Protein phosphatase 2A is required for the initiation of chromosomal DNA replication

Protein phosphatase 2A (PP2A) is an abundant, multifunctional serine/threon ine-specific phosphatase that stimulates simian virus 40 DNA replication. T he question as to whether chromosomal DNA replication also depends on PP2A was addressed by using a cell-free replication system derived from Xenopus laevis eggs. Immunodepletion of PP2A from Xenopus egg extract resulted in s trong inhibition of DNA replication. PP2A was required for the initiation o f replication but not for the elongation of previously engaged replication forks. Therefore, the initiation of chromosomal DNA replication depends not only on phosphorylation by protein kinases but also on dephosphorylation b y PP2A.