Abstact

The fertilized egg relies almost entirely on maternal stores in the oocyte to ensure the successful initiation of development(1). The cytoplasmic lattices (CPLs) in mammalian oocytes store maternal-expressed proteins and play an essential role in embryogenesis(2,3). Impairing multiple CPL members leads to early embryonic arrest (EEA), resulting in infertility in mammals. However, the mechanism underlying the assembly and storage of CPLs remains largely unknown. Here, we report the cryo-EM structure of a native mouse CPL repeating unit (~ 4 MDa) at 3.74 A resolution. This repeating unit exhibits a tripartite architecture comprising a framework, extended linkers, and a CPL core. The external framework is built from PADI6 decamers and the subcortical maternal complexes (SCMC). Two linkers formed by NLRP4F polymerize the frameworks into an extended filament. In CPL core, the epigenetic regulator UHRF1 is trapped by PADI6, UBE2D, and NLRP14 in a compact, autoinhibited conformation that prevents nuclear entry and ubiquitin ligase activity. Moreover, the CPL core stores GTP-bound alpha/beta-tubulin heterodimers and inactive SCF E3-ubiquitin ligase components (FBXW-SKP1 complex) in a poised but restrained state. These features establish CPLs as a dynamic regulatory pool that enables rapid microtubule assembly and tightly controlled ubiquitination during the oocyte-to-embryo transition. Together, this semi-in-situ structure illuminates CPL assembly and storage-module organization, and establishes CPLs as specialized proteostasis organelles for maternal regulation in oocytes and early embryonic development.