Characterization of cyclin L1 and L2 interactions with CDK11 and splicing factors - Influence of cyclin L isoforms on splice site selection

Although it has been reported that cyclin L1 alpha and L2 alpha proteins interact with CDK11(p110), the nature of the cyclin L transcripts, the formation of complexes between the five cyclin L and the three CDK11 protein isoforms, and the influence of these complexes on splicing have not been thoroughly investigated. Here we report that cyclin L1 and L2 genes generate 14 mRNA variants encoding six cyclin L proteins, one of which has not been described previously. Using cyclin L gene-specific antibodies, we demonstrate expression of multiple endogenous cyclin L proteins in human cell lines and mouse tissues. Moreover, we characterize interactions between CDK11(p110), mitosis-specific CDK11(p58), and apoptosis-specific CDK11(p46) with both cyclin L alpha and -beta proteins and the co-elution of these proteins following size exclusion chromatography. We further establish that CDK11(p110) and associated cyclin L alpha/beta proteins localize to splicing factor compartments and nucleoplasm and interact with serine/arginine-rich proteins. Importantly, we also determine the effect of CDK11-cyclin L complexes on pre-mRNA splicing. Pre-incubation of nuclear extracts with purified cyclin L alpha and -beta isoforms depletes the extract of in vitro splicing activity. Ectopic expression of cyclin L1 alpha, L1 beta, L2 alpha, or L2 beta or active CDK11(p110) individually enhances intracellular intron splicing activity, whereas expression of CDK11(p58/p46) or kinase-dead CDK11(p110) represses splicing activity. Finally, we demonstrate that expression of cyclins L alpha and -beta and CDK11(p110) strongly and differentially affects alternative splicing in vivo. Together, these data establish that CDK11(p110) interacts physically and functionally with cyclin L alpha and -beta isoforms and SR proteins to regulate splicing.