Phase-separated CCER1 coordinates histone-to-protamine transition and fertility

Idiopathic fertility disorders are associated with mutations in various genes. Here, we report that coiled-coil glutamate-rich protein 1 (CCER1), a germline-specific and intrinsically disordered protein (IDP), mediates post-meiotic sperm differentiation, while CCER1 deficiency results in defective sperm chromatin compaction and infertility in mice. CCER1 increases transition protein (Tnp1/2) and protamine (Prm1/2) transcription and serves as a multifaceted mediator for multiple epigenetic modification events of histones during the histone to protamine transition (HTP). Immiscible with heterochromatin in the nucleus, CCER1 self-assembles into polymer droplet and acts as a liquid-liquid phase separated condensate in the nucleus. Notably, we identified the loss-of function variants of human CCER1 (hCCER1) in five patients with non-obstuctive azoospermia (NOA) while absent in 2,713 fertile controls. The mutants led to premature termination or frameshift of CCER1 translation and disrupted condensates in vitro. In conclusion, we propose that nuclear CCER1 is a novel phase-separated condensate that links histone epigenetic modifications, sperm HTP transitions, chromatin condensation, and male fertility.