Characterization of Cyclin L1 and L2 Interactions with CDK11 and Splicing Factors

Abstract

Although it has been reported that cyclin L1␣ and L2␣ 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␣ and -␤ proteins and the co-elution of these proteins following size exclusion chromatography. We further establish that CDK11 p110 and associated cyclin L␣/␤ 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. Preincubation of nuclear extracts with purified cyclin L␣ and -␤ isoforms depletes the extract of in vitro splicing activity. Ectopic expression of cyclin L1␣, L1␤, L2␣, or L2␤ 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␣ and -␤ 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␣ and -␤ isoforms and SR proteins to regulate splicing.It has become apparent over the past decade that several cyclin-dependent kinases (CDKs) 4 and their cyclin regulatory partners participate in regulating mRNA production (1). Thus far, CDK7, CDK8, and CDK9 functions are ascribed to transcriptional initiation and elongation, and CDK12 (CrkRS) and CDK13 (CDC2L5) functions are related to pre-mRNA splicing (2-4). Interestingly, CDK11 p110 plays roles in both transcription and splicing, suggesting that this CDK may link the two processes (5, 6). In addition, the CDK11 p110 partner proteins cyclins L1 and L2 also influence splicing (7,8). Two distinct genes, Cdc2L1 and Cdc2L2 (acronym for Cell division control 2 Like), encode the human p110 and p58 PITSLRE protein kinases (9 -12). These kinases were renamed CDK11 p110 and CDK11 p58 when cyclins L1 and L2 were identified as regulatory subunits of CDK11 p110 (13). Expression of the CDK11 p110 isoforms is ubiquitous and constant throughout the cell cycle (11). In contrast, CDK11 p58 is expressed and functions specifically in G 2 /M via an internal ribosome entry site (IRES) located within the CDK11 p110 mRNA (14 -17). During apoptosis, a third isoform, CDK11 p46 , is generated by caspase-dependent cleavage of CDK11 p110 and CDK11 p58 , leaving the catalytic domain intact (18,19).A role for CDK11 p...

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