Cytoplasmic lattices store developmentally poised degradative and cytoskeletal complexes in mammalian eggs.

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 metaphase II 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 UHRF1-UBE2D3 E3-E2 ubiquitination module and three distinct FBXW-SKP1 E3 ubiquitin ligase components, notably all in activity-excluded states. The CPL further contains αβ-tubulin heterodimers in a GTP-bound state, indicating microtubule-assembly-competent tubulin held in reserve. CPL filaments are capped by a terminal unit that lacks a PADI6 dimer, a scaffold component, suggesting a structural mechanism that prevents further oligomerization. Interactions between neighboring CPL filaments promote the assembly of a three-dimensional network 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.