Stress granules (SGs) are highly dynamic and reversible cytoplasmic biomolecular condensates formed via liquid‐liquid phase separation (LLPS) under various stresses. As inherently heterogeneous assemblies, SGs possess distinct stable cores (initial nucleation seeds), substructures, or microphases. However, the mechanisms governing the formation and heterogeneity of SG nucleation seeds, and their dynamic integration, remain largely unclear. Here, we demonstrate that LENG8 is recruited to SGs under multiple stress conditions and is indispensable for SG assembly. Upon stress exposure, nuclear LENG8 granules disassemble, enabling LENG8 to translocate into the cytoplasm and undergo LLPS to form independent initial nucleation foci distinct from canonical G3BP1/TIA1‐dependent seeds. Subsequently, these LENG8‐initiated foci merge into growing SGs through a direct interaction between the prion‐like domain of LENG8 and TIA1, facilitating SG expansion and maturation. Depletion of LENG8 or disruption of the LENG8‐TIA1 interaction markedly impairs SG formation. Using conditional Leng8 knockout mice, we further establish that LENG8 deficiency attenuates stress‐induced SG assembly and increases cellular apoptosis in germ cells. Collectively, our study identifies LENG8 as a previously unrecognized SG nucleator, revealing the hierarchical assembly and integration mechanism of distinct nucleation modules during early SG biogenesis.

Spermatogenesis is a metabolically intensive process that is highly sensitive to perturbations in proteostasis. The integrated stress response (ISR) and its central effector, ATF4, orchestrate adaptive responses to maintain cellular homeostasis under stress; however, the functional significance of ATF4 in mammalian spermatogenesis has not been established. To investigate this, we engineered a conditional knockout mouse model with germ cell-specific deletion of the Atf4 gene. Results showed that Atf4 deletion did not impair spermatogenesis or male fertility, with knockout mice exhibiting normal testicular histology and standard sperm parameters. Proteomic analysis, however, revealed that ATF4 contributes to testicular protein expression homeostasis, as its deficiency caused marked dysregulation of the testicular proteome, especially impacting SQSTM1/p62 downregulate through endoplasmic reticulum (ER) stress pathway. We conclude that ATF4’s role in regulating proteostatic balance is functionally decoupled from its necessity for the core progression of spermatogenesis. These findings define ATF4 as a potential resilience agent safeguarding testicular function under ER stress, rather than a direct regulator of male germ cell development.