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Brain Res Mol Brain Res May 20, 2005; 136 (1-2): 199-211.
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The circadian clock-containing photoreceptor cells in Xenopus laevis express several isoforms of casein kinase I.

Constance CM , Fan JY , Preuss F , Green CB , Price JL .

The frog (Xenopus laevis) retina has been an important model for the analysis of retinal circadian rhythms. In this paper, several isoforms of X. laevis casein kinase I (CKI) were analyzed to address whether they are involved in the phosphorylation and degradation of period protein (PER), as they are in the circadian oscillators of other species. cDNAs encoding two splice variants of CKI(delta) (a full-length form and deletion isoform, which is missing an exon that encodes a putative nuclear localization signal and two evolutionarily conserved protein kinase domains) were isolated and analyzed, together with a previously isolated CKI(epsilon) isoform. Both CKI(delta) and CKI(epsilon) were shown to be constitutively expressed in the photoreceptors of the retina, where a circadian clock has been localized. Both the full-length CKI(delta) and CKI(epsilon) were shown to have kinase activity in vitro, and the full-length CKI(delta) phosphorylated and degraded Drosophila PER when expressed in Drosophila S2 cells. The expression and biochemical characteristics of these CKIs are consistent with an evolutionarily conserved role for CKI in the Xenopus retinal clock. The CKI(delta) deletion isoform did not exhibit kinase activity and did not trigger degradation of PER. Subcellular localization of both CKI(delta) isoforms was cytoplasmic in several cell culture lines, but the full-length CKI(delta) , and not the deletion CKI(delta) isoform, was localized to both the nucleus and the cytoplasm in Drosophila S2 cells. These results indicate that the sequences missing in the deletion CKI(delta) isoform are important for the nuclear localization and kinase activity of the full-length isoform and that one or both of these features are necessary for degradation of Drosophila PER.

PubMed ID: 15893604
Article link: Brain Res Mol Brain Res
Grant support: [+]

Species referenced: Xenopus laevis
Genes referenced: clock csnk1d csnk1g2 dbt mapt myc npm1 ret rpe tptep2-csnk1e

Article Images: [+] show captions