The casein kinase 1 (CK1) family of serine / threonine protein kinases are involved in the regulation of multiple physiological and pathological processes in tissue cells, and coordinate the orderly progression of life through different signal transduction pathways. Hedgehog (Hh), Hippo, and Wnt/β-catenin pathways are the main signaling pathways for CK1 to regulate cell growth and proliferation, embryonic development, energy metabolism, circadian rhythm and other life activities. Studies have found that CK1 members play an important role in regulating upstream signals of Wnt, Hh and other pathways, and in particular, they have an important role in regulating enzyme activity in signaling pathways. CK1 family members are highly expressed in many cells. At the same time, they regulate the various signaling pathways to make the body's various life activities in an orderly manner.

Figure 1. Protein structure of CK1.

Casein kinase 1 family

Serine-threonine protein kinases of the CK1 family have been found in eukaryotes from yeast to humans. Mammals have seven family members (sometimes called isoforms, but encoded by different genes): α, β1, γ1, γ2, γ3, δ, and ε. Isomers, ranging from 22 to 55 kDa, have been identified in the membrane, nucleus, and cytoplasm of eukaryotes, and in mitotic spindles of mammalian cells. These family members have the highest homology in their kinase domain (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 the kinase domain VIII. This family member appears to have similar substrate specificity in vitro, and substrate selection is thought to be regulated in vivo through subcellular localization and docking sites in specific substrates. A common phosphorylation site is S/Tp-XXS/T, where S/Tp represents phosphoserine or phosphothreonine, X represents any amino acid, and underlined residues represent the target site. Therefore, this CKI consensus site needs to be triggered by another kinase. CKI also phosphorylates a related unprimed site, which ideally contains an acidic amino acid cluster at the N-terminus of the target S/T, including acid residues at n-3 and a hydrophobic region at the C-terminus of the target S/T. A single acidic residue at position n-3 is not sufficient to achieve CKI phosphorylation. In contrast, among several important targets, NF-AT and β-catenin, CKI does not require n-3 initiation, but is the first serine in the phosphorylation sequence SLS, followed by a cluster of acidic residues, albeit with low efficiency in the best position.

Wnt / β-catenin signaling pathway

The significance of CK1 in the Wnt/β-catenin signaling pathway was first reported in the late 1990s. CK1e was isolated and cloned to express the gene in Xenopus. Xenopus was then used to differentiate into secondary embryonic axes. It was confirmed that CK1 plays a positive regulatory role in the pathway. It was later discovered that CK1 members are actively involved in the regulation of Wnt/β-catenin signaling pathways, and at the same time, they have found that they may play a negative regulatory role in the signaling pathways. CK1-mediated phosphorylation occurs in multiple steps of Wnt/β-catenin signaling, and different CK1 subunits exert different regulatory effects on Wnt signaling. In overexpression experiments, it was found that CK1 subunits such as CK1a, CKd, and CKe bind to endogenous Axin protein complexes in mammalian cells and form part of the β-catenin "destruction complex". Among them, CK1a is mainly responsible for initiating β-catenin phosphorylation and degradation through the combination of residue 228231 and Axin and in the "destruction complex". In the absence of Wnt, CK1 can cause continuous phosphorylation of β-catenin gene sites T41 and S37 through phosphorylation of β-catenin gene site S45 and GSK3 gene site S33. In CK1 dysfunctional mammalian cells, fruit flies, mice, and CK1a knockout mice, CK1a has been shown to negatively regulate the Wnt signaling pathway through phosphorylation of β-catenin.

Figure 2. Protein structure of Wnt8.

Hh signaling classical pathway

Hh signaling receptor complex is mainly composed of 12 transmembrane receptors Patched (Ptc) and Hh, and 7 transmembrane protein Smoothened (Smo) is an essential signal transduction molecule in Hh signaling. When Hh signal is absent, Ptch1 can inhibit the activity of G protein-coupled receptor (GPCR) and signal transduction molecule Smo through some mechanism. When the Hh signal is combined with the receptor PtcIhog, it can reduce the inhibition of Ptc on Smo, promote the envelope aggregation and reveal the cytoplasmic C-terminus of Smo, and make a variety of kinases such as CK1, protein kinase A (PKA), and GPCR Kinase 2 is involved in phosphorylation and activation of Smo, and at the same time promotes Smo-mediated activation of the transcription factor (Ci)/Gli zinc finger structure, thereby inducing Hh target gene expression. The activity of the Smo transduction molecule is closely related to its phosphorylation level. Among the kinases identified so far, CK1 and PKA, especially CK1, play an important role in the phosphorylation of the C-terminal serine residues of the Smo peptide chain.

Conclusions

CK1 participates in the regulation of multiple activities of the living body through different signal pathways, and together with a variety of signals maintains the orderly movement of the body. Now the research on CK1 is being further deepened. In addition to the signaling pathways described above, CK1 may also interact with protein P469674 to form an upstream / downstream protein regulatory network to regulate the body's metabolism, circadian rhythm, and growth. Among the proteins involved are expected to include DVL, β-catenin gene sequence, APC, β-TrCP gene, PER (Pereod), CRYY (Crytochrome), etc. In summary, members of the highly conserved CK1 family play important regulatory roles in many cells, including DNA proliferation, processing and repair, cytoskeletal dynamics, vesicle transport, apoptosis, and cell differentiation. The regulatory effect of CK1 on different pathways and its mechanism need to be further explored in order to provide new ideas for the treatment of different diseases.

Reference

1. Bingham EW; et al. Casein kinase from the Golgi apparatus of lactating mammary gland. The Journal of Biological Chemistry. 1974, 249 (11): 3647–51.