Cloning and characterization of p10, an alternatively spliced form of p15 cyclin-dependent kinase inhibitor

We have cloned an alternatively spliced form of cyclin-dependent kinase (CDK) inhibitor p15 from human placenta. The alternative splice arises from an alternative 5' donor site in intron I. An in-frame stop codon within the new exon, called exon 1β, leads to translation of a M(r) 10,000 protein identical to the NH₂ terminus of p15 hut contains a novel, basic COOH terminus. The alternatively spliced form, termed here as pl0, is ubiquitously expressed in normal and tumor cell lines as shown by Northern hybridization and reverse transcription-PCR. Transforming growth factor β1 induces the expression of pill similarly to p15 in human HaCaT keratinocytes. Expression and analysis of p15 and epitope-tagged p10 in cells by immunohistochemistry showed similar localization of both to the cytoplasm and nucleus in mink epithelial cells and cytoplasmic localization in mouse fibroblasts. Analysis of the effects of p10 and p15 on cell growth indicated that both were transiently growth inhibitory in Mv1Lu and NIH 3T3 cells, but their stable expression did not significantly reduce the number of cell colonies. In contrast to p15, CDK4 and CDK6 did not coimmunoprecipitate p10 in transient expression assays in COS-7 cells. Furthermore, overexpression of p10 together with p15 in COS-7 cells did not interfere with the complex formation of p15 with CDK4 or CDK6. Thus, in the absence of detectable CDK binding, p10 is transiently able to restrain cell cycling, indicating that the alternative splicing of the CDK inhibitors presents further complexity in their regulation and functions.