Nuclear Importation of Mariner Transposases among Eukaryotes: Motif Requirements and Homo-Protein Interactions

Mariner-like elements (MLEs) are widespread transposable elements in animal genomes. They have been divided into at least five sub-families with differing host ranges. We investigated whether the ability of transposases encoded by Mos1, Himar1 and Mcmar1 to be actively imported into nuclei varies between host belonging to different eukaryotic taxa. Our findings demonstrate that nuclear importation could restrict the host range of some MLEs in certain eukaryotic lineages, depending on their expression level. We then focused on the nuclear localization signal (NLS) in these proteins, and showed that the first 175 N-terminal residues in the three transposases were required for nuclear importation. We found that two components are involved in the nuclear importation of the Mos1 transposase: an SV40 NLS-like motif (position: aa 168 to 174), and a dimerization sub-domain located within the first 80 residues. Sequence analyses revealed that the dimerization moiety is conserved among MLE transposases, but the Himar1 and Mcmar1 transposases do not contain any conserved NLS motif. This suggests that other NLS-like motifs must intervene in these proteins. Finally, we showed that the over-expression of the Mos1 transposase prevents its nuclear importation in HeLa cells, due to the assembly of transposase aggregates in the cytoplasm.

类Mariner转座子（Mariner-like elements, MLEs）是广泛分布于动物基因组中的转座因子，目前已根据宿主范围差异至少划分为五个亚家族。本研究探究了Mos1、Himar1及Mcmar1编码的转座酶的核输入能力在不同真核生物类群宿主间是否存在差异。研究结果表明，核输入可能会限制部分MLEs在特定真核进化谱系中的宿主范围，这一限制作用取决于转座酶的表达水平。随后，我们针对这些蛋白中的核定位信号（nuclear localization signal, NLS）展开研究，证实三种转座酶的前175个N端残基是核输入所必需的。进一步研究发现，Mos1转座酶的核输入涉及两个元件：一个位于氨基酸168至174位的SV40样核定位基序，以及一个位于前80个残基内的二聚化亚结构域。序列分析显示，该二聚化结构域在MLE转座酶中保守存在，但Himar1与Mcmar1转座酶未携带任何保守的NLS基序，这提示此类蛋白中必然存在其他类NLS基序发挥功能。最后，我们证实当Mos1转座酶过表达时，会因转座酶在细胞质中聚集形成聚集体，从而阻碍其在海拉细胞中的核输入。