Deregulation of poly(A) polymerase interferes with cell growth

Vertebrate poly(A) polymerase (PAP) contains a catalytic domain and a C-terminal Ser-Thr-rich regulatory region. Consensus and nonconsensus cyclin-dependent kinase (cdk) sites are conserved in the Ser-Thr-rich region in vertebrate PAPs. PAP is phosphorylated by cdc2-cyclin B on these sites in vitro and in vivo and is inactivated by hyperphosphorylation in hi-phase cells, when cdc2-cyclin B is active. In the experiments described here, we undertook a genetic approach in chicken DT40 cells to study the function of PAP phosphorylation. We found that PAP is highly conserved in chicken and is essential in DT40 cells. While cells could tolerate reduced levels of PAP, even modest overexpression of either wild-type PAP or a mutant PAP with two consensus cdk sites mutated (cdk(-) PAP) was highly deleterious and at a minimum resulted in reduced growth rates. Importantly, cells that expressed cdk(-) PAP had a significantly lower growth rate than did cells that expressed similar levels of wild-type PAP, which was reflected in increased accumulation of cells in the G(0)-G(1) phase of the cell cycle. We propose that the lower growth rate is due to the failure of hyperphosphorylation and thus M-phase inactivation of cdk(-) PAP.