N‐terminal lysines are essential for protein translocation via a modified ERAD system in complex plastids
N‐terminal lysines are essential for protein translocation via a modified ERAD system in complex plastids
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DOI：
10.1111/mmi.12959
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发表时间：
2015-05
影响因子：
3.6
通讯作者：
Julia Lau;Simone Stork;D. Moog;Maik S. Sommer;U. Maier
中科院分区：
文献类型：
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作者：
Julia Lau;Simone Stork;D. Moog;Maik S. Sommer;U. Maier
核编码的前蛋白被输入到红藻起源的复杂质体中，必须穿过多达五层膜。因此，通过第二外层膜或质周膜（PPM）的运输是由SELMA（共生体特异性ERAD样机制）促进的，SELMA是一种内质网相关降解（ERAD）衍生的机制。SELMA的核心成分是参与泛素化的酶（E1 - E3）、Cdc48 atp酶复合物和Derlin蛋白。这些成分存在于所有研究的具有红藻起源的四种膜结合复杂质体的生物体中，表明底物的泛素依赖易位过程在机制上类似于ERAD中的逆行易位过程。即使根据目前的模型，通过SELMA进行的易位不会以经典的多泛素化结束，但易位的机制可能需要前蛋白的瞬时单/寡泛素化。我们研究了SELMA的进口机制，并能够证明蛋白质在PPM中的运输取决于前蛋白N端而不是C端部分的赖氨酸。这些赖氨酸被预测为易位过程中泛素化的目标。由于缺乏N端赖氨酸的蛋白质被卡在PPM中，可以设想并初步表征易位过程的“冷冻中间体”。
Nuclear‐encoded pre‐proteins being imported into complex plastids of red algal origin have to cross up to five membranes. Thereby, transport across the second outermost or periplastidal membrane (PPM) is facilitated by SELMA (symbiont‐specific ERAD‐like machinery), an endoplasmic reticulum‐associated degradation (ERAD)‐derived machinery. Core components of SELMA are enzymes involved in ubiquitination (E1 – E3), a Cdc48 ATPase complex and Derlin proteins. These components are present in all investigated organisms with four membrane‐bound complex plastids of red algal origin, suggesting a ubiquitin‐dependent translocation process of substrates mechanistically similar to the process of retro‐translocation in ERAD. Even if, according to the current model, translocation via SELMA does not end up in the classical poly‐ubiquitination, transient mono‐/oligo‐ubiquitination of pre‐proteins might be required for the mechanism of translocation. We investigated the import mechanism of SELMA and were able to show that protein transport across the PPM depends on lysines in the N‐terminal but not in the C‐terminal part of pre‐proteins. These lysines are predicted to be targets of ubiquitination during the translocation process. As proteins lacking the N‐terminal lysines get stuck in the PPM, a ‘frozen intermediate’ of the translocation process could be envisioned and initially characterized.