Substrate-specific adapter for the anaphase promoting complex/cyclosome (APC/C) E3 ubiquitin-protein ligase complex. Associates with the APC/C in late mitosis, in replacement of CDC20, and activates the APC/C during anaphase and telophase. The APC/C remains active in degrading substrates to ensure that positive regulators of the cell cycle do not accumulate prematurely. At the G1/S transition FZR1 is phosphorylated, leading to its dissociation from the APC/C. Following DNA damage, it is required for the G2 DNA damage checkpoint: its dephosphorylation and reassociation with the APC/C leads to the ubiquitination of PLK1, preventing entry into mitosis. Acts as an adapter for APC/C to target the DNA-end resection factor RBBP8/CtIP for ubiquitination and subsequent proteasomal degradation. Through the regulation of RBBP8/CtIP protein turnover, may play a role in DNA damage response, favoring DNA double-strand repair through error-prone non-homologous end joining (NHEJ) over error-free, RBBP8-mediated homologous recombination (HR). Protein modification; protein ubiquitination. The unphosphorylated form interacts with APC/C during mitosis. Interacts with NINL. Interacts (in complex with the anaphase promoting complex APC) with MAD2L2; inhibits FZR1-mediated APC/C activation (By similarity). Interacts with SIRT2 (PubMed:22014574). Interacts with USP37. Interacts (via WD repeats) with MAK. Interacts with RBBP8/CtIP; this interaction leads to RBBP8 proteasomal degradation. Interacts with HECW2 (By similarity). Interacts with SASS6; the interaction is regulated by CENATAC and leads to SASS6 proteasomal degradation (By similarity). Interacts (via N-terminus) with CCNF (By similarity). Interacts with CDC6 (By similarity). Interacts with TK1 (via the KEN box) (By similarity). Acetylated. Deacetylated by SIRT2 at Lys-69 and Lys-159; deacetylation enhances the interaction of FZR1 with CDC27, leading to activation of anaphase promoting complex/cyclosome (APC/C). Phosphorylated during mitosis, probably by maturation promoting factor (MPF), leading to its dissociation of the APC/C. Following DNA damage, it is dephosphorylated by CDC14B in G2 phase, leading to its reassociation with the APC/C, and allowing an efficient G2 DNA damage checkpoint. Phosphorylated by MAK (By similarity). Belongs to the WD repeat CDC20/Fizzy family.