Phase separation of the LINE-1 ORF1 protein is mediated by the N-terminus and coiled-coil domain

Long Interspersed Nuclear Element-1 (LINE-1 or L1) is a retrotransposable element that autonomously replicates in the human genome, resulting in DNA damage and genomic instability. Activation of L1 in senescent cells triggers a type I interferon response and age-associated inflammation. Two open reading frames encode an ORF1 protein functioning as mRNA chaperone and an ORF2 protein providing catalytic activities necessary for retrotransposition. No function has been identified for the conserved, disordered N-terminal region of ORF1. Using microscopy and NMR spectroscopy, we demonstrate that ORF1 forms liquid droplets in vitro in a salt-dependent manner and that interactions between its N-terminal region and coiled-coil domain are necessary for phase separation. Mutations disrupting blocks of charged residues within the N-terminus impair phase separation while some mutations within the coiled-coil domain enhance phase separation. Demixing of the L1 particle from the cytosol may provide a mechanism to protect the L1 transcript from degradation. Statement of significanceOver half of the human genome is comprised of repetitive sequences. The Long Interspersed Nuclear Element-1 (L1) is an autonomous mobile DNA element that can alter its genomic location, resulting in genomic instability and DNA damage. L1 encodes two proteins that are required for this function: the ORF1 RNA chaperone and the enzymatic ORF2. Here, we demonstrate that ORF1 forms liquid-liquid phase separated states in vitro, which is mediated by electrostatic interactions between the conserved, disordered N-terminus and coiled-coil domain. This work provides a framework to explore how L1 phase separation may enhance the ability of the retrotransposable element to colonize the genome by preventing degradation of the L1 transcript and evasion of host immune responses.