Casein kinase 1

The Casein kinase 1 family (EC 2.7.11.1) of protein kinases are serine/threonine-selective enzymes that function as regulators of signal transduction pathways in most eukaryotic cell types. CK1 isoforms are involved in Wnt signaling, circadian rhythms, nucleo-cytoplasmic shuttling of transcription factors, DNA repair, and DNA transcription. The Casein kinase 1 family (EC 2.7.11.1) of protein kinases are serine/threonine-selective enzymes that function as regulators of signal transduction pathways in most eukaryotic cell types. CK1 isoforms are involved in Wnt signaling, circadian rhythms, nucleo-cytoplasmic shuttling of transcription factors, DNA repair, and DNA transcription. By the early 1950s it was known from metabolic labeling studies using radioactive phosphate that phosphate groups attached to phosphoproteins inside cells can sometimes undergo rapid exchange of new phosphate for old. In order to perform experiments that would allow isolation and characterization of the enzymes involved in attaching and removing phosphate from proteins, there was a need for convenient substrates for protein kinases and protein phosphatases. Casein has been used as a substrate since the earliest days of research on protein phosphorylation. By the late 1960s, cyclic AMP-dependent protein kinase had been purified, and most attention was centered on kinases and phosphatases that could regulate the activity of important enzymes. Casein kinase activity associated with the endoplasmic reticulum of mammary glands was first characterized in 1974, and its activity was shown to not depend on cyclic AMP. The CK1 family of monomeric serine–threonine protein kinases is found in eukaryotic organisms from yeast to humans. Mammals have seven family members (sometimes referred to as isoforms, but encoded by distinct genes): alpha, beta 1, gamma 1, gamma 2, gamma 3, delta, and epsilon. Isoforms range from 22 to 55 kDa and have been identified in the membranes, nucleus, and cytoplasm of eukaryotes and additionally in the mitotic spindle in mammalian cells. The family members have the highest homology in their kinase domains (53%–98% identical) and differ from most other protein kinases by the presence of the sequence S-I-N instead of A-P-E in kinase domain VIII. The family members appear to have similar substrate specificity in vitro, and substrate selection is thought to be regulated in vivo via subcellular localization and docking sites in specific substrates. One consensus phosphorylation site is S/Tp-X-X-S/T, where S/Tp refers to a phospho-serine or phospho-threonine, X refers to any amino acid, and the underlined residues refer to the target site. Thus, this CKI consensus site requires priming by another kinase. CKI also phosphorylates a related unprimed site, which optimally contains a cluster of acidic amino acids N-terminal to the target S/T including an acidic residue at n − 3 and a hydrophobic region C-terminal to the target S/T. A single acidic residue in the n − 3 position is not sufficient for CKI phosphorylation. In contrast, in several important targets, NF-AT and beta-catenin, CKI does not require n − 3 priming but, instead, phosphorylates the first serine in the sequence S-L-S, which is followed by a cluster of acidic residues, albeit less efficiently than the optimal sites.