Cyclin E Associates with Components of the Pre-mRNA Splicing Machinery in Mammalian Cells

ABSTRACT

Cyclin E-cdk2 is a critical regulator of cell cycle progression from G₁ into S phase in mammalian cells. Despite this important function little is known about the downstream targets of this cyclin-kinase complex. Here we have identified components of the pre-mRNA processing machinery as potential targets of cyclin E-cdk2. Cyclin E-specific antibodies coprecipitated a number of cyclin E-associated proteins from cell lysates, among which are the spliceosome-associated proteins, SAP 114, SAP 145, and SAP 155, as well as the snRNP core proteins B′ and B. The three SAPs are all subunits of the essential splicing factor SF3, a component of U2 snRNP. Cyclin E antibodies also specifically immunoprecipitated U2 snRNA and the spliceosome from splicing extracts. We demonstrate that SAP 155 serves as a substrate for cyclin E-cdk2 in vitro and that its phosphorylation in the cyclin E complex can be inhibited by the cdk-specific inhibitor p21. SAP 155 contains numerous cdk consensus phosphorylation sites in its N terminus and is phosphorylated prior to catalytic step II of the splicing pathway, suggesting a potential role for cdk regulation. These findings provide evidence that pre-mRNA splicing may be linked to the cell cycle machinery in mammalian cells.

ACKNOWLEDGMENTS

We thank Michelle Garrett and Ali Fattaey for the generous gift of recombinant p21 and Bill Lane and the Harvard Microchemistry Department for peptide sequencing. We also thank M. McMahon, N. Solvason, D. Mahony, D. Parry, and P. Rickert for suggestions and critical review of the manuscript and Gary Burget and Maribel Andonian for assistance with graphics.

R.R. was supported by an NIH and Tobacco Research Council Grant. The DNAX Research Institute is supported by the Schering-Plough Corporation.