Catalytic activity of protein kinase CK1δ (casein kinase 1δ) is essential for its normal subcellular localization

Mammalian casein kinase 1 delta (CK1 delta) is a homologue of the S.cereuisiae Hrr25p protein kinase, Hrr25p is involved in regulating diverse events including vesicular trafficking, gene expression, DNA repair, and chromosome segregation. In contrast to Hrr25p, little ii known about the function, regulation, or subcellular localization of CK1 delta, In the present study, we show that CK1 delta in mammalian cells is mainly cytoplasmic and enriched within the Golgi and/or ER-Golgi transport vesicles, consistent with a role in vesicular trafficking. Transient expression of green fluorescent protein (GFP)- or FLAG peptide-tagged CK1 delta showed localization similar to that of the endogenous CK1 delta. GFP-CK1 delta was also enriched at the centrosomes in interphase cells. Strikingly, two inactive mutant CK1 delta proteins (K38M and T176I) showed almost exclusive nuclear staining, suggesting that protein kinase activity is required for normal localization of CK1 delta and prevention of nuclear accumulation. The nuclear export inhibitor leptomycin B promoted nuclear enrichment of CK1 delta indicating that nuclear localization of CK1 delta occurs physiologically. Both endogenous CK1 delta and GFP-CK1 delta are enriched on the spindle poles in mitotic cells, consistent with a role in regulating spindle formation. Localization is a property of the protein kinase domain and is independent of the C-terminal noncatalytic domain. These data are consistent with roles for CK1 delta in mammalian cells analogous to those of its yeast counterparts. (C) 2001 Academic Press.