Stenglein Mark D, Harris Reuben S
Department of Biochemistry, Molecular Biology and Biophysics, The Institute for Molecular Virology and The Beckman Center for Transposon Research, the University of Minnesota, Minneapolis, Minnesota 55455.
Department of Biochemistry, Molecular Biology and Biophysics, The Institute for Molecular Virology and The Beckman Center for Transposon Research, the University of Minnesota, Minneapolis, Minnesota 55455.
J Biol Chem. 2006 Jun 23;281(25):16837-16841. doi: 10.1074/jbc.M602367200. Epub 2006 Apr 28.
The most common transposable genetic element in humans, long interspersed element 1 (L1), constitutes about 20% of the genome. The activity of L1 and related transposons such as Alu elements causes disease and contributes to speciation. Little is known about the cellular mechanisms that control their spread. We show that expression of human APOBEC3B or APOBEC3F decreased the rate of L1 retrotransposition by 5-10-fold. Expression of two related proteins, APOBEC3D or APOBEC3G, had little effect. The mechanism of L1 inhibition did not correlate with an obvious subcellular protein distribution as APOBEC3B appeared predominantly nuclear and APOBEC3F was mostly cytosolic. Two lines of evidence indicated that these APOBEC3 proteins use a deamination-independent mechanism to inhibit L1. First, a catalytically inactive APOBEC3B mutant maintained L1 inhibition activity. Second, cDNA strand-specific C --> T hypermutations were not detected among L1 elements that had replicated in the presence of APOBEC3B or APOBEC3F. In addition, lower levels of retrotransposed L1 DNA accumulated in the presence of APOBEC3B and APOBEC3F. Together, these data combined to suggest a model in which APOBEC3B or APOBEC3F provide a preintegration barrier to L1 retrotransposition. A particularly high level of APOBEC3F protein in human testes and an inverse correlation between L1 activity and APOBEC3 gene number suggest the relevance of this mechanism to mammals.
人类中最常见的可移动遗传元件——长散在核元件1（L1），约占基因组的20%。L1及相关转座子（如Alu元件）的活性会引发疾病并促进物种形成。对于控制它们传播的细胞机制，人们了解甚少。我们发现，人类载脂蛋白B mRNA编辑酶催化多肽样蛋白3B（APOBEC3B）或载脂蛋白B mRNA编辑酶催化多肽样蛋白3F（APOBEC3F）的表达使L1逆转录转座率降低了5至10倍。两种相关蛋白——载脂蛋白B mRNA编辑酶催化多肽样蛋白3D（APOBEC3D）或载脂蛋白B mRNA编辑酶催化多肽样蛋白3G（APOBEC3G）的表达则几乎没有影响。L1抑制机制与明显的亚细胞蛋白分布无关，因为APOBEC3B主要出现在细胞核中，而APOBEC3F大多存在于细胞质中。有两条证据表明，这些APOBEC3蛋白利用一种不依赖脱氨基作用的机制来抑制L1。首先，一种无催化活性的APOBEC3B突变体保持了L1抑制活性。其次，在APOBEC3B或APOBEC3F存在的情况下复制的L1元件中，未检测到cDNA链特异性的C→T超突变。此外，在APOBEC3B和APOBEC3F存在的情况下，积累的逆转录转座L1 DNA水平较低。综合这些数据表明了一种模型，即APOBEC3B或APOBEC3F为L1逆转录转座提供了一种整合前屏障。人类睾丸中APOBEC3F蛋白的含量特别高，以及L1活性与APOBEC3基因数量之间的负相关表明了该机制与哺乳动物的相关性。
