Four casein kinase I isoforms are differentially partitioned between nucleus and cytoplasm.
The casein kinase I (CKI) family consists of at least seven vertebrate genes, some of which can be alternatively spliced. Previously, we have studied the four splice variants of the chicken CKIalpha gene. The four proteins differ only by the presence or absence of two peptides, a 28-amino-acid "L" insert in the catalytic domain and a 12-amino-acid "S" insert near the extreme C-terminus. Here cells were transfected with DNA encoding all four isoforms fused to the green fluorescent protein (GFP) and the localization of each protein was examined. We noted that the L insert includes the sequence PVGKRKR, which has the characteristics of a nuclear localization signal (NLS), and we show that the CKIalphaL and CKIalphaLS isoforms which contain this sequence are targeted to the nucleus, where a fraction becomes associated with nuclear speckles. In contrast the two isoforms lacking the L insert remain predominantly cytoplasmic. Mutation of the first lysine in the putative NLS to asparagine prevented the nuclear entry of GFP-CKIalphaL. Therefore different CKIalpha isoforms are targeted to different cellular compartments in a fashion modulated by alternate transcription and in these locations presumably phosphorylate and regulate different cellular substrates.
Fu
Z
Z
Department of Neuroscience, University of Florida, Gainesville, Florida 32610, USA.
Chakraborti
T
T
Morse
S
S
Bennett
G S
GS
Shaw
G
G
D016475:3T3 Cells; D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D001216:Asparagine / Q000737:chemistry; D001483:Base Sequence; D015153:Blotting, Western; D019556:COS Cells; D047388:Casein Kinases; D002467:Cell Nucleus / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism*; D003599:Cytoskeleton / Q000378:metabolism; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000378:metabolism; D008239:Lysine / Q000737:chemistry; D051379:Mice; D008856:Microscopy, Fluorescence; D008957:Models, Genetic; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D019913:Nuclear Localization Signals; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D020033:Protein Isoforms; D011494:Protein Kinases / Q000096:biosynthesis* / Q000737:chemistry*; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D014162:Transfection
D008164:Luminescent Proteins; D019913:Nuclear Localization Signals; D020033:Protein Isoforms; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D001216:Asparagine; D011494:Protein Kinases; D047388:Casein Kinases; D008239:Lysine