Deadenylation of poly(A) tails is the critical first step that initiates mRNA degradation in eukaryotic cells. In mammalian cells, the PAN2‑PAN3 and CCR4‑NOT complexes are the two major deadenylases, responsible for shortening long and short poly(A) tails, respectively. Previous studies have established that BTG4 and CNOT6L recruit CCR4‑NOT to execute maternal mRNA deadenylation and degradation at distinct stages of the maternal‑to‑zygotic transition (MZT). However, the in vivo physiological function of the PAN2‑PAN3 complex, which acts upstream of CCR4‑NOT to initiate the deadenylation cascade on long poly(A) tails, has long remained unclear.
A research team led by Prof. Heng-Yu Fan at Zhejiang University in collaboration with Dr. Falong Lu from the Institute of Genetics and Developmental Biology at the Chinese Academy of Sciences and Dr. Shuyan Ji from Zhejiang University, has now uncovered the indispensable role of maternal PAN2 during oocyte‑to‑early‑embryo development. This study, published in Science Bulletin , reveals that PAN2 functions as a critical molecular homeostat that precisely trims mRNA poly(A) tails, clearing maternal transcripts to allow zygotic genome activation (ZGA).
Using an oocyte‑specific Pan2 conditional knockout (cKO) mouse model alongside multi‑omics techniques including PAIso‑seq2, Smart‑seq2, and mass spectrometry‑based proteomics, the researchers systematically decoded PAN2 function during MZT. They found that Pan2 mRNA is stored in germinal vesicle (GV) oocytes, but its translation is kept repressed until metaphase II (MII) by a 3'‑UTR-encoded molecular timer. This timed activation ensures that PAN2 functions within the appropriate developmental window.
Although oocyte maturation and ovulation appeared normal in Pan2 cKO female mice, most embryos arrested at the 2‑cell stage. Transcriptomic analysis showed that Pan2 deficiency caused the abnormal upregulation of 1668 genes and downregulation of 1815 genes at the 2‑cell stage. Notably, nearly three-quarters of the downregulated transcripts were key ZGA genes, whereas over 60 percent of the upregulated transcripts represented uncleared maternal mRNAs. Proteomic profiling confirmed that this failure of transcript clearance led to the abnormal accumulation of proteins such as BTG4 and DIRAS2, causing developmental toxicity that halted embryonic progression.
Mechanistically, PAIso‑seq2 profiling revealed PAN2 selectively targets poly(A) tails with low guanylate (G) and high uridylate (U) content. Poly(A) tails containing G residues (mixed tails generated by TENT4A/B) were resistant to PAN2‑mediated decay. Structural modeling further demonstrated that PAN2 forms a ternary complex with PAN3 and PABPC, allowing PAN2 to be specifically recruited to long poly(A) tails coated with multiple PABPC1 molecules.
Furthermore, global Pan2 knockout mice exhibited intrauterine lethality around embryonic day 10 with severe heart developmental abnormalities, demonstrating that PAN2 function extends beyond early cleavage stages to organogenesis.
Together with the team’s prior discoveries on BTG4‑CCR4‑NOT (2016, Nature Structural & Molecular Biology ), CNOT6L (2018, The EMBO Journal ), PABPN1L (2020, EMBO Reports ), and TUT4/7‑DIS3L2 (2019, Nucleic Acids Research ; 2020, Nature Communications ), this study completes a four-stage model of maternal mRNA cascade degradation. PAN2‑PAN3 initiates the trimming of long poly(A) tails (>150 nt), BTG4 recruits CCR4‑NOT to further shorten the tails, TUT4/7 add terminal uridylation and DIS3L2 executes final degradation. This “stage‑specific, tail‑composition‑dependent” cascade ensures that maternal mRNAs are cleared cleanly and on schedule, establishing a new paradigm for understanding post-transcriptional control in early development.
Science Bulletin