Checkpoint Kinase 1 (CHEK1)

Chk1 is a kinase that phosphorylates cdc25, an important phosphatase in cell cycle control, particularly for entry into mitosis. Cdc25, when phosphorylated on serine 216 by chk1 becomes bound by an adaptor protein in the cytoplasm. Therefore it is inhibited from removing the inhibiting phosphate from MPF (mitotic/maturation promoting factor) added by Wee1. Consequently, a cell is prevented from entering mitosis.CHK1 is expressed ubiquitously as an approximately 2.4-kb mRNA, with the most abundant expression in thymus, testis, small intestine, and colon. Antibodies against CHK1 recognized a 54-kD protein on immunoblots of mammalian cell extracts. However, the protein had altered mobility when isolated from cells treated with ionizing radiation (IR), indicating that CHK1 is modified in response to DNA damage.