Identification of Four Alternatively Spliced Isoforms of Chicken Casein Kinase I Alpha That Are All Expressed in Diverse Cell Types

Casein kinase I (CKI) is a family of widely expressed protein kinases. It is previously shown in mammalian tissues that CKIalpha exists as two or three alternatively spliced isoforms (Rowles et al.,1991; Zhang et al., 1996; Kuret et al., 1997). We now report that four alternatively spliced isoforms of CKIalpha are expressed in many chicken cells and tissues. A partial cDNA clone was isolated from a chicken brain library, using a probe derived from a bovine CKIalpha cDNA. The translated sequence of this clone was 100% identical to the bovine homolog containing the 'L' insert, with the addition of 12 amino acids just before the C terminus that had previously been reported in human and Xenopus CKIalpa. After completing the missing portion of the coding sequence by 5' RACE (rapid amplification of cDNA ends), full-length cDNA was PCR amplified from chicken brain cDNA, yielding four different products. These were cloned and sequenced and found to correspond to the four CKIalpha isoforms: CKIalpha, CKIalphaL, CKIalphaS and CKILalphaLS, where 'S' is the insert consisting of the 12 human/Xenopus C-terminal amino acids. Using reverse transcription and polymerase chain reaction (Rt-PCR), it was shown that the four isoforms are all expressed in neurons, fibroblasts and several tissues. This represents the first demonstration that four splice variants exist and are all expressed in a single type of cell.