LINE-1 ribonucleoprotein complexes bind DNA to enable nuclear entry during mitosis

Long Interspersed Nuclear Element-1 (LINE-1) is an autonomous retrotransposon that makes up a significant portion of the human genome, contributing to genetic diversity and genome evolution. LINE-1 encodes two proteins, ORF1p and ORF2p, both essential for successful retrotransposition. ORF2p has endonuclease and reverse transcription activity, while ORF1p binds RNA. Many copies of ORF1p assemble onto the LINE-1 RNA to form a ribonucleoprotein complex (RNP). However, the functional role of RNPs in the LINE-1 life cycle is unclear. Using reconstitution assays on DNA curtains, we show that L1 RNPs gain DNA binding activity only when RNA is super-saturated with ORF1p. In cells, L1 RNPs bind to chromosomes during mitosis. Mutational analysis reveals that DNA binding is crucial for nuclear entry and LINE-1 retrotransposition activity. Thus, a key function of ORF1p is to form an RNP that gains access to the genome through DNA binding upon nuclear envelope breakdown.