is an activator of Anaphase promoting complex/Cyclosome (APC/C), best known for its role as E3 ubiquitin ligase that drives the cell cycle. APC/C activity is regulated by CDK-mediated phosphorylation of FZR1 during mitotic cell cycle. Although the critical role of FZR1 phosphorylation has been shown mainly in yeast and in vitro cell culture studies, its biological significance in mammalian tissues in vivo remained elusive. Here, we examined the in vivo role of FZR1 phosphorylation using a mouse model, in which non-phosphorylatable substitutions were introduced in the putative CDKphosphorylation sites of FZR1. Although ablation of FZR1 phosphorylation did not show substantial consequences in mouse somatic tissues, it led to severe testicular defects resulting in male infertility. In the absence of FZR1 phosphorylation, male juvenile germ cells entered meiosis normally but failed to enter meiosis II or form differentiated spermatids. In aged testis, male mutant germ cells were overall abolished, showing Sertoli cell-only phenotype. In contrast, female mutants showed apparently normal progression of meiosis. The present study demonstrated that phosphorylation of FZR1 is required for temporal regulation of APC/C activity at meiosis II entry, and for maintenance of spermatogonia, which raised an insight into the sexual dimorphism of FZR1-regulation in germ cells. Anaphase promoting complex/Cyclosome (APC/C) controls timely transitions of mitotic cell cycle phases by promoting ubiquitylation and degradation of many key cell cycle regulators 1. APC/C activity is regulated by either of two co-activators CDC20 and FZR1(CDH1), which determine the substrate specificity of ubiquitylation for each cell cycle phase 2-5. APC/C CDC20 activity appears in metaphase-to-anaphase transition, when it has an essential function in promoting chromosome segregation by mediating cyclin B1 and securin degradation. APC/C FZR1 is thought to regulate a wide range of cell cycle events, in which significant number of proteins have been identified as APC/C FZR1 substrates 6,7. While CDC20 plays a role in APC/C activity at metaphase, when the cyclin-dependent kinase 1 (CDK1) activity is high, FZR1 contributes to APC/C activity when CDK1 activity is sustained at a low level 4,8. APC/C FZR1 activity is negatively regulated by CDK-mediated phosphorylation of FZR1 during mitotic cell cycle. At mitotic exit, reduction of CDK1 activity leads to phosphatase-mediated dephosphorylation of FZR1 which then binds and activates APC/C until late G1 phase 9. In yeast mitotic cell cycle, FZR1 is phosphorylated by increased level of CDK activity after late G1/S onward, which subsequently leads to dissociation of FZR1 from APC/C and its inactivation 10-15. In mammals, mitotic cells transiently transfected with mutant FZR1 (CDH1) that lacked potential CDK1 phosphorylation sites, resulted in premature reduction of cyclin A and cyclin B with decrease in G2/M phase-cells 16,17. Thus, CDK-mediated phosphorylation of FZR1 plays a crucial role in temporal regulation of
