Rigogliuso Giuseppe, Biniossek Martin L, Goodier John L, Mayer Bettina, Pereira Gavin C, Schilling Oliver, Meese Eckart, Mayer Jens
1Department of Human Genetics, Medical Faculty, University of Saarland, Homburg, Germany.
2Institute of Molecular Medicine and Cell Research, University of Freiburg, Freiburg, Germany.
Mob DNA. 2019 Aug 22;10:36. doi: 10.1186/s13100-019-0178-z. eCollection 2019.
A considerable portion of the human genome derives from retroviruses inherited over millions of years. Human endogenous retroviruses (HERVs) are usually severely mutated, yet some coding-competent HERVs exist. The HERV-K(HML-2) group includes evolutionarily young proviruses that encode typical retroviral proteins. HERV-K(HML-2) has been implicated in various human diseases because transcription is often upregulated and some of its encoded proteins are known to affect cell biology. HERV-K(HML-2) Protease (Pro) has received little attention so far, although it is expressed in some disease contexts and other retroviral proteases are known to process cellular proteins.
We set out to identify human cellular proteins that are substrates of HERV-K(HML-2) Pro employing a modified Terminal Amine Isotopic Labeling of Substrates (TAILS) procedure. Thousands of human proteins were identified by this assay as significantly processed by HERV-K(HML-2) Pro at both acidic and neutral pH. We confirmed cleavage of a majority of selected human proteins in vitro and in co-expression experiments in vivo. Sizes of processing products observed for some of the tested proteins coincided with product sizes predicted by TAILS. Processed proteins locate to various cellular compartments and participate in diverse, often disease-relevant cellular processes. A limited number of HERV-K(HML-2) reference and non-reference loci appears capable of encoding active Pro.
Our findings from an approach combining TAILS with experimental verification of candidate proteins in vitro and in cultured cells suggest that hundreds of cellular proteins are potential substrates of HERV-K(HML-2) Pro. It is therefore conceivable that even low-level expression of HERV-K(HML-2) Pro affects levels of a diverse array of proteins and thus has a functional impact on cell biology and possible relevance for human diseases. Further studies are indicated to elucidate effects of HERV-K(HML-2) Pro expression regarding human substrate proteins, cell biology, and disease. The latter also calls for studies on expression of specific HERV-K(HML-2) loci capable of encoding active Pro. Endogenous retrovirus-encoded Pro activity may also be relevant for disease development in species other than human.
人类基因组的相当一部分源自数百万年来遗传的逆转录病毒。人类内源性逆转录病毒（HERV）通常发生严重突变，但仍存在一些具有编码能力的HERV。HERV-K（HML-2）组包含编码典型逆转录病毒蛋白的进化上较年轻的前病毒。HERV-K（HML-2）与多种人类疾病有关，因为其转录常常上调，并且已知其一些编码蛋白会影响细胞生物学。HERV-K（HML-2）蛋白酶（Pro）迄今很少受到关注，尽管它在某些疾病背景下表达，并且已知其他逆转录病毒蛋白酶可加工细胞蛋白。
我们着手采用改良的底物末端胺同位素标记（TAILS）方法来鉴定作为HERV-K（HML-2）Pro底物的人类细胞蛋白。通过该测定法鉴定出数千种人类蛋白在酸性和中性pH下均被HERV-K（HML-2）Pro显著加工。我们在体外和体内共表达实验中证实了大多数选定人类蛋白的切割。一些测试蛋白观察到的加工产物大小与TAILS预测的产物大小一致。加工后的蛋白定位于各种细胞区室，并参与各种通常与疾病相关的细胞过程。有限数量的HERV-K（HML-2）参考和非参考基因座似乎能够编码活性Pro。
我们将TAILS与体外和培养细胞中候选蛋白的实验验证相结合的方法得出的结果表明，数百种细胞蛋白是HERV-K（HML-2）Pro的潜在底物。因此可以想象，即使HERV-K（HML-2）Pro的低水平表达也会影响多种蛋白的水平，从而对细胞生物学产生功能影响，并可能与人类疾病相关。需要进一步研究以阐明HERV-K（HML-2）Pro表达对人类底物蛋白、细胞生物学和疾病的影响。后者还需要对能够编码活性Pro的特定HERV-K（HML-2）基因座进行表达研究内源逆转录病毒编码的Pro活性也可能与人类以外物种的疾病发展有关。
