The interferon-stimulated gene product HERC5 inhibits human LINE-1 retrotransposition with an ISGylation-independent mechanism

Mobilization of long interspersed element-1 (LINE-1 or L1) compromises genome stability and can cause sporadic genetic disease. Accordingly, cells have evolved multiple mechanisms to restrict L1 retrotransposition. Several interferon-stimulated genes (ISGs) that interact with cytoplasmic L1 ribonucleoproteins (RNPs), which contain the L1-encoded proteins ORF1p and ORF2p, have been identified as suppressors of L1 retrotransposition. We previously reported that the ISG protein HECT and RLD domain containing E3 ubiquitin-protein ligase 5 (HERC5) efficiently inhibits L1 retrotransposition. While HERC5 is known to restrict numerous viruses through ISGylation, how HERC5 inhibits L1 remains to be elucidated. Here, we show that HERC5 inhibits L1 retrotransposition through an ISGylation-independent mechanism. HERC5 interacts with L1 RNA and reduces ORF1p levels, a function that requires the ORF1 coding sequence. We further demonstrate that HERC5 decreases L1 translation efficiency and prevents the assembly of L1 RNPs. Our comparative analysis suggests that HERC5 may have acquired its L1-inhibitory function during the evolution of the small HERC family. These findings uncover a previously unidentified mechanism by which an ISG protein recognizes and inhibits L1 and suggest a role for HERC5 as an evolutionarily adapted restriction factor that expands the repertoire of cellular defenses against retrotransposons.