Wee 1 and increases Wee 1 activity. It is known that both Cdc25C and Wee 1 phosphorylation cooperatively reduce Cdk1 cyclin B1 activity leading to G2 M arrest.
In mammalian cells, three members of the Aurora family have been identified: Aurora A, B, and C. Among them, Aurora A is associated with the centrosome and microtubules. Aurora A is essential for controlling multiple steps in the cell cycle from late S phase through M phase, including centrosome maturation and separation, mitotic spindle formation, and mitotic entry and exit. Aurora A mediates its multiple functions by interacting with other centrosome proteins including p53, centrosomin, centromere protein A, Eg5, and BRCA1. Plk1, which is the best studied member of the Plk family in mammalian cells, is involved in various events in mitotic progression. Plk1 increases during S and G2 M. Plk1 phosphorylates and activates Cdc25, which leads to activation of Cdk1 cyclin B1 and G2 M checkpoint .
Plk1 also plays a role in mitosis exit by regulating the anaphase promoting complex. In response to DNA damage, Plk1 activity is inhibited in an ATM ATR dependent manner, preventing mitosis entry. Nek2, which is a member of the Nek kinase family, has a role in regulation of the G2 M checkpoint and is localized to the centrosome. Nek2 has two splice variants: Nek2A and Nek2B. Nek2A is required for centrosome separation at the G2 M transition and forms a complex with the catalytic subunit of protein phosphatase 1 and a large coiled coil protein called C Nap1. Nek2 can phosphorylate its substrates, C Nap1 and Nlp, contributing to their displacement from the centrosome, which is an essential step for subsequent splitting of the centrosome . Survivin is a member of the inhibitor of apoptosis protein family that plays an essential role in the control of cell divisi