Two closely related casein kinase I (CKI) isoforms, CKI␦ and CKI⑀, are ubiquitously expressed in many human tissues, but their specific biologic function remains to be clarified. Here, we provide the first evidence that CKI⑀ is involved in hematopoietic cell differentiation. CKI⑀, but not CKI␦, was down-regulated along with human granulocytic differentiation. The specific down-regulation was observed in granulocyte colony-stimulating factor (G-CSF)-induced cell differentiation of murine interleukin-3 (IL-3)-dependent myeloid progenitor 32D cells.
Introduction of wild-type (WT)-
IntroductionMembers of the casein kinase I (CKI) family of monomeric serine/threonine kinases are highly conserved from yeast to human and are ubiquitously expressed in different cell types. 1,2 In mammals, 7 isoforms (␣, , ␥1-3, ␦, and ⑀) have been identified. [3][4][5][6] These isoforms share a high degree of similarity within the NH 2 -terminal catalytic domains but show considerable variation in their carboxy-terminal (C-terminal) noncatalytic domains. Their variable C-terminal domains are responsible for substrate specificity and serve to promote differential subcellular localization of individual isoforms and to modulate kinase activity. 4,[7][8][9][10] Studies of CKI homologs in yeast have shown the biologic role of CKI in the regulation of DNA repair and normal cell cycle progression, vesicular trafficking, and cytokinesis. [11][12][13][14][15] The identification of potential substrates for CKI in vitro also possibly inferred that CKI might be involved in a wide variety of cellular functions in mammals. For example, CKI is likely to regulate DNA and RNA metabolism, cellular morphology, vesicular trafficking, DNA damage response and repair, and the activity of various transmembrane receptors. [16][17][18][19][20][21][22][23][24] From the diverse cellular functions of CKI isoforms arose a possibility that CKI is likely to regulate the stability of their substrates, protein turnover, and transport-dependent cellular processes. [25][26][27][28] However, there have been only a few reports in which the phosphorylation by CKI is shown to be essential for the biologic function of the substrates. Unexpectedly, recent genetic analyses in diverse fields have demonstrated that CKI⑀ plays an essential role in regulating several critical in vivo processes such as circadian rhythm, embryogenesis, and morphogenesis in various species. [29][30][31][32] The homolog of CKI␦ and CKI⑀ was first cloned in a screen for a budding yeast mutant, hrr25. 11 Human CKI␦ and CKI⑀ encoded on 2 independent genes localized at chromosome 17q25 and 22q12-13 are basic polypeptides of 49 kDa and 47 kDa, respectively. 4,5 They are the closest isoforms in the CKI family, because their amino acid sequences are 98% identical over their kinase domains and 53% identical over their C-terminal domains. The autophosphorylation of the C-terminal domain has been shown to inhibit its kinase activity. 33 Successful complementation of hrr25 mutants in budding yeast by human CKI␦ ...