MAT1-Modulated CAK Activity Regulates Cell Cycle G₁ Exit

Abstract

The cyclin-dependent kinase (CDK)-activating kinase (CAK) is involved in cell cycle control, transcription, and DNA repair (E. A. Nigg, Curr. Opin. Cell. Biol. 8:312–317, 1996). However, the mechanisms of how CAK is integrated into these signaling pathways remain unknown. We previously demonstrated that abrogation of MAT1 (ménage à trois 1), an assembly factor and targeting subunit of CAK, induces G₁ arrest (L. Wu, P. Chen, J. J. Hwang, L. W. Barsky, K. I. Weinberg, A. Jong, and V. A. Starnes, J. Biol. Chem. 274:5564–5572, 1999). This result led us to investigate how deregulation of CAK by MAT1 abrogation affects the cell cycle G₁ exit, a process that is regulated most closely by phosphorylation of retinoblastoma tumor suppressor protein (pRb). Using mammalian cellular models that undergo G₁arrest evoked by antisense MAT1 abrogation, we found that deregulation of CAK inhibits pRb phosphorylation and cyclin E expression, CAK phosphorylation of pRb is MAT1 dose dependent but cyclin D1/CDK4 independent, and MAT1 interacts with pRb. These results suggest that CAK is involved in the regulation of cell cycle G₁ exit while MAT1-modulated CAK formation and CAK phosphorylation of pRb may determine the cell cycle specificity of CAK in G₁progression.

ACKNOWLEDGMENTS

We thank Yuen Kai Fung for providing the GSTpRb-A/B/C construct and W. French Anderson for providing the retroviral pG1xSvNa vector.