Cytoplasmic lattices (CPL) are critical storage assemblies found in oocytes that are essential for early embryo development. Recent structural studies reveal key components in the CPL structure, including PADI6, NLRP4F, SCMC, ubiquitin machinery, and α/β-tubulin heterodimers. These studies provide insight into the importance of the CPL and the role of these stored proteins in oocyte maturation.
The oocyte is one of the largest and longest-lived cells in animals and is required to effectively store the core material needed for proper embryonic development. The oocyte uses different compartment storage mechanisms to stockpile these essential building blocks, such as mRNAs, proteins, and molecular machines(reviewed in 1).
Importantly, the growing body of work suggests that a cache of maternal protein is essential for proper embryo development, and this pool of maternal protein is not transcribed; rather, it is stored in a cytoplasmic lattice-like structure(reviewed in 1). The protein storage structure, which was named the cytoplasmic lattice and first discovered in the 1960s, is an organelle-like assembly in oocytes that can comprise up to 10% of the ooplasm volume (see Figure 1)2-6.
Disruption of CPL assembly results in impaired embryonic development beyond the two-cell stage7-9, and dysfunction of key CPL resident proteins is linked to female infertility10-12. Below, we take a closer look at recent novel findings revealing new structural and functional features of the CPL in mammalian oocytes.
Early studies on CPLs proposed that it was composed of ribosomes or intermediate filaments13,14. However, other studies have identified specific proteins, such as peptidylarginine deiminase 6 (PADI6)7 and subunits of the subcortical maternal complex (SCMC), including -factor located in oocytes, permitting embryonic development (FLOPED)15, NLR family pyrin domain containing 5 (NLRP5)9,16, NLRP4F17, transducin-like enhancer of split 6 (TLE6)17, and zinc-finger BED domain-containing protein 3 (ZBED3)18- as components of the CPL.
A recent cryo-electron tomography (cryo-ET) reconstruction ruled out the involvement of intermediate filaments and ribosomes in CPLs; rather, they identified helical-natured fibers comprised of filaments containing PADI6 and SCMC6.
Knockout of PADI6 or SCMC subunit proteins in oocytes led to a reduction in hundreds of maternal proteins6. Furthermore, proteins like α-tubulin, chromatin-regulating proteins, cell signaling proteins, and ubiquitin proteins were shown to be depleted in PADI6 knockout oocytes6, and while some evidence suggested that enrichment of some of these proteins occurred in CPLs, it was unclear how this was occurring.
More recently, three Nature papers utilized cryo-electron microscopy (cryo-EM) to deduce the approximately 2.7 - 4 MDa CPL structure from mouse oocytes that were shown to contain between 13 - 16 unique proteins19-21. At its basic level, the CPL repeating unit is comprised of an external framework comprised of PADI6, SCMC core and SCMC-associated proteins; a linker region made of NLRP4F, and a core region containing ubiquitin-conjugating enzyme E2 D3 (UBE2D3), ubiquitin-like with PHD and RING finger domains 1(UHRF1), NLRP14, FBXW–SKP1 complex, and α/β-tubulin heterodimers19-21.
These studies illuminate the structure of the CPL and the potential functions of key CPL storage proteins, which we discuss below.
The UHRF1 protein, which has classically been shown to regulate DNA methylation in oocytes22, not only associates with the CPL, but its expression levels appear to be controlled by PADI6 in oocytes6. Furthermore, UHRF1 null oocytes displayed disorganized CPLs, which were not dependent on their DNA methylation function23. So, how does UHRF1 affect CPLs, and is the interaction between UHRF1 and PADI6 important?
Li et al. identified a complex with PADI6, UHRF1, and UBE2D, called the MPU complex, which had ubiquitin-regulating functions in oocytes and developing embryos24. Specifically, they found that UHRF1, which is a classic RING-type E3 ubiquitin ligase, can transfer ubiquitin from the E2 ubiquitin conjugating enzyme, UBE2D, to a target substrate. They also showed that PADI6 in the MPU complex actually functions as an inhibitor of both UHRF1 and UBE2D.
In the recent paper by Chi et al. They show that the UHRF1 molecule in the CPL core is extensively interlocked with other CPL proteins like PADI6 and NLRP14, and the five functional domains of the UHRF1 molecule are bound via multiple intramolecular contacts, thus locking it in an inhibitory state in the CPL core19.
Interestingly, the NLRP14 protein identified in the aforementioned structural studies19-21, is preferentially expressed in oocytes and early embryos, and was previously shown to associate with UHRF1 and protect it from degradation25. Recent findings suggest that NLRP14 modulates the E3 ubiquitin ligase activity of both UHRF1 and SKPI26.
The role of UHRF1 ubiquitination in the oocyte has not been extensively investigated, but one of the key functions of the MPU is to assist in the removal of aggregated proteins24. While the recent structural studies did not further examine the ubiquitin-dependent function of the UHRF1 protein, it does help to explain why it is predominantly localized in the cytoplasm in oocytes.
Additionally, the FBXWs–SKP1 ubiquitin complex was also shown to be localized to the CPL core as well19-21. Importantly, these studies showed that the ubiquitin machinery proteins were not just being sequestered in the CPL cores, but they also appeared to provide some structural stability to the CPL as well.
One theory proposed by these studies suggested that the ubiquitin machinery is sequestered in the CPL core, but primed for rapid regulation of oocyte proteins during early embryogenesis. This is supported by studies showing that some of these ubiquitin proteins regulate oocyte competence, and dysfunction of these proteins is associated with fertility disorders6,23,25. It will be of interest to identify the proteins that are targeted by these E3 ubiquitin ligases during oocyte maturation and how they affect embryo development.
The oocyte and early embryo require ample tubulin as it undergoes several rounds of cell division, making it a logical candidate maternal protein to store in CPLs. Several studies in which the CPL was compromised or abolished also reported reductions in tubulin6,23,27, further linking it to the CPL. But why doesn’t the oocyte simply overexpress tubulin if excess levels are needed?
Importantly, one hypothesis for storing key maternal proteins in CPLs is to prevent negative expression feedback loops. Indeed, this may be highly relevant for tubulin, which is tightly regulated by negative feedback loops28,29.
These recent cryo-EM studies were the first to identify tubulin proteins in the CPL core; specifically, they showed that α-tubulin interacts with and is stabilized by NLRP14, while β-tubulin interacts with the FBXWs–SKP1 complex19-21. These interactions maintain the α/β-tubulin heterodimers in a poised but restrained state that is ready for rapid microtubule assembly during oocyte maturation.
The Leung group calculated that upwards of 109 to 1010 a/β tubulin heterodimers may be concentrated in CPLs based on the presence of two a/β tubulin heterodimers per core20, and this may be an underrepresentation as the Deng Lab’s structure showed three a/β tubulin heterodimers present in each core19. In either case, the result is a cache of micromolar quantities of tubulin heterodimers that are primed for use, while not impacting normal tubulin expression in the oocyte.
It will be of interest to better understand how the tubulin in the CPL is activated, and the precise functions performed by CPL-sequestered tubulin.
Several of the studies above showed that mutating or depleting CPL proteins leads to compromised CPL structures, dysfunctional oocytes, and even female infertility issues. On a related topic, vitrification and warming of embryos occur during in vitro fertilization, but how this affects embryo development is not well understood. A recent study showed that the process of vitrification and warming led to disrupted CPLs and ruptured mitochondrial membranes, which could contribute to developmental delays30. Gaining a better understanding of CPLs and the role of the ubiquitin and tubulin proteins stored in them may be highly beneficial towards understanding oocyte maturation and early embryo development.
Ubiquitin Antibody Mouse Monoclonal (Cat # AUB01)
Ubiquitination Affinity Beads (Cat # UBA01B-beads)
Signal-Seeker® Ubiquitin Detection Kit (Cat # BK161)
Signal-Seeker® Acetyl-Lysine Detection Kit - AAC03-Beads (Cat # BK164)
Tubulin protein (>99% pure): porcine brain (Cat # T240)
SiR-Tubulin Kit (Cat # CY-SC002)
1. Jentoft, I.M.A. and M. Schuh, Protein Storage in Oocytes: Implications for Oocyte Quality, Embryonic Development, and Female Fertility. Annu Rev Cell Dev Biol, 2025. 41(1): p. 15-43.
2. Hadek, R., Cytoplasmic whorls in the golden hamster oocyte. J Cell Sci, 1966. 1(3): p. 281-2.
3. Weakley, B.S., Investigations into the structure and fixation properties of cytoplasmic lamellae in the hamster oocyte. Z Zellforsch Mikrosk Anat, 1967. 81(1): p. 91-9.
4. Weakley, B.S., Initial stages in the formation of cytoplasmic lamellae in the hamster oocyte and the identification of associated electron-dense particles. Z Zellforsch Mikrosk Anat, 1969. 97(3): p. 438-48.
5. Gallicano, G.I., R.W. McGaughey, and D.G. Capco, Cytoskeletal sheets appear as universal components of mammalian eggs. J Exp Zool, 1992. 263(2): p. 194-203.
6. Jentoft, I.M.A., et al., Mammalian oocytes store proteins for the early embryo on cytoplasmic lattices. Cell, 2023. 186(24): p. 5308-5327 e25.
7. Esposito, G., et al., Peptidylarginine deiminase (PAD) 6 is essential for oocyte cytoskeletal sheet formation and female fertility. Mol Cell Endocrinol, 2007. 273(1-2): p. 25-31.
8. Li, L., B. Baibakov, and J. Dean, A subcortical maternal complex essential for preimplantation mouse embryogenesis. Dev Cell, 2008. 15(3): p. 416-425.
9. Tong, Z.B., et al., Mater, a maternal effect gene required for early embryonic development in mice. Nat Genet, 2000. 26(3): p. 267-8.
10. Docherty, L.E., et al., Mutations in NLRP5 are associated with reproductive wastage and multilocus imprinting disorders in humans. Nat Commun, 2015. 6: p. 8086.
11. Xu, Y., et al., Mutations in PADI6 Cause Female Infertility Characterized by Early Embryonic Arrest. Am J Hum Genet, 2016. 99(3): p. 744-752.
12. Lin, J., et al., Expanding the genetic and phenotypic spectrum of female infertility caused by TLE6 mutations. J Assist Reprod Genet, 2020. 37(2): p. 437-442.
13. Burkholder, G.D., D.E. Comings, and T.A. Okada, A storage form of ribosomes in mouse oocytes. Exp Cell Res, 1971. 69(2): p. 361-71.
14. Capco, D.G., et al., Cytoskeletal sheets of mammalian eggs and embryos: a lattice-like network of intermediate filaments. Cell Motil Cytoskeleton, 1993. 24(2): p. 85-99.
15. Tashiro, F., et al., Maternal-effect gene Ces5/Ooep/Moep19/Floped is essential for oocyte cytoplasmic lattice formation and embryonic development at the maternal-zygotic stage transition. Genes Cells, 2010. 15(8): p. 813-28.
16. Kim, B., et al., Potential role for MATER in cytoplasmic lattice formation in murine oocytes. PLoS One, 2010. 5(9): p. e12587.
17. Qin, D., et al., The subcortical maternal complex protein Nlrp4f is involved in cytoplasmic lattice formation and organelle distribution. Development, 2019. 146(20).
18. Gao, Z., et al., Zbed3 participates in the subcortical maternal complex and regulates the distribution of organelles. J Mol Cell Biol, 2018. 10(1): p. 74-88.
19. Chi, P., et al., Structure of the mouse cytoplasmic lattice. Nature, 2026. 654(8118): p. 523-531.
20. Kilic, Z.I., et al., Cytoplasmic lattices are megadalton storage complexes in mammalian oocytes. Nature, 2026. 654(8118): p. 541-546.
21. Liu, S., et al., Molecular basis of oocyte cytoplasmic lattice assembly. Nature, 2026. 654(8118): p. 532-540.
22. Maenohara, S., et al., Role of UHRF1 in de novo DNA methylation in oocytes and maintenance methylation in preimplantation embryos. PLoS Genet, 2017. 13(10): p. e1007042.
23. Uemura, S., et al., UHRF1 is essential for proper cytoplasmic architecture and function of mouse oocytes and derived embryos. Life Sci Alliance, 2023. 6(8).
24. Li, J., et al., The maternal PADI6-UHRF1-UBE2D complex regulates ubiquitination during oocyte maturation and embryogenesis. Nat Struct Mol Biol, 2026. 33(3): p. 512-524.
25. Zhang, W., et al., NLRP14 deficiency causes female infertility with oocyte maturation defects and early embryonic arrest by impairing cytoplasmic UHRF1 abundance. Cell Rep, 2023. 42(12): p. 113531.
26. Liu, S., et al., NLRP14 modulates the activity of E3 ubiquitin ligases during the oocyte-to-embryo transition. Nat Commun, 2026. 17(1).
27. Kan, R., et al., Regulation of mouse oocyte microtubule and organelle dynamics by PADI6 and the cytoplasmic lattices. Dev Biol, 2011. 350(2): p. 311-22.
28. Lin, Z., et al., TTC5 mediates autoregulation of tubulin via mRNA degradation. Science, 2020. 367(6473): p. 100-104.
29. Gasic, I., S.A. Boswell, and T.J. Mitchison, Tubulin mRNA stability is sensitive to change in microtubule dynamics caused by multiple physiological and toxic cues. PLoS Biol, 2019. 17(4): p. e3000225.
30. Barroso, M.T., et al., Vitrification-warming delays preimplantation development and impairs mitochondrial function and cytoplasmic lattices integrity in mouse embryos. iScience, 2026. 29(6): p. 115859.
