Abstract
In some fibroblasts, casein kinase 1α (CK1α) is localized to nuclear speckles, which are sub-nuclear compartments supplying splicing factors, whereas it is recruited on keratin filaments in colorectal cancer cells such as DLD1 cells. In order to obtain a deeper understanding of why CK1α is localized to these different subcellular sites, we herein elucidated the mechanisms underlying its localization to nuclear speckles. CK1α and FAM83H were localized to nuclear speckles in RKO and WiDr colorectal cancer cells, which do not express simple epithelial keratins, and in DLD1 cells transfected with siRNAs for type I keratins. The localization of FAM83H to nuclear speckles was also detected in colorectal cancer cells with a poorly organized keratin cytoskeleton in colorectal cancer tissues. Using an interactome analysis of FAM83H, we identified SON, a protein present in nuclear speckles, as a scaffold protein to which FAM83H recruits CK1α. This result was supported by the knockdown of FAM83H or SON delocalizing CK1α from nuclear speckles. We also found that CK1δ and ε are localized to nuclear speckles in a FAM83H-dependent manner. These results suggest that CK1 is recruited to nuclear speckles by FAM83H and SON in the absence of an intact keratin cytoskeleton.
Highlights
Casein kinase 1 (CK1) is a family of serine/threonine protein kinases[1,2]
In our previous study[8], we immunohistochemically examined colorectal cancer tissues collected from 111 patients using an anti-FAM83H antibody, and noted that staining for FAM83H was preferentially detected in the nucleus in a small subset of colorectal cancer tissues (Fig. 1a); we hypothesized that FAM83H is present in the nucleus and may recruit CK1αto the nucleus rather than on keratin filaments
We demonstrated that CK1αrequires FAM83H and SON to be localized to nuclear speckles
Summary
FAM83H is localized to nuclear speckles in a small subset of colorectal cancer tissues. These results support the hypothesis, proposed by in vivo experiments, that CK1αand FAM83H are localized to nuclear speckles in colorectal cancer cells without an intact keratin cytoskeleton. We confirmed the binding of these CK1 members to FAM83H-FLAG in RKO-F3 cells in co-immunoprecipitation assays using an anti-FLAG antibody (Fig. 5g) These results indicate that CK1δand εas well as CK1αare localized to nuclear speckles in a FAM83H-dependent manner. In order to examine whether the phosphorylation of SR proteins is promoted by the accumulation of CK1 members in nuclear speckles, we performed a Western blot analysis of RKO-F1, 2, and 3 cells, in which CK1α, δ, and εstrongly accumulated in nuclear speckles, as already shown in Fig. 5d–f, using the mAb104 antibody. Further studies are needed in order to clarify the function of CK1 and FAM83H in nuclear speckles in colorectal cancer cells
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