Mita P, Wudzinska A, Sun X, Andrade J, Nayak S, Kahler DJ, Badri S, LaCava J, Ueberheide B, Yun CY, Fenyö D, Boeke JD. LINE-1 protein localization and functional dynamics during the cell cycle.
eLife 2018;
7:30058. [PMID:
29309036 PMCID:
PMC5821460 DOI:
10.7554/elife.30058]
[Citation(s) in RCA: 82] [Impact Index Per Article: 13.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2017] [Accepted: 01/04/2018] [Indexed: 01/12/2023] Open
Abstract
LINE-1/L1 retrotransposon sequences comprise 17% of the human genome. Among the many classes of mobile genetic elements, L1 is the only autonomous retrotransposon that still drives human genomic plasticity today. Through its co-evolution with the human genome, L1 has intertwined itself with host cell biology. However, a clear understanding of L1’s lifecycle and the processes involved in restricting its insertion and intragenomic spread remains elusive. Here we identify modes of L1 proteins’ entrance into the nucleus, a necessary step for L1 proliferation. Using functional, biochemical, and imaging approaches, we also show a clear cell cycle bias for L1 retrotransposition that peaks during the S phase. Our observations provide a basis for novel interpretations about the nature of nuclear and cytoplasmic L1 ribonucleoproteins (RNPs) and the potential role of DNA replication in L1 retrotransposition.
Only two percent of our genetic material or genome are occupied by genes, while between 60-70 percent are made up of hundreds of thousands of copies of very similar DNA sequences. These repetitive sequences evolved from genetic elements called transposons.
Transposons are often referred to as ‘jumping genes’, as they can randomly move within the genome and thereby create dangerous mutations that may lead to cancer or other genetic diseases. LINE-1 is the only remaining active transposon in humans, and it expands by copying and pasting itself to new locations via a process called 'retrotransposition'. To do so, it is first transcribed into RNA – the molecules that help to make proteins – and then converted back into identical DNA sequences.
Previous research has shown that LINE-1 can form complexes with a series of proteins, including the two encoded by LINE-1 RNA itself: ORF1p and ORF2p. The LINE-1 complexes can enter the nucleus of the cell and insert a new copy of LINE-1 into the genome. However, until now it was not known how they do this. To investigate this further, Mita et al. used human cancer cells grown in the lab and tracked LINE-1 during the different stages of the cell cycle.
The results showed that LINE-1 enters the nucleus as the cell starts to divide and the membrane of the nucleus breaks down. The LINE-1 complexes are then retained in the nucleus while the membrane of the nucleus reforms. Later, as the cell duplicates its genetic material, LINE-1 starts to copy and paste itself. Mita et al., together with another group of researchers, also found that during this process, only LINE-1 RNA and ORF2p were found in the nucleus.
This shows that the cell cycle dictates both where the LINE-1 complexes gather and when LINE-1 is active. A next step will be to further investigate how the ‘copy and paste’ mechanisms of LINE-1 and the two LINE-1 proteins are regulated during the cell cycle. In future, this may help to identify LINE-1’s role in processes like aging or in diseases such as cancer.
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