Four casein kinase I isoforms are differentially partitioned between nucleus and cytoplasm

The casein kinase I (CKI) family consists of at least seven vertebrate gene s, some of which can be alternatively spliced. Previously, we have studied the four splice variants of the chicken CKI alpha gene. The four proteins d iffer 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 extr eme C-terminus. Here cells were transfected with DNA encoding all four isof orms fused to the green fluorescent protein (GFP) and the localization of e ach protein was examined. We noted that the L insert includes the sequence PVGKRKR, which has the characteristics of a nuclear localization signal (N- LS), and we show that the CKI alphaL and CKI alpha LS isoforms which contai n this sequence are targeted to the nucleus, where a fraction becomes assoc iated with nuclear speckles. In contrast the two isoforms lacking the L ins ert remain predominantly cytoplasmic. Mutation of the first lysine in the p utative NLS to asparagine prevented the nuclear entry of GFP-CKI alphaL. Th erefore different CKIa isoforms are targeted to different cellular compartm ents in a fashion modulated by alternate transcription and in these locatio ns presumably phosphorylate and regulate different cellular substrates. (C) 2001 Academic Press.