CK1δ the so-called SLS motif (Ser-Leu-Ser) has been described, which can be found in β-catenin and nuclear factor of activated T-cells (NFAT). In several sulfatide and cholesterol-3-sulfate (SCS)-binding proteins the consensus motif Lys/Arg-X-Lys/Arg-X-X-Ser/Thr has been identified and phosphorylation of this motif has been demonstrated for myelin basic protein (MBP), the Ras homolog family member A (RhoA), and tau. Subcellular localization Within living cells CK1δ can be detected in both, the cytoplasm and the nucleus, and increased levels of CK1δ can be found in close proximity to the Golgi apparatus and the trans Golgi network (TGN). Temporarily, CK1δ can also be localized to membranes, receptors, transport vesicles, components of the cytoskeleton, centrosomes or spindle poles. While the present NLS is not sufficient for nuclear localization of CK1δ, the presence of the kinase domain and even its enzymatic activity are needed for proper subcellular localization of CK1δ. Interaction with cellular proteins Localization of CK1δ to certain subcellular compartments can furthermore be initiated by its interaction with cellular proteins. In order to mediate interaction with CK1δ appropriate docking motifs need to be present in the respective proteins. Docking motif Phe-X-X-X-Phe has been identified in NFAT, β-catenin, PER, and proteins of the FAM83 family. As an example, nuclear CK1δ can be localized to nuclear speckles by its interaction with FAM83H. Another interaction motif is represented by the sequence Ser-Gln-Ile-Pro, which is present in microtubule plus-end-binding protein 1 (EB1). Numerous interaction partners for CK1δ have been described within recent years, forming strong interactions with CK1δ and therefore being more than simple substrate proteins. As mentioned above, interactions with CK1δ have been shown for AKAP450 and DDX3X. By initially performing yeast two-hybrid screens, interaction could also be confirmed for the Ran-binding protein in the microtubule-organizing center (RanBPM), microtubule-associated protein 1A, and snapin, a protein associated with neurotransmitter release in neuronal cells. Interactions with CK1δ have also been detected for the development-associated factors LEF-1 (lymphocyte enhancer factor-1) and the proneural basic helix-loop-helix (bHLH) transcription factor Atoh1. Finally, interaction of CK1δ with PER and CRY circadian clock proteins have been demonstrated, facilitating nuclear translocation of PERs and CRYs. Cellular functions Circadian rhythm CK1δ seems to be involved in the circadian rhythm, the internal cellular clock, which permits a rhythm of about 24 h. The circadian rhythm mainly consists of a negative feedback loop mediated by (PER) and crypthochrome (CRY) proteins, which can dimerize and shuttle into the nucleus.