A Truncated Form of the p27 Cyclin-Dependent Kinase Inhibitor Translated from Pre-mRNA Causes G 2 -Phase Arrest.

Pre-mRNA splicing is an indispensable mechanism for eukaryotic gene expression. Splicing inhibition causes cell cycle arrest at the G and G/M phases, and this is thought to be one of the reasons for the potent antitumor activity of splicing inhibitors. However, the molecular mechanisms underlying the cell cycle arrest have many unknown aspects. In particular, the mechanism of G/M-phase arrest caused by splicing inhibition is completely unknown. Here, we found that lower and higher concentrations of pladienolide B caused M-phase and G-phase arrest, respectively. We analyzed protein levels of cell cycle regulators and found that a truncated form of the p27 cyclin-dependent kinase inhibitor, named p27*, accumulated in G-arrested cells. Overexpression of p27* caused partial G-phase arrest. Conversely, knockdown of p27* accelerated exit from G/M phase after washout of splicing inhibitor. These results suggest that p27* contributes to G/M-phase arrest caused by splicing inhibition. We also found that p27* bound to and inhibited M-phase cyclins, although it is well known that p27 regulates the G/S transition. Intriguingly, p27*, but not full-length p27, was resistant to proteasomal degradation and remained in G/M phase. These results suggest that p27*, which is a very stable truncated protein in G/M phase, contributes to G-phase arrest caused by splicing inhibition.