SummaryTranslational regulation of mRNAs is crucial for promoting various 
cellular and developmental processes. Pumilio1 (Pum1) has been shown to play key 
roles in translational regulation of target mRNAs in many systems of diverse 
organisms. In zebrafish immature oocytes, Pum1 was shown to bind to cyclin B1 
mRNA and promote the formation of cyclin B1 RNA granules. This Pum1-mediated RNA 
granule formation seemed critical to determine the timing of translational 
activation of cyclin B1 mRNA during oocyte maturation, leading to activation of 
maturation/M-phase-promoting factor (MPF) at the appropriate timing. Despite its 
fundamental importance, the mechanisms of translational regulation by Pum1 
remain elusive. In this study, we examined the phosphorylation of Pum1 as a 
first step to understand the mechanisms of Pum1-mediated translation. SDS-PAGE 
analyses and phosphatase treatments showed that Pum1 was phosphorylated at 
multiple sites during oocyte maturation. This phosphorylation began in an early 
period after induction of oocyte maturation, which preceded the polyadenylation 
of cyclin B1 mRNA. Interestingly, depolymerization of actin filaments in 
immature oocytes caused phosphorylation of Pum1, disassembly of cyclin B1 RNA 
granules, and polyadenylation of cyclin B1 mRNA but not translational activation 
of the mRNA. Overexpression of the Pum1 N-terminus prevented the phosphorylation 
of Pum1, disassembly of cyclin B1 RNA granules, and translational activation of 
the mRNA even after induction of oocyte maturation. These results suggest that 
Pum1 phosphorylation in the early period of oocyte maturation is one of the key 
processes for promoting the disassembly of cyclin B1 RNA granules and 
translational activation of target mRNA.