Cyclin-Dependent Kinase-Associated Proteins Cks1 and Cks2 Are Essential during Early Embryogenesis and for Cell Cycle Progression in Somatic Cells

Abstract

Cks proteins associate with cyclin-dependent kinases and have therefore been assumed to play a direct role in cell cycle regulation. Mammals have two paralogs, Cks1 and Cks2, and individually deleting the gene encoding either in the mouse has previously been shown not to impact viability. In this study we show that simultaneously disrupting CKS1 and CKS2 leads to embryonic lethality, with embryos dying at or before the morula stage after only two to four cell division cycles. RNA interference (RNAi)-mediated silencing of CKS genes in mouse embryonic fibroblasts (MEFs) or HeLa cells causes cessation of proliferation. In MEFs CKS silencing leads to cell cycle arrest in G₂, followed by rereplication and polyploidy. This phenotype can be attributed to impaired transcription of the CCNB1, CCNA2, and CDK1 genes, encoding cyclin B1, cyclin A, and Cdk1, respectively. Restoration of cyclin B1 expression rescues the cell cycle arrest phenotype conferred by RNAi-mediated Cks protein depletion. Consistent with a direct role in transcription, Cks2 is recruited to chromatin in general and to the promoter regions and open reading frames of genes requiring Cks function with a cell cycle periodicity that correlates with their transcription.

ACKNOWLEDGMENTS

We thank James Roberts (Fred Hutchinson Cancer Research Center) for providing the retrovirus expressing the HPV E6 oncogene, David Morgan (UC San Francisco) for providing the adenovirus expressing human cyclin B1, and Alan Saluk for help with the laser-scanning cytometry experiments.

This work was supported by the National Institutes of Health grant CA74224 to S.I.R.; fellowships of the Susan G. Komen Breast Cancer Foundation, Roche Foundation, Novartis Jubiläumsstiftung, and Swiss Foundation for Medical-Biological Grants to B.G.; fellowships from the Lance Armstrong Foundation and U.S. Department of Defense to V.L.; a fellowship from the Lance Armstrong Foundation to S.R.C.; and a fellowship from the Leukemia and Lymphoma Society of America to C.H.S.