The article describes the identification and characterization of a novel ubiquitination factor named E4, which is essential for efficient multiubiquitin chain assembly. E4, previously known as UFD2 in yeast, binds to preformed ubiquitin conjugates and catalyzes the assembly of long ubiquitin chains in conjunction with the ubiquitin-conjugating enzyme E2 and the substrate-specific ubiquitin-protein ligase E3. The authors demonstrate that E4 is a distinct protein family member, including human homologs and the Dictyostelium regulatory protein NOSA, which is required for fruiting body development. E4 activity is linked to stress tolerance in yeast, suggesting that it mediates the degradation of aberrant proteins under stress conditions. The study also reveals that E4 interacts with the AAA ATPase CDC48, which may function in the proteolysis pathway after E4-catalyzed ubiquitination. These findings highlight a novel mechanism for regulating protein degradation through ubiquitin chain assembly.The article describes the identification and characterization of a novel ubiquitination factor named E4, which is essential for efficient multiubiquitin chain assembly. E4, previously known as UFD2 in yeast, binds to preformed ubiquitin conjugates and catalyzes the assembly of long ubiquitin chains in conjunction with the ubiquitin-conjugating enzyme E2 and the substrate-specific ubiquitin-protein ligase E3. The authors demonstrate that E4 is a distinct protein family member, including human homologs and the Dictyostelium regulatory protein NOSA, which is required for fruiting body development. E4 activity is linked to stress tolerance in yeast, suggesting that it mediates the degradation of aberrant proteins under stress conditions. The study also reveals that E4 interacts with the AAA ATPase CDC48, which may function in the proteolysis pathway after E4-catalyzed ubiquitination. These findings highlight a novel mechanism for regulating protein degradation through ubiquitin chain assembly.