The cytoplasmic lattice in mammalian eggs is a highly ordered macromolecular assembly of 16 distinct proteins organized into three functional modules—maternal-effect protein scaffolds, ubiquitination machinery, and tubulin heterodimers—that sequesters ubiquitin ligases in activity-excluded states and stores assembly-competent GTP-bound tubulin, thereby creating a structured maternal reservoir that prevents premature enzymatic activity and spontaneous microtubule polymerization while maintaining these components poised for rapid deployment during the egg-to-embryo transition.

The cytoplasmic lattice (CPL) in mammalian eggs is essential for early embryonic development, but its molecular components, structural organization, and functional capacity have remained elusive. Here, using cryo-electron microscopy, we show that the CPL filament in mouse eggs contains repeating units with a periodicity of ∼37 nm, and determine its high-resolution, native structure and complete subunit composition. The CPL architecture organizes maternal-effect proteins, ubiquitination machinery, and tubulin into a highly structured reservoir. Maternal-effect proteins form the scaffold of the CPL to sequester a UBE2D–UHRF1 E2-E3 ubiquitination module and three distinct FBXW E3 ubiquitin ligases, notably all in activity-excluded states. The CPL further contains an αβ-tubulin heterodimer in a GTP-bound state with a calcium ion coordinated to α-tubulin, suggesting microtubule assembly-competent tubulin held in reserve. The CPL structure is capped at each end by a terminal unit that lacks a PADI6 dimer, a scaffold component, thereby preventing further oligomerization. Interactions between maternal-effect proteins in adjacent CPL units promote the assembly of a three-dimensional lattice in the egg cytoplasm. Taken together, our work defines how CPL assembly and architecture prime mammalian eggs for ubiquitin-mediated protein degradation and cytoskeletal remodeling during the egg-to-embryo transition.